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Screening

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									       Screening

         Puja Myles
Puja.myles@nottingham.ac.uk
         Learning outcomes
You should be able to:
• Define screening
• Describe the criteria that should be met
  before implementing a screening
  programme
• Discuss the properties of a screening test
• Outline the benefits and disadvantages of
  screening programmes
             Lecture outline
•   What is screening?
•   UK National Screening Programmes
•   Screening Criteria
•   Properties of a screening test
•   Disadvantages of screening
       What is screening?
“Screening is the presumptive
identification of unrecognised disease or
defect by the application of tests,
examinations or other procedures that can
be applied rapidly”

                US Commission on Chronic Illness, 1957
        What is screening?
Screening is a public health service in which
members of a defined population, who do not
necessarily perceive they are at risk of, or are
already affected by a disease or its
complications, are asked a question or offered a
test, to identify those individuals who are more
likely to be helped than harmed by further tests
or treatment to reduce the risk of a disease or its
complications.
                  – UK National Screening Committee
           UK National Screening
               Programmes
•   Cervical cancer
•   Breast cancer
•   Bowel cancer
•   Diabetic retinopathy
•   Antenatal screening
    – e.g. infectious diseases, foetal anomalies
• Neonatal screening
    – e.g. blood spot (hypothyroidism, PKU, sickle cell),
      neonatal hearing
     Principles for introducing a
       screening programme
• Originally proposed by Wilson and Junger
  in 1968.
• Subsequently modified by a number of
  authors.
• NHS National Screening Committee
  criteria:
  http://www.nsc.nhs.uk/uk_nsc/uk_nsc_ind.htm
       Principles for introducing a
         screening programme

•   The condition
•   The test
•   The treatment
•   The programme
Criteria for introducing a screening
              programme
The condition
  – Should be an important health problem i.e.
    substantial burden of morbidity or mortality
  – Natural history should be known
  – Should be a detectable preclinical phase:
     • Asymptomatic disease, e.g. breast cancer
     • Markers of risk, e.g. pre-malignant changes in
       cervical cancer
  – Cost-effective primary prevention should have
    been implemented
Criteria for introducing a screening
              programme
The test
  – Safe
  – Acceptable to population
  – Validated
  – Relatively low cost
Criteria for introducing a screening
              programme
The treatment
  – Effective treatment available
  – Evidence that early treatment gives better
    outcomes
Criteria for introducing a screening
              programme
The programme
 – Evidence from RCTs that the screening
   programme reduces mortality or morbidity
 – Evidence that programme is clinically, socially
   and ethically acceptable
 – Economic costs should be considered
   Properties of screening tests
1. Let‟s start with an imaginary population of 1000
2. 100 people have asymptomatic disease
  (prevalence=__%)
3. The screening test correctly identifies 90 of
  these 100 people as having disease (sensitivity
  of the test= 90%)
  Sensitivity is the proportion of people with the
  disease who are correctly identified as being
  positive (true positive)
Properties of screening tests
What happens to the 10 people with
disease who are missed?
The screening test wrongly identifies them
as negative (false negative)
         Screening Tests
           Disease +   Disease -



Test +



Test -
         Screening Tests
           Disease +        Disease -



Test +     True positive    False positive



Test -     False negative   True negative
                Specificity
• Ability of test to identify correctly all those who
  are free of the disease in the screened
  population
• The proportion of people free of the disease in
  whom a screening test gives a negative result
• High specificity means low false positives
• Negative consequences for people who test
  false positive: anxiety, unnecessary
  diagnostic tests which may be invasive
     Sensitivity and Specificity
         Disease +       Disease -     Total
Test +    a               b            a+b
Test -    c               d            c+d
Total     a+c (total      b+d (total   a+b+c+d
          disease)        healthy)
• Sensitivity = those who screen positive/ total
  number with disease
• Specificity = those who screen negative/ total
  number of disease-free people
               Test yourself!
         Disease +    Disease -   Total
Test +    90           90          180
Test -    ?           810          ?
Total    100          900         1000


• Sensitivity = ?
• Specificity = ?
                PPV and NPV
• Positive predictive value (PPV)
   – The proportion of those who test positive who actually
     have the disease
• Negative predictive value (NPV)
   – The proportion of those who test negative who re free
     of the disease


• Influenced by the sensitivity and specificity of the
  test AND by prevalence of disease in population
    Two problems with cancer
          screening…
Length time bias:
Screening is more effective at detecting
slow developing cancers which are less
likely to be the cause of an individual‟s
death but by detection can cause
considerable anxiety
                             Lead time bias
                              Time



             Survival time from diagnosis with screening



                                       Survival time from diagnosis
                                            without screening




Screening                    Diagnosis                                Death


            Symptoms
   Disadvantages of screening
• Significant harm to false positives:
  communication of anxiety producing
  information; follow-up tests which may be
  risky (e.g. colonoscopy in the case of
  bowel cancer)
              Final word…
• Screening is not a diagnostic test. It just
  separates individuals into groups who
  have either a low probability or a high
  probability of disease being present.
• It can be misunderstood by the public that
  a positive result is „bad news‟ and a
  negative result is „all clear‟.
           Revision points
You should be able to:
• Define screening
• Describe the criteria that should be met
  before implementing a screening
  programme
• Discuss the properties of a screening test
• Outline the benefits and disadvantages of
  screening programmes

								
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