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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

Bacteriologic examination

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.

Table 988.1 Classification and causes of chronic diarrhea
1. Watery diarrhea
  1. Osmotic
    • Carbohydrate malabsorption
    • Osmotic laxatives
  2. Secretory
    • Bacterial toxins
    • Bile acid malabsorption
    • Laxative abuse
    • Hormonal disorders: VIPoma, carcinoid syndrome, gastrinoma, hyperthyroidism
2. Inflammatory diarrhea
  1. Invasive bacterial and parasitic infections
  2. Inflammatory bowel disease
  3. Pseudomembranous colitis
  4. Infectious diseases
  5. Neoplasia
3. Fatty diarrhea
  • Malabsorption syndromes

Figure 988.1 Classification of causes of acute diarrheaFigure 988.1 Classification and causes of acute diarrhea

Evaluation of dysentery

Differentiate between bacillary dysentery and amebic dysentery is done by the identification of the causative organism in the stool. See also: Microscopic Examination of Feces

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 examination

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

Figure 988.2 Preliminary evaluation of acute diarrhea. Examination of feces for white blood cells is helpful in narrowing differential diagnosis in intestinal infections in acute diarrhea
Figure 988.2 Preliminary evaluation of acute diarrhea. Examination of feces for white blood cells is helpful in narrowing differential diagnosis in intestinal infections in acute diarrhea
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