Chapter 6 Coding and Classification by qingyunliuliu

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									                      Chapter 6 Coding and Classification
6.1 Introduction
06.01a Traditional patient record

In the traditional patient record, data are available in written format only, mainly as free
text, but sometimes also as numeric data, such as laboratory test results. The patient record
is primarily used for patient care itself, that is, for diagnosis, therapy, and prognosis.
Reconstructing the patient history from such a handwritten patient record by a clinician
other than the original author is hindered by the fact that many medical terms are
ill-defined and are perhaps even ambiguous.

06.01b Computer-based patient record

Since many patient data are becoming available in computer-based patient records ( CPRs)
(see Chapter 7), use of these data for purposes other than traditional archiving and
reporting is becoming feasible. Reasons for storing medical data in a computer are given in
Panel 6.1. Decision-support systems may support care providers in making decisions based
on CPR data (see Chapters 15 and 16). For instance, prescription of a drug may trigger a
decision-support system that checks for contraindications or drug interactions. Such a
system will be able to operate properly only if all the diseases and symptoms of a patient
are recorded in a standardized and consistent way.

06.01c Need for classification

Many data in health care, such as diagnoses, patient history data, physical examination data,
or the reporting of X-ray pictures, are expressed as free text (see also Chapters 2 and 3).
This leads to an infinite list of possible expressions. However, statistical overviews and
decision-support systems can cope with only a finite number of classes. Rules for assigning
expressions from the patient record to classes must be well defined by objective criteria.
Assigning such an expression to a class always implies data reduction (i.e., loss of
information), but this is not necessarily a disadvantage.

06.01d Type of classification

The appropriate level of detail and the structure of the classification system depend on the
purpose for which the classification system has been designed. A classification of
diagnoses for health statistics, for example, may require categories other than
classifications for planning patient care in a hospital ward. On the other hand, it must be
possible to present all medically relevant expressions in CPRs without any data reduction.
Therefore, standardized terminologies are used in these type of applications.

06.01e Terminology for coding

In this chapter, we will follow as much as possible the standard terminology used in the
International Standards Organization ( ISO) International Electrotechnical Commission
(IEC) Technical Report TR 9789 (Information Technology; Guidelines for the
Organization and Representation of Data Elements for Data Interchange. Coding Methods
and Principles) (see also Chapter 34). This means that three basic elements are used in the
so-called semantic triangle: (1) object, (2) concept, and (3) term.

      Objects, also called referents, are particular things in reality, and they are concrete
       (e.g., the stomach), as well as abstract (e.g., the mind).
      A concept is a unit of thought formed by using the common properties of a set of
       objects (e.g., an organ).
      A term is a designation by a linguistic expression of a concept or an object in a
       specific language.

6.2 Classifications
06.02a Classifying

The term classifying has two different meanings:

   1. the process of designing a classification, and
   2. the coding or description of an object by using codes or terms that are designators
      of the concepts in a classification.

06.02b Definition

Here, we use only the first meaning of classifying. A classification is an ordered system of
concepts within a domain, with implicit or explicit ordering principles. The way in which
classes are defined depends on their intended use. A classification is based on prior
knowledge and forms a key to the extension of knowledge (see also Fig. 1.1).

06.02c Purpose

The purpose of a classification is, for example, to support the generation of health care
statistics or to facilitate research. Examples are the classification of abnormalities of
electrocardiograms or diagnoses of patients into disease classes.

06.02d Concepts

In a classification, concepts are ordered according to generic relations. Generic relations
are relations of the type "A is a kind of B," for example, pneumonia is a kind of lung
disease, where pneumonia represents the narrower concept and lung disease represents the
broader concept. Classifications contain concepts within a certain domain. Examples of
domains are reason for encounter, diagnosis, and medical procedure. In this respect the
International Classification of Diseases, 9th edition (ICD-9), which will be discussed in
Section 5.1, is a classification of diagnoses. A classification allows one to compare
findings collected in different environments. For instance, if we want to compute the
number of beds required per age category in a hospital, we could use the following age

                                babies        age 0-3
                                children      age 4-12
                                teenagers     age 13-18
                                 adults        age 19-64
                                 elderly       age 65+

In this hypothetical example, defining the classes is a relatively simple task and the
requirements for a classification are easily met (see Panel 6.2). Classifying is done
according to a single criterion: age; that is, age is used as a differentiating criterion.

6.2.1 Ordering Principles
06.02.01a ordering principles

In classifications that use more than one ordering principle, the situation is more
complicated. In classifying diseases we deal with the following aspects, among others:

       anatomic location,
       etiology,
       morphology, and
       dysfunction.

Each of these aspects can be used for a different ordering. Such an ordering throughout a
classification is called an axis. Multiaxial classifications use several orderings
simultaneously. In the International Classification of Primary Care (ICPC), for instance,
the diagnoses are classified along two axes, one for the organ system (an alphabetic
character) and one for the components (a number; see Table 6.1). ICPC has primarily been
designed for epidemiological purposes. Therefore, the classes were chosen in such a way
that for health care studies in primary care, each class will contain a sufficient number of
cases. This is why all tropical diseases are grouped together. This classification may be of
use in areas such as Europe or North America, but it is definitely impractical for general
practitioners operating in tropical areas, such as Africa, Central and South America, India,
or Indonesia.

6.2.2 Nomenclatures and Thesauri
06.02.02a Thesaurus

One of the problems of uniform registration in health care is the lack of a common
terminology. A thesaurus is a list of terms used for a certain application area or domain.
Examples are a list of diagnostic terms or a list of terms for laboratory tests. A thesaurus is
always intended to be complete for its domain. For practical usage, thesauri that also
contain a list of synonyms for each preferred term have also been developed. In this way, a
thesaurus stimulates the usage of standardized terminology. A restricted set of preferred
terms used within an organization for a given purpose is called a controlled vocabulary.

06.02.02b Nomenclature

In a nomenclature, codes are assigned to medical concepts, and medical concepts can be
combined according to specific rules to form more complex concepts. This leads to a large
number of possible code combinations.

06.02.02c Differences between classification and nomenclature
The difference between a classification system and a nomenclature is that in the former
possible codes are predefined, whereas in the latter a user is free to combine codes for all
aspects involved. The retrieval of records for patients whose data fulfill certain
classification codes from a large database is relatively easy; retrieving records for patients
stored by using a nomenclature is more difficult because of the high degree of freedom,
leading to very complex codes. A nomenclature, however, is useful in producing
standardized reports, such as discharge letters.

06.02.02d Nomenclature of diseases

In 1933, the New York Academy of Medicine started work on a database of medical terms,
the Standard Classified Nomenclature of Diseases. The American Medical Association
continued this work in 1961, and in 1965 the Systematic Nomenclature of Pathology
(SNOP) coding system was published by the American College of Pathologists. SNOP
formed the basis for the development of the Systematized Nomenclature of Human and
Veterinary Medicine ( SNOMED), which is an example of such a nomenclature (see
Section 5.4).

6.2.3 Codes
06.02.03a Coding

Coding is the process of assigning an individual object or case to a class, or to a set of
classes in the case of a multiaxial classification. In most classifications, classes are
designated by codes. Coding is, in fact, interpretation of the aspects of an object. Codes
may be formed by numbers, alphabetic characters, or both. The following list describes
different types of codes.

06.02.03b Number codes

       Number codes may be issued sequentially. This means that each new class will be
        given the next unused number. The advantage is that new classes can easily be
       Numbers could be issued at random to avoid any patient-specific information is
        hidden in the code.
       Series of numbers can be reserved for sets of classes. Issuing this type of number is
        only of use with a fixed set of classes, that is, when no expansion of the set of
        classes is expected.

06.02.03c Mnemonic codes

A mnemonic code is formed from one or more characters of its related class rubric. This
helps users to memorize codes. However, for classifications with many classes this may
lead either to long codes or codes with no resemblance to the class rubrics. Therefore,
mnemonic codes are generally used for limited lists of classes. For example, hospital
departments are often indicated by a mnemonic code, such as ENT for the Department of
Ear, Nose, Throat, CAR for Cardiology, or OB-GYN for the Department of Obstetrics and
06.02.03d Hierarchical codes

Hierarchical codes are formed by extending an existing code with one or more additional
characters for each additional level of detail. A hierarchical code thus bears information on
the level of detail of the related class and on the hierarchical relation with its parent class.
This way of coding bears resemblance to the structure of hierarchical databases (Chapter 4),
with "parents" at the higher level, and "children" at the lower levels. This implies that
patient data can be retrieved by using hierarchical codes at a certain level, even when
significant extensions or modifications are made at lower levels. An example of
hierarchical codes are the codes used in ICD-9.

06.02.03e Juxtaposition codes

Juxtaposition codes are composite codes consisting of segments. Each segment provides a
characteristic of the associated class. In ICPC, for instance, a diagnostic code is formed by
using a code consisting of one letter of the alphabet (a mnemonic code for the tract),
followed by a two-digit number. For instance, all codes with the character "D" are related
to the tractus digestivus and all codes starting with an "N" describe disorders of the
nervous system. In the example of ICPC, two independent characteristics are coded
simultaneously, and each characteristic has its own position in the code.

06.02.03f Combination codes

Another example is a classification of medical procedures using ordering principles: action,
equipment, aim, and anatomical site (see Fig. 6.1). The combination of 100 anatomical
sites with 20 different actions, 10 different instruments, and 5 different purposes results in
a classification system with a potential of a 100,000 classes and codes. A way to cope with
this explosion is the use of a combination code. By using a six-digit combination code
consisting of four segments, with segments dedicated to action (two digits), equipment
(two digits), aim (one digit), and anatomical site (one digit), respectively, a coding clerk
has to distinguish only 135 codes, with which 100,000 combinations can be generated.

06.02.03g Value addition codes

In value addition codes in general only powers of 2 are used as a representation of a data
item or class. Just as in a combination code, several characteristics can be coded. In this
case, however, only one number instead of a segment for each characteristic is used as a
code. This is easily illustrated if we code the presence or absence of risk factors, such as:

        20 = 1 for smoker/0 for nonsmoker,
        21 = 2 for overweight/0 for no overweight,
        22 = 4 for increased cholesterol/0 for not increased cholesterol.

By using the codes 1 to 7 we can sum all the three risk factors mentioned above. A smoker
who is overweight but with no increased cholesterol level is coded as 3, and a nonsmoker
who is overweight and who has an increased cholesterol level is coded as 6.

6.2.4 Taxonomy
06.02.04a Taxonomy

Taxonomy is the theoretical study of classification, including its basic principles,
procedures, and rules. The term taxonomy is known from Linnaeus's work in classifying
biological organisms. The term taxonomy is also used to designate the end product of a
taxonomic design process and is then frequently synonymous with classification. In this
book we will use the term taxonomy for the first definition: the science of classification.
The term classification is used for the end product of the design process.

Taxonomy is concerned with classifications in general. All objects in a group have some
features in common, that is, they fall within the boundaries of a group. All mammals form
one group, to which people, cats, and whales belong. A group may be further subdivided
on the basis of another feature or character. The lion, the tiger, and Felix domestica (the cat
in our home) all belong to the group (or set) of cats. In a disease classification system such
as ICD-9, the classification and subdivision are performed by the grouping of diseases in
organ systems or by etiology. The different "chapters" (main disease categories or
etiological categories) of ICD-9 are subdivided into groups, the groups are divided into
three-digit classes, and so on (see Section 5.1 for a description of ICD-9).

6.2.5 Nosology
06.02.05a Nosology

Nosology is usually defined as the science of the classification of diseases. Since
nosological discussions usually involve symptoms, syndromes, disorders, and injuries, as
well as diseases, it would be more appropriate to define nosology as the science of the
classification of diagnostic terms, that is, the taxonomy of diagnostic terms.

Increasing information needs in health care have highlighted many nosological problems. It
seems that the impressive expansion of the diagnostic vocabulary during the last century
has not been matched by the development of a precise meta-language for describing
relations between diagnostic terms. Although meta-terms such as disease, disorder and
syndrome are widely used, there is much confusion as to their proper meaning. A
meta-language for describing nosological relations is either lacking or unused.

06.02.05b Nosography

Nosology is usually distinguished from nosography, which is the science of the description
of diseases. The difference between the definition and the description of disease is usually
explained as follows: A disease definition gives only essential characteristics of the disease,
whereas a description includes accidental characteristics, that is, characteristics that are
empirically correlated with the essence of the disease, such as the so-called classification
criteria of rheumatoid arthritis by the American Rheumatism Association (ARA) (see
Table 6.2). There are no essential characteristics in this definition; all characteristics are
accidental. This kind of definition, in which a set of accidental characteristics is used, is
called polythetic. Apparently, the essential characteristic of rheumatoid arthritis is
something that medical science has not yet discovered.
There is a growing feeling that classifications such as ICD, SNOMED and the Diagnostic
and Statistical Manual for Mental Disorders ( DSM-IV) do no justice to the way in which
diagnostic terms are actually used in health care and that a new paradigm is needed.

6.3 History of Classification
06.03a History of classification

In health care, the most widely used classification system is ICD and the classifications
derived from it. The first attempt at registration was the London Bills of Mortality in 1629.
The first edition of the International List of Causes of Death, as it was then called, was
presented by Jacques Bertillon at a meeting of the International Statistical Institute (ISI) in
1893 in Chicago, and it was officially accepted in 1900. This list was regularly revised
under the supervision of ISI until its fifth edition in 1938. Until then, the code list was
primarily used for mortality statistics. Health insurance companies, hospitals, medical
services, the military, and other agencies felt a growing need to extend the list with codes
for morbidity. The International Health Conference, held in New York City in 1946,
entrusted the Interim Commission of the World Health Organization (WHO) with the
responsibility of undertaking the necessary preparatory work to extend the International
List of Causes of Death with an International List for the Causes of Morbidity.

6.4 Classification and Coding Problems
06.04a Classification and coding problems

Classification problems should be distinguished from coding problems:

       Classification problems concern the ordering of concepts in a way that is logically
        sound, elegant, and well-suited for the potential users of the classification.
       Coding problems concern the technical support that must be provided to enable
        coding clerks to assign an individual case to the right class and produce the right
        code in an efficient and reliable way.

6.4.1 Classification Problems
06.04.01a Combination of categories

A problem of juxtaposition, combination codes, and value addition codes is that not all
combinations that can be generated are sensible. In the example of the medical procedures,
a "transplantation to remove an abscess" is not sensible. Combination codes also give
ambiguous results. The combination of a code for larynx, a code for removal, and a code
for tube is ambiguous. It is unclear whether the tube or the larynx is removed, because the
code lacks semantic information about how the items are related. In a prestandard for the
classification of surgical procedures of the Comité Européen de Normalisation (CEN) (see
Chapter 34), this ambiguity problem is tackled by the use of both semantic categories and
syntactic categories.

06.04.01b Classification Problems

When developing a classification of diseases, etiology, location, and pathophysiological
mechanism can be useful ordering principles. However, we cannot always apply each
ordering principle to all diseases. Using etiology as the ordering principle, we can classify
"viral pneumonia" as a viral disease, but we cannot classify "pneumonia" with the same
degree of certainty to any etiological class. Therefore, pneumonia will be classified, using
an anatomical ordering principle, as a pulmonary disease. Most classifications combine
several ordering principles on one level. The overlap of disease classes that then results
violates the rule of mutual exclusiveness (see Panel 6.2). The class "pulmonary disease"
intersects with the class "viral disease." When a disease is already classified elsewhere, an
exclusion statement is used to indicate that the disease is considered a member of one class
only. However, this will cause problems in statistical analysis. If we want to compute the
number of cases resulting from a viral disease, we cannot simply count the members in the
class "viral diseases", since "viral pneumonia" is also a viral disease but those cases are
classified in the class "pulmonary diseases". Adding the two classes will include cases of
nonviral pulmonary diseases as well.

06.04.01c Maintenance of classification

The dynamic nature of classification explains the continuous need for maintenance of
classifications such as ICD and SNOMED. The classification of acquired immune
deficiency syndrome (AIDS) as a viral disease was preceded by the classification of AIDS
as an immune deficiency disease. The question whether AIDS was a viral disease was
accompanied by much discussion. Nowadays, the hypothesis that AIDS is caused by
infection with human immunodeficiency virus (HIV) is widely accepted.
Creutzfeldt-Jakob's disease is currently regarded to be a prion disease, but it used to be
regarded as a slow virus disease. Because diagnostic terms can disappear or serve different
diagnostic goals over periods of time, one should be aware that statistical analysis of
existing data may require a different algorithm each time a change is made to the
classification scheme.

6.4.2 Coding Problems
06.04.02a Differences between coding language an clinical practice

Browsing large medical classifications of diagnoses and procedures is required to encode a
patient's condition for medicoeconomic purposes. The basic problem with this kind of
browsing is the fact that the language used in the classification is rather different from the
clinical language found in the patient record. Regardless of who encodes the patient's
condition, there are difficulties in terms of mismatches between the terms in the
classification and the overall representation of the patient. This gap can be bridged by
using adequate computer programs.

06.04.02b Intelligent assistance

Two different techniques are used to provide clinicians and encoders with intelligent help.
The first type is morpho-semantic analysis of the input languages to extract all underlying
concepts. This analysis decomposes all compound words into their parts: prefixes, stems,
and suffixes. It then groups similar stems into more general categories. On this basis, an
analysis of all available sentences around the classification in use is performed and a
corresponding indexing is precomputed. Any future query of the browsing process will be
handled in this context. The net result is a somewhat conceptual indexing of the
classification, which has been shown to be much more valuable than usual lexical

The other type of assistance is the incorporation of a thesaurus with synonym expressions
that all point to an existing entry in the classification. Such a thesaurus, which may be
hidden to the user, is part of the corpus on which the indexing is done. By using a large
thesaurus, the overall performance of the browser may be dramatically increased. Local
thesauri that use expressions and other medical terms specific to a language or country are
also possible. In general, synonyms may include equivalent expressions (e.g., proper names)
or subexpressions that represent a specialization of the initial expression. At the
implementation level, browsers for medical classifications are readily available for use on
personal computers, and they usually have adequate response times.

6.5 Classification Systems
6.5.1 ICD - International Classification of Diseases
06.05.01a ICD

As discussed in Section 3, ICD is the archetypal coding system for patient record
abstraction. The first edition was published in 1900, and it is being revised at
approximately 10-year intervals. The most recent version is ICD-10, which was published
in 1992. WHO is responsible for its maintenance. Most present registration systems,
however, are still based on ICD-9 or its modification, ICD-9-CM, which contains more
detailed codes. ICD consists of a core classification of three-digit codes, which are the
minimum requirement for reporting mortality statistics to WHO. An optional fourth digit
provides an additional level of detail. At all levels, the numbers 0 to 7 are used for further
detail, whereas the number 8 is reserved for all other cases and the number 9 is reserved
for unspecified coding.

The basic ICD is meant to be used for coding diagnostic terms, but ICD-9 as well as
ICD-10 also contain a set of expansions for other families of medical terms. For instance,
ICD-9also contains a list of codes starting with the letter "V" for reasons for encounter or
other factors that are related to someone's health status. A list of codes starting with the
letter "E" is used to code external causes of death. The nomenclature of the morphology of
neoplasms is coded by the "M" list.

The disease codes of both ICD-9 and ICD-10 are grouped into chapters. For example, in
ICD-9, infectious and parasitic diseases are coded with the three-digit codes 001 to 139,
and in ICD-10 the codes are renumbered and extended as codes starting with the letters A
or B; for tuberculosis the three-digit codes 010 to 018 are used in ICD-9, and the codes
A16 to A19 are used in ICD-10. The four-digit levels and optional five-digit levels enable
the encoder to provide more detail. Table 6.3 gives examples of some codes in the ICD-9

The U.S. National Center for Health Statistics published a set of clinical modifications to
ICD-9, known as ICD-9-CM. It is fully compatible with ICD-9, but it contains an extra
level of detail where needed (see Table 6.3). In addition, ICD-9-CM contains a volume III
on medical procedures.

6.5.2 ICPC - International Classification of Primary Care
06.05.02a ICPC-International Classification of Primary Care

The World Organization of National Colleges, Academies and Academic Associations of
General Practitioners/Family Physicians (WONCA) did not accept ICD-9, but came up
with its own classification. The granularity of this system is less than that of ICD-9. It is
not only used for coding diagnoses but it also contains codes for reasons for encounter
(RfE) and for therapies and laboratory tests. In most primary care information systems, the
laboratory test results are directly entered as coded numerical values, so there is no need
for manual coding, and a drug prescription module automatically stores the generic code
for the drug and other prescription data.

ICPC is compatible with earlier WONCA classifications, such as the ICHPPC-2-Defined
(International Classification of Health Care Problems in Primary Care) and the
IC-Process-PC. For codes derived from ICHPPC-2-Defined, inclusion criteria (for further
specification of the code) are used.

ICPC is a two-axis system (see Table 6.1). The first axis, primarily oriented toward body
systems (the tracts), is coded by a letter, and the second axis, the component, is coded by
two digits. The component axis contains seven code groups. In this system the diagnosis
pneumonia is coded R81 (R for respiratory tract and 81 for the diagnostic component).
Codes that can be applied to more than one tract are described only as a two-digit
component. For instance, the procedure code 42 (electrical tracing) can be used for
electrocardiograph registration by using the code K42. These codes require the
combination with a tract letter.

06.05.02b SOAP

ICPC is used to encode encounters structured according to the SOAP principle (S for
subjective information, e.g., complaints; O is for objective information, e.g., test and lab
results; A is for assessment, e.g., diagnosis; and P is for plan, e.g., diagnostic tests,
treatment, medication, etc.; see also Chapter 7). Optionally, a fourth digit is used for some
cases when an extra level of detail is required or to specify synonyms, which is a mixture
of coding principles. ICPC can be used in the RfE mode (i.e., for coding the reason for
encounter or the complaints), the diagnostic mode, or the process mode, where further
actions, such as laboratory tests and therapies are coded. The process mode is not coded
directly, since most of its components are already incorporated as alphanumeric values.

06.05.02c Disease episode

An attractive way to organize patient-oriented information is by disease episodes. ICPC
can be used to organize the registration of a disease episode over time, from its onset to its
resolution. A disease episode may include several encounters. Each problem in an
encounter should be coded separately. The same holds for complications of primary
diseases. The committee that developed ICPC also produced conversions to and from
ICD-9 and ICD-10. For several diagnoses of ICPC, criteria derived from
ICHPPC-2-Defined have been defined.

6.5.3 DSM - Diagnostic and Statistical Manual for Mental Disorders
06.05.03a DSM

A specialist code designed by the American Psychiatric Association is the Diagnostic and
Statistical Manual for Mental Disorders (DSM) coding system. The first edition (DSM-I)
was published in 1952. In developing DSM-II the decision was made to base it on the then
newly developed ICD-8. Both systems became effective in 1968. DSM-IV has been
coordinated with the development of ICD-10.

The chapter on mental disorders of ICD-9-CM was compatible with DSM-III-R, its revised
third edition. The fourth edition, DSM-IV, is compatible with the chapter on mental
disorders in ICD-10. The classification is intended to be used by psychiatrists. However,
the etiology or the pathophysiological processes are only known for some mental disorders.
The approach taken in DSM-III, DSM-III-R, and DSM-IV is nontheoretical with regard to
etiology or the pathophysiological process except for disorders for which the etiology or
the pathology is established. In these cases etiology and pathology are included in the
definition of the disorder. For instance, it is believed that phobic disorders represent some
displacement of anxiety, resulting from the breakdown of defense mechanisms that keep
internal conflicts out of one's consciousness. Others explain phobic disorders on the basis
of acquired or learned avoidance responses to conditional anxiety. Still others believe that
certain phobias result from a dysregulation of basic biological systems that mediate
separation anxiety. Clinicians, however, agree on the clinical manifestations. Since it is not
possible to define a theory for each disorder, let alone know the etiology, DSM is designed
to describe the clinical manifestations of the disease along several axes. Therefore, DSM is
a multiaxial classification system. Like ICPC, DSM also uses definitions for the disorders,
including criteria for assigning a diagnosis.

Disorders in the DSM systems are classified along five axes:

    1.   clinical syndromes,
    2.   personality disorders and special developmental disorders,
    3.   relevant physical conditions,
    4.   severity of psychological stressors, and
    5.   overall psychological functioning.

6.5.4 SNOMED - Systematized Nomenclature of Human and Veterinary
06.05.04a SNOMED

SNOMED allows for the coding of several aspects of a disease. SNOMED was first
published in 1975 and was revised in 1979. Its current version is called SNOMED
International (Systematized Nomenclature of Human and Veterinary Medicine). SNOMED
is also a multiaxial system. SNOMED II was a code with 7 axes, and SNOMED
International has 11 axes or modules. Each of these axes forms a complete hierarchical
classification system (see Table 6.4).

A diagnosis in SNOMED may consist of a topographic code, a morphology code, a living
organism code, and a function code. When a well-defined diagnosis for a combination of
these four codes exists, a dedicated diagnostic code is defined. For example, the disease
code D-13510 (Pneumococcal pneumonia) is equivalent to the combination of:
      T-28000 (topology code for Lung, not otherwise specified),
      M-40000 (morphology code for Inflammation, not otherwise specified), and
      L-25116 (for Streptococcus pneumoniae) along the living organism axis.

Tuberculosis (D-14800), for instance, could also be coded as Lung (T-28000) + Granuloma
(M-44000) + Mycobacterium tuberculosis (L-21801) + Fever (F-03003). However, this can
be confusing since tuberculosis is not only restricted to the lung.

SNOMED is also able to combine medical concepts, using so-called combination or
juxtaposition codes, to form more complex concepts. Linkage between concepts, for
instance, can be expressed by "is caused by." In SNOMED International, almost all
diagnostic terms of ICD-9-CM are incorporated in the disease/diagnostic module (D-codes).
Rules for combining SNOMED terms to form complex entities or complex concepts have
not yet been developed. Any SNOMED term may be combined with any other SNOMED
term. This means that there are often multiple ways to express a code for the same valid
concept; however, these are not always meaningful. This freedom of combining codes for
all axes allows for meaningless codes; checking such codes for correctness by a computer
is almost impossible.

6.5.5 ICD-O - International Classification of Diseases for Oncology
06.05.05a ICD-O

In 1976, WHO published the first edition of the International Classification of Diseases for
Oncology ( ICD-O) after extensive field testing. It was based on ICD-9. The second edition,
published in 1990, is an extension of the draft neoplasm chapter of ICD-10. ICD-O
combines a four-digit topography code based on ICD with a morphology code that
includes a neoplasm behavior code and a code for histological grading and differentiation.
These neoplasm morphology codes have been adopted in the morphology axes of
SNOMED and SNOMED International. ICD-O is widely used for cancer registrations.

6.5.6 CPT - Current Procedural Terminology
06.05.06a CPT

Another coding system used in the United States for billing and reimbursement is the
Current Procedural Terminology (CPT) code. It provides a coding scheme for diagnostic
and therapeutic procedures that define procedures with codes based on the cost.

6.5.7 ICPM - International Classification of Procedures in Medicine
06.05.07a ICPM

ICPM was published in 1976 by WHO for trial purposes. It contained chapters on
diagnostic, laboratory, preventive, surgical, other therapeutic, and ancillary procedures.
Originally, WHO planned to add chapters on radiology and drugs and to revise the
classification after some years on the basis of comments received from users.
Unfortunately, this never happened. Nevertheless, ICPM has been a source of inspiration
for a number of other procedural classifications. The procedural part of ICD-9-CM and
CCP were both based on ICPM. In Germany and The Netherlands, extensions of ICPM are
mandatory in hospitals for reimbursement and administration purposes.
6.5.8 RCC - Read Clinical Classification
06.05.08a RCC

The Read Clinical Classification (RCC), or Read code, was developed privately in the
early 1980s by a British GP (James Read), and was adopted by the British National Health
Service (NHS) in 1990. RCC has been further expanded by the Clinical Terms Project. The
Clinical Terms Project is a working group chaired by the chief executive of NHS, which
consists of representatives from the Royal College of Medicine, the Joint Consultants
Committee, the General Medical Services Committee of the British Medical Association,
and the NHS executive. RCC tries to cover the entire field of health care (see Table 6.5).

06.05.08b RCC and CPR

RCC has especially been developed for use by CPR systems. It aims to cover all terms that
may be written in a patient record. They are arranged in chapters that cover all aspects of
care. Each code represents a clinical concept and an associated "preferred term." Each code
can also be linked to a number of synonyms, acronyms, eponyms, and abbreviations,
which allows for the use of natural language. The concepts are arranged in a hierarchical
structure, with each successive level representing greater detail. RCC uses a five-digit
alphanumeric code which, in principle, allows for more than 650 million possible codes.
RCC is compatible with and is cross-referenced to all widely used standard classifications,
such as ICD-9 (see Table 6.6), ICD-9-CM, OPCS-4, CPT-4, and Diagnosis-Related
Groups (DRGs). RCC has a one-to-one cross-reference, or mapping, to all the terms in the
classifications mentioned (Table 6.6).This hierarchy in coding detail is found in all code
categories. In RCC version 3, terms may have multiple parents in the hierarchy. Version
3.1 adds the ability to combine terms in a specific, controlled way.

6.5.9 ATC - Anatomic Therapeutic Chemical Code
06.05.09a ATC

The Anatomic Therapeutic Chemical Code (ATC) has been developed for the systematic
and hierarchical classification of drugs. In the early 1970s, the Norwegian Medicinal Depot
expanded the existing three-level anatomic and therapeutic classification system of the
European Pharmaceutical Market Research Association and added two chemical levels.
Later, the WHO Drug Utilization Research Group accepted the ATC classification as a
standard. Presently, the WHO Collaborating Center for Drug Statistics Methodology in
Oslo is responsible for maintaining the ATC codes. ATC is an acronym for anatomical (A),
the organ system in the body for which the drug is given; therapeutic (T), the therapeutic
purpose for which the drug is used; and chemical (C), the chemical class to which the drug
belongs. Table 6.7 provides an example of an ATC code and its composition, while Table
6.8 provides a listing of the definitions used in the ATC code.

06.05.09b Advantages and disadvantages of ATC

All classifications have disadvantages. No coding system fulfills all needs of all users. The
advantages of the ATC are as follows:
      It identifies a drug product, including the active substance, the route of
       administration, and if relevant, the dose;
      It is therapeutically as well as chemically oriented, a feature that most other
       systems lack;
      Its hierarchical structure allows for a logical grouping;
      It is accepted as the international WHO standard for drug utilization research.

A disadvantage is that it does not cover combination products, dermatological preparations,
and locally compounded preparations.

06.05.09c ATC and national drug database

In some countries national drug databases often contain the ATC code for each drug
product. This allows pharmaceutical information systems to select alternative drugs. It also
provides decision-support systems with information to check for drug interactions, double
medication, and dosage control.

6.5.10 MeSH - Medical Subject Headings
06.05.10a MeSH

The Medical Subject Headings (MeSH) classification is developed and maintained by the
National Library of Medicine (NLM) in the United States. It is generally used to index the
world medical literature. Within the hierarchy of MeSH, a concept may appear as narrower
concepts of more than one broader concept. For example, pneumonia is listed as a
respiratory tract infection as well as a lung disease. MeSH forms the basis for the Unified
Medical Language System (UMLS) also developed by NLM (see Panel 6.3).

6.5.11 DRG - Diagnosis Related Groups
06.05.11a DRG

The DRG classification is based on ICD-9-CM codes and other factors not included in
ICD-9. The grouping of ICD codes is based on factors that affect the cost of treatment and
the length of stay in the hospital, such as severity, complications, and type of treatment.
The resulting classes are homogeneous with respect to costs and they are medically
recognized. DRGs may thus be used for budgeting. Because factors related to the delivery
of care are included, their usefulness for budgeting is disputable. Some disease groups are
clustered further, which is called case mix.

6.6 Current Developments
06.06a GALEN and UMLS

The American Society for Testing and Materials ( ASTM) is working on the
standardization of an extensive nomenclature system. In Europe, standardization efforts are
undertaken by the European Union (see Chapter 34). The GALEN project, for example,
aims at the development of a reference model for medical concepts, which will be
independent of language and existing coding systems and which will be independent of the
data model used by computer-based patient record systems.
NLM is developing UMLS (see Panel 6.3). UMLS contains a meta-thesaurus with medical
concepts and a semantic network, which provides information on the semantic
relationships between medical concepts. These concepts are taken from established
vocabularies such as SNOMED, ICD-9-CM, and MeSH. NLM is developing methods to
enhance the use of UMLS for encoding clinical data.

6.7 Conclusion
06.07a Conclusion

There are many overlapping classifications not only for the coding of diagnoses but also
for the classification of medical events. Although most diagnostic coding systems try to be
compatible with the ICD family, ICD itself represents only a limited view and is unable to
fulfill the needs of all users. Another problem is that all coding systems require
well-defined criteria, but a standardized medical terminology is still lacking.

Systems such as SNOMED have much more expressive power than the more rigid systems
such as ICD-9-CM. In studies in which several coding schemes were compared with
respect to their expressive powers, SNOMED scored much higher than ICD-9-CM. On the
other hand, the use of coded data in database queries for statistical overviews and for use
by expert systems is more complicated.

Wide acceptance of a coding system is essential for the development of decision-support
systems. International institutions such as WHO with its recognized collaborating centers
play an important role in the standardization process.

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