About Stool Tests
Stool (or feces) is usually thought of as nothing but waste — something to quickly flush
away. But bowel movements can provide doctors with valuable information as to what's
wrong when a child has a problem in the stomach, intestines, or another part of
the gastrointestinal system.
A doctor may order a stool collection to test for a variety of possible conditions,
allergy or inflammation in the body, such as part of the evaluation of milk protein
allergy in infants
infection, as caused by some types of bacteria, viruses, or parasites that invade
the gastrointestinal system
digestive problems, such as the malabsorption of certain sugars, fats, or nutrients
bleeding inside of the gastrointestinal tract
The most common reason to test stool is to determine whether a type of bacteria or
parasite may be infecting the intestines. Many microscopic organisms living in the
intestines are necessary for normal digestion. If the intestines become infected with
harmful bacteria or parasites, though, it can cause problems like certain types of bloody
diarrhea, and testing stool can help find the cause.
Stool samples are also sometimes analyzed for what they contain; for instance,
examining the fat content. Normally, fat is completely absorbed from the intestine, and
the stool contains virtually no fat. In certain types of digestive disorders, however, fat is
incompletely absorbed and remains in the stool
1.Fecal pH test
A fecal pH test is one where a specimen of feces is tested for acidity in order to
diagnose a medical condition. Human feces is normally alkaline. An acidic stool can
indicate a digestive problem such as lactose intolerance or a contagion such as E.
coli or Rotavirus.
The test is fast and can be performed in a doctor's office. A patient must not be
receiving antibiotics. At least half a milliliter of feces is collected and a strip
of nitrazine paper is dipped in the sample and compared against a color scale. A pH of
less than 5.5 indicates an acidic sample.
2. Fecal fat test
In medicine, the fecal fat test is a diagnostic test for fat malabsorption conditions, which
lead to excess fat in the feces (steatorrhea).
(Malabsorption is a state arising from abnormality in absorption of food
nutrients across the gastrointestinal (GI) tract.Impairment can be of single or multiple
nutrients depending on the abnormality. This may lead to malnutrition and a variety
In the small intestine, dietary fat (primarily triglycerides) is digested by enzymes such
as pancreatic lipase into smaller molecules which can be absorbed through the wall of
the small intestine and enter the circulation for metabolism and storage. As fat is a
valuable nutrient, human feces normally contain very little undigested fat. However, a
number of diseases of the pancreas and gastrointestinal tract are characterized by fat
Examples of such diseases are:
disorders of exocrine pancreatic function, such as chronic pancreatitis, cystic
fibrosis and Shwachman-Diamond syndrome (these are characterized by deficiency
of pancreatic digestive enzymes)
celiac disease (in which the fat malabsorption in severe cases is due to
inflammatory damage to the integrity of the intestinal lining)
short bowel syndrome (in which much of the small intestine has had to be
surgically removed and the remaining portion cannot completely absorb all of the
small bowel bacterial overgrowth syndrome
In the simplest form of the fecal fat test, a random fecal specimen is submitted to
the hospital laboratory and examined under a microscope after staining with a Sudan
III or Sudan IV dye ("Sudan staining"). Visible amounts of fat indicate some degree of
Quantitative fecal fat test
Quantitative fecal fat tests measure and report an amount of fat. This usually done over
a period of three days, the patient collecting all of their feces into a container.
The container is thoroughly mixed to homogenize the feces, this can be done with
a paint mixer. A small sample from the feces is collected. The fat content is extracted
with solventsand measured by saponification (turning the fat into soap).
Normally up to 7 grams of fat can be malabsorbed in people consuming 100 grams of
fat per day. In patients with diarrhea, up to 12 grams of fat may be malabsorbed since
the presence of diarrhea interferes with fat absorption, even when the diarrhea is not
due to fat malabsorption.
3. Stool guaiac test
The stool guaiac test or guaiac fecal occult blood test (gFOBT) is one of several
methods that detect the presence of fecal occult blood(FOB). Fecal occult
blood is blood present in the feces that is not visibly apparent.
The term guaiac denotes the name of the paper surface used in the test which has a
phenolic compound, alpha-guaiaconic acid, that is extracted from the wood resin
of Guaiacum trees
The stool guaiac test involves fasting from iron supplements, red meat (the blood it
contains can turn the test positive), certain vegetables (which contain a chemical with
peroxidase properties that can turn the test positive), and vitamin C and citrus fruits
(which can turn the test falsely negative) for a period of time before the test. It has been
suggested that cucumber, cauliflower and horseradish, and often other vegetables,
should be avoided for three days before the test.
In testing, feces are applied to a thick piece of paper attached to a thin film coated with
guaiac. Either the patient or medical professional smears a small fecal sample on to the
film. The fecal sample can be obtained by digital rectal examination or by wiping soiled
toilet tissue on the film. Only a small sample for smearing is necessary; a large sample
of stool may impede an accurate test.
Both sides of the test card can be peeled open, to access the inner guaiac paper. One
side of the card is marked for application of the stool and the other is for the developer
After applying the feces, one or two drops of hydrogen peroxide are then dripped on to
the other side of the film, and it is observed for a rapid blue color change.
When the hydrogen peroxide is dripped on to the guaiac paper, it oxidizes the alpha-
guaiaconic acid to a blue colored quinone. Normally, when no blood and no peroxidases
or catalases from vegetables are present, this oxidation occurs very slowly. Heme, a
component of hemoglobin found in blood, catalyzes this reaction, giving a result in
about two seconds. Therefore, a positive test result is one where there is a quick and
intense blue color change of the film.
The guaiac test can often be false-positive which is a positive test result when there is in
fact no source of bleeding. This is particularly common if the recommended dietary
preparation is not followed, as the heme in red meat or the peroxidase or catalase
activity in vegetables, especially if uncooked, can cause analytical false positives.
Vitamin C can cause analytical false negatives due to its anti-oxidant properties
inhibiting the color reaction.
If the card has not been promptly developed, the water content of the feces decreases,
and this can reduce the detection of blood. Although rehydration of stored samples can
reverse this effect this is not recommended because the test becomes unduly
analytically sensitive and thus much less specific.
Some stool specimens have a high bile content that causes a green color to show after
applying the developer drops. If entirely green, such samples are negative, but if
questionnably green to blue, such samples are designated positive.
The package insert guidelines from the manufacturers, for example Hemoccult
SENSA, recommend that nonsteroidal anti-inflammatory drugs (NSAID), such as
ibuprofen and aspirin, and iron supplements be discontinued for at least several days
before the tests. There is a concern that these agents may irritate the body and cause
biologically positive tests even in the absence of a more substantial illness, but there is
some doubt about how frequently this occurs with NSAID medication. Although both
iron and bismuth containing products such as antacids and antidiarrheals can cause
dark stools that are occasionally confused as containing blood, actual bleeding from iron
There is no consensus on whether to stop warfarin before a guaiac test. Even when using
anticoagulants a high proportion of positive guaiac tests were found to be due to diagnosable
lesions, suggesting anticoagulants may not cause bleeding unless there is an abnormality.