- 18 May 2018
Waste products discharged from the digestive tract are composed of up to 75% water, food which is digested but not absorbed, indigestible residue, undigested food, epithelial cells, bile, bacteria, secretion from the digestive tract and inorganic bacteria. Normally an adult human excretes 100-200 grams of feces in a day.
Examination of stool is very helpful in the diagnosis of disease of the gastrointestinal tract as listed below.
Detection of parasites
Stool examination is performed for the detection and identification of worms (adult worms, larvae, segments of worms, ova) and protozoa (cyst or trophozoites). See also: Microscopic Examination of Feces
Stool culture is performed for the evaluation of bacterial infection such as Clostridium difficile, Yersinia, Salmonella, Shigella or Vibrio. Bacterial toxins (such as those released by Clostridium difficile or Clostridium botulinum) can also be identified. See also: Microscopic Examination of Feces
Evaluation of chronic diarrhea
Chronic diarrhea defined as a passage of three or more liquid or loose stools in a day lasting for more than four weeks. Acute diarrhea refers to the passing of three or more liquid or loose stools in a day for less than four weeks. In diarrhea, stool examination is very important part of laboratory investigations. Depending on the nature of the investigation, either a random stool sample or 72- sample or 48-hour sample is collected. A random stool sample is used for the tests of occult blood, pH, fat, white blood cells, microscopy, or culture. A 72- or 48-hour sample is collected and examined for the weight, carbohydrate, fat content, osmolality, or chymotrypsin activity. Causes of chronic and acute diarrhea are listed in Table 988.1 and Figure 988.1 respectively.
1. Watery diarrhea
2. Inflammatory diarrhea
3. Fatty diarrhea
Evaluation of dysentery
Identification of Rotavirus
In infants and young children, Rotavirus is the most common cause of diarrhea. Rotavirus can be identified by the electron microscopic examination of stool. Other techniques, such as latex agglutination, immunofluorescence, or enzyme-linked immunosorbent assay (ELISA) are also used for the detection of Rotavirus in stool.
Chemical tests can be applied on feces to detect excess fat excretion (malabsorption syndrome), occult blood (in ulcerated lesions of the gastrointestinal tract, especially occult carcinoma of the colon) and presence or absence of urobilinogen (obstructive jaundice). See also: Chemical Examination of Feces
Differentiating infection by invasive bacteria (like Salmonella or Shigella) from that due to toxin-producing bacteria (like Vibrio cholerae or Escherichia coli)
Feces is examined for the presence of white blood cells. Increased numbers of polymorphonuclear neutrophils (identified by methylene blue stain from the presence of granules in their cytoplasm) are seen as shown in Figure 988.2. See also: Causes, symptoms, diagnosis, and treatment of Cholera
- 30 Aug 2017
- Brown: Normal
- Black: Bleeding in upper gastrointestinal tract (proximal to cecum), Drugs (iron salts, bismuth salts, charcoal)
- Red: Bleeeding in large intestine, undigested tomatoes or beets
- Clay-colored (gray-white): Biliary obstruction
- Silvery: Carcinoma of ampulla of Vater
- Watery: Certain strains of Escherichia coli, Rotavirus enteritis, cryptosporidiosis
- Rice water: Cholera
- Unformed with blood and mucus: Amebiasis, inflammatory bowel disease
- Unformed with blood, mucus, and pus: Bacillary dysentery
- Unformed, frothy, foul smelling, which float on water: Steatorrhea.
- Sedimentation techniques: Ova and cysts settle at the bottom. However, excessive fecal debris may make the detection of parasites difficult. Example: Formolethyl acetate sedimentation procedure.
- Floatation techniques: Ova and cysts float on surface. However, some ova and cysts do not float at the top in this procedure. Examples: Saturated salt floatation technique and zinc sulphate concentration technique.
- 30 Aug 2017
- Occult blood
- Excess fat excretion (malabsorption)
- Reducing sugars
- Fecal osmotic gap
- Fecal pH
- Intestinal diseases: hookworms, amebiasis, typhoid fever, ulcerative colitis, intussusception, adenoma, cancer of colon or rectum.
- Gastric and esophageal diseases: peptic ulcer, gastritis, esophageal varices, hiatus hernia.
- Systemic disorders: bleeding diathesis, uremia.
- Long distance runners.
- Ingestion of peroxidase-containing foods like red meat, fish, poultry, turnips, horseradish, cauliflower, spinach, or cucumber. Diet should be free from peroxidase-containing foods for at least 3 days prior to testing.
- Drugs like aspirin and other anti-inflammatory drugs, which increase blood loss from gastrointestinal tract in normal persons.
- Foods containing large amounts of vitamin C.
- Conversion of all hemoglobin to acid hematin (which has no peroxidase-like activity) during passage through the gastrointestinal tract.
- Deficiency of pancreatic lipase (insufficient lipolysis): chronic pancreatitis, cystic fibrosis.
- Deficiency of bile salts (insufficient emulsification of fat): biliary obstruction, severe liver disease, bile salt deconjugation due to bacterial overgrowth in the small intestine.
- Diseases of small intestine: tropical sprue, celiac disease, Whipple’s disease.
- Microscopic stool examination after staining for fat: A random specimen of stool is collected after putting the patient on a diet of >80 gm fat per day. Stool sample is stained with a fat stain (oil red O, Sudan III, or Sudan IV) and observed under the microscope for fat globules (Figure 845.2). Presence of ≥60 fat droplets/HPF indicates steatorrhea. Ingestion of mineral or castor oil and use of rectal suppositories can cause problems in interpretation.
- Quantitative estimation of fecal fat: The definitive test for diagnosis of fat malabsorption is quantitation of fecal fat. Patient should be on a diet of 70-100 gm of fat per day for 6 days before the test. Feces are collected over 72 hours and stored in a refrigerator during the collection period. Specimen should not be contaminated with urine. Fat quantitation can be done by gravimetric or titrimetric method. In gravimetric method, an accurately weighed sample of feces is emulsified, acidified, and fat is extracted in a solvent; after evaporation of solvent, fat is weighed as a pure compound. Titrimetric analysis is the most widely used method. An accurately weighed stool sample is treated with alcoholic potassium hydroxide to convert fat into soaps. Soaps are then converted to fatty acids by the addition of hydrochloric acid. Fatty acids are extracted in a solvent and the solvent is evaporated. The solution of fat made in neutral alcohol is then titrated against sodium hydroxide. Fatty acids comprise about 80% of fecal fat. Values >7 grams/day are usually abnormal. Values >14 grams/day are specific for diseases causing fat malabsorption.