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Tests for specific substances in feces

Fecal samples can be chemically analyzed for avariety of substances including occult blood ,qualitative fats ,trypsin ,urobilinogen ,and bile .In addition ,estimates of carbohydrate utilization can be made .

Occult blood:

The most frequently performed test of feces is chemical screening for occult blood .The purpose of the test is to detect pathological lesion (e.g.,carcinoma)before they produce symptoms and whilethe condition is stillamenable to treatment .Inded ,such testingis widely used in mass screening programs for colorectal cancer ,with 75 percent of such cancers detected 

Blood is  One of the main problems of testing stools for occult

The number of false –positive results that occur.Adiet high in meat, for example ,may cause stools to test positive for blood ,as do diets high in certain vegetables . In addition ,bleeding from the gums or nasal passage may produse positive results for occult blood in stools.

Therapy with may drugs may lead to positive results because of direct or indirect drug effects on the gastrointestinal tract.

These drugs include aspirin ,anticoaculante,adrenocorticosteriods ,and phenylbutazone.Incontrast ,ascorbic acid may lead to false –negative results.

Numerous pathological conditions may cause bleeding into the intestinal tract such as atrophic gastritis, carcinoma and hemorrhoids.

In such case ,stools may remain black for as long as 5 days after the initial bleeding occurred .If, however, upper GIbleeding is massive and the volume increases GI motility (e.g.,as occur in bleeding from easophageal varices), stols may be red or maroon and somewhat liquid in consistency .Generally ,though ,bright red stools are associated with lower GI bleeding from hemorrhoids ,ulcerative colitis,and carcinoma.

Quantitative fats:As noted previously ,the definitive test for excessive fecal fat is the 72-hour quantitative measure ,with the amount of fat present expressed as apercentage of solid material .Because fat out put may vary on aday –to-day basis ,the 3-day collection is believed to be the most reliable approach. In adults,controlled fat diet of 100g per day also is followed during the collection.In in fants and children ,for whom

such as diet could not be used ,results of the collection are based on the estimated intake of dietary fat.

Thus ,it is important to know which foods were ingested during the test period .

Trypsin:

   Trypsin is an enzyme secreted by the pancreas.Itis normally not present in stools except in children under 2 years of ago.If it is absent from the stools of children under age 2 years ,pancreatic deficiency is indicated .Note that trypsin may not be detected if the child is constipated ,because of the prolonged action of normal intestinal bacteria on the enzyme .

Carbohydrate Utilization:

    I ndividual with various disorders related to malabsorption may have difficulty with carbohydrateabsorption as wellas fat absorption .thus, although investigation of the finding of steatorrhea (excess fat in the stool)include evalution of carbohydrate metabolism .

Such an evalution includes performing oral and intravenous(IV)glucose tolerance tests and comparing the results .persons with carbohydrate malabsorption have normal result on IV glucose tolerance tests but not on oral glucose tolerance tests.If carbohydrates can not be absorbed normally ,excessive amounts appear in the stool.

Urobilinogen :

      Urobilinogen is produced from bilirubin ,abreakdown product of red blood cells ,and normally appears in urine and feces.Because blood and urine samples for products of bilirobin metabolism are more easily obtained than stool samples , this test is rarely used.Factors that may lead to falsely decreased values include antibiotic therapy and exposure of the sample to light.