Stool Analysis Part 2: Microscopic Examination

Microscopic Examination

• Microscopic analysis of stool specimens includes examining the sample for: • Leukocytes • Epithelial cells • Blood • Qualitative fat • Parasites (eggs, larvae, cysts)
• Characteristic microscopic findings: • Shigellosis: large number of leukocytes (> 50 cells per high power field) with macrophages and erythrocytes • Salmonellosis and invasive E. coli: smaller number of pus cells (< 20 per high power field) • Cholera, EPEC, ETEC, and viral diarrhea: few leukocytes (< 5 cells per high power field)
• Abnormal epithelial cell presence indicates intestinal mucosa scratch

Laboratory Diagnosis of Parasitic Infections

• Methods for examining stool: • Saline wet mount examination • Iodine preparation • Buffered methylene blue stain • Concentration techniques
• Detection of parasites: • Live motile trophozoites (protozoa) • Cyst (inactive stage of protozoa) • Eggs and larvae (helminths)
• Saline wet mount: • Uses normal saline to maintain osmotic pressure • Examined under microscope with specific objectives (X4, X10, X40) • Detects trophozoites and cysts of protozoa, eggs and larvae of helminths • Useful for detecting live motile trophozoites of protozoa (e.g., E. histolytica) and helminths larvae (e.g., Strongyloides stercolaris)
• Iodine staining method: • Uses iodine to stain nuclei of some cyst forms of protozoa (e.g., Entamoeba histolytica) • Kills motile trophozoites, making it unsuitable for detecting live forms

Stool Testing

• Concentration methods: • Sedimentation: detects Ascaris lumbricoides, Schistosoma mansoni • Floatation: detects Enterobius vermicularis, Ancylostoma
• Chemical testing: • pH: normal range is 6-7.5, acidic in amoebic dysentery, and alkaline in bacillary dysentery • Occult blood (fecal occult blood test or stool occult blood test): • Uses benzidine powder and hydrogen peroxide • Detects occult blood in stool samples • Reducing factors: • Mono sugar and di sugar levels in stool (normal range: 6mg/g) • Elevated levels indicate disturbance in sugar-digesting enzymes (e.g., lactase, sucrase)

Stool Culturing

• Normal flora of the bowel includes certain bacteria
• Pathological bacteria can produce diarrhea and systemic infection
• Stool cultures evaluate diarrhea of unknown etiology to identify causative bacteria
• Bacteria produce diarrhea through: • Invasion of the intestinal wall • Production of toxins in the intestine • Ingestion of toxins produced by bacteria
• Culture media: • MacConkey's Agar: inhibits gram-positive organisms, differentiates lactose fermenters and non-lactose fermenters • Xylose Lysine Deoxycholate (XLD) agar: selective medium for Salmonella and Shigella isolation • Thiosulfate Citrate Bile Salt Sucrose (TCBS) agar: excellent selective medium for Cholerae isolation • Sorbitol MacConkey agar: useful for E. coli O157 screening