Basic 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 laboratory.
Stool 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.
Fecal 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.
Probably 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, 2000).
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.