examination of the stool includes inspecting the specimen for consistency and
color and testing for occult (not visible) blood. Special tests, including
tests for fecal urobilinogen, fat, nitrogen, parasites, pathogens, food
residues, and other substances, require that the specimen be sent to the
samples are usually collected on a random basis unless a quantitative study
(eg, fecal fat, urobilinogen) is to be performed. Random specimens should be
sent promptly to the laboratory for analysis. The quantitative 24- to 72-hour
collections must be kept refrigerated until they are taken to the laboratory.
Some stool collections require the patient to follow a special diet or to
refrain from taking certain medications before the collection. It is important
to follow test guidelines closely for accurate results.
occult blood testing is one of the most commonly per-formed stool tests. It can
be useful in initial screening for several disorders. It tests only for the
presence of blood, so other follow-up testing is required. It is most
frequently used in cancer screen-ing programs and for early cancer detection
(Chart 34-1). The test can be performed at the bedside, in the laboratory, or
at home. It tests for heme, the iron-containing portion of the hemoglobin
molecule that is altered during transit through the intestines.
the most widely used occult blood test is the Hema-test. It is inexpensive and
noninvasive, and it carries no risk to the patient. It should not, however, be
performed when there is hem-orrhoidal bleeding. The test can be performed at
home as well as in the doctor’s office. The patient provides a stool specimen,
and the physician smears it on a dry, guaiac-impregnated paper slide. If the
test is done at home, the patient mails the slide to the physi-cian in an
envelope provided for that purpose. The stool specimen is then examined for
occult blood. Serial 3- to 6-day testing is rec-ommended. The test is not
perfect, because certain factors inter-fere with its sensitivity and
specificity. False-positive results may occur if the patient has eaten rare
meat, liver, poultry, turnips, broccoli, cauliflower, melons, salmon, sardines,
or horseradish within 7 days before testing. Medications that can cause gastric
ir-ritation, such as aspirin, ibuprofen, indomethacin, colchicine,
cor-ticosteroids, cancer chemotherapeutic agents, and anticoagulants, may also
cause false-positive results. Extensive research has demon-strated that
therapeutic doses of iron preparations do not cause false-positive results.
Ingestion of vitamin C from supplements or foods can cause false-negative
results. Therefore, a careful assess-ment of the patient’s diet and medication
regimen is essential to reduce incorrect interpretation of results (Ahmed,
Other occult blood tests that may yield more specific and more sensitive readings include Hematest II SENSA and HemoQuant. Immunologic tests are more specific to human hemoglobin and decrease the problem of dietary interference. Hemoporphyrin assays detect the broadest range of blood derivatives, but a strict dietary protocol is essential. Immunochemical tests using anti-human antibodies that are extremely sensitive to human hemo-globin are also available.