Microscopic Stool Analysis

Stools Analysis - Part 2

Microscopic Examination

• Microscopic analysis of stool specimens includes examining the sample for leukocytes, epithelial cells, blood, qualitative fat, and parasites (eggs, larvae, cysts).
• A large number of leukocytes (> 50 cells per high power field) along with macrophages and erythrocytes are typical of shigellosis.
• A smaller number of pus cells (< 20 per high power field) are found in salmonellosis, and in infections caused by invasive E. coli.
• Few leukocytes (< 5 cells per high power field) are present in cholera, EPEC, and ETEC and viral Diarrhea.
• Large numbers of epithelial cells or large amounts of mucus indicate that the intestinal mucosa is scratched.

Epithelial cells

• Normally, small to moderate numbers of epithelial cells are present in feces.
• The laboratory diagnosis of most parasitic infections is by demonstrating the ova of the parasite in the infected person's stools.

Stool Examination

• Techniques used to examine the stool include saline wet mount examination, iodine preparation, and buffered methylene blue stain.
• Saline wet mount is used for the detection of parasitic trophozoites and cysts of protozoa, and eggs and larvae of helminths.
• Iodine staining method is used to detect cysts of protozoa, but it kills the motile trophozoites.

Stool Testing

Concentration Methods

• Sedimentation is used for detecting Ascaris lumbricoides and Schistosoma mansoni.
• Floatation is used for detecting Enterobius vermicularis and Ankylostoma.

Chemical Testing of Stool

• pH: Normal stool pH is 6-7.5, acidic in amoebic dysentery, and alkaline in bacillary dysentery.
• Occult Blood: May be present in a variety of conditions, including malignancy of the gastrointestinal tract.
• Benzidine test: Used to detect occult blood in the stool sample.

Reducing Factors

• Levels of mono and disaccharide in stool (6mg/g) can indicate disturbances in enzymes that digest sugar.

Stool Culturing

• Stool cultures are undertaken to evaluate diarrhea of unknown etiology to identify possible causative bacteria.
• Bacteria produce diarrhea in three main ways:
  • Organisms invade the intestinal wall, damaging tissue.
  • Organisms produce toxins within the intestine that alter gastrointestinal motility.
  • Toxins produced by bacteria are ingested and produce diarrhea, although the organisms themselves are not detected in feces.

Culture Media

• MacConkey's Agar: Inhibits most gram-positive organisms, differentiating between lactose fermenters and non-lactose fermenters.
• Xylose Lysine Deoxycholate (XLD) Agar: Used for the isolation of Salmonella and particularly Shigella from fecal samples.
• Thiosulphate Citrate Bile Salt Sucrose (TCBS) Agar: Excellent selective medium for the primary isolation of Cholerae.
• Sorbitol MacConkey's Agar: Used for screening E. coli O157.