14.4 Fecal Analysis: Macroscopic, Microscopic, and Chemical Examination of Stool

Fecal Analysis: Macroscopic, Microscopic, and Chemical Examination of Stool

Feces carries 10% of the Clinical Microscopy blueprint, which is itself 10% of the MTLE. Routine fecalysis is one of the most frequently requested tests in Philippine laboratories, usually for diarrhea and parasite screening. Stool parasite techniques are covered in the parasitology sections. This section covers the rest of fecal analysis: collection, gross examination, occult blood testing, fat and meat-fiber screening, fecal leukocytes, carbohydrate testing, the APT test for swallowed maternal blood, and fecal enzyme and inflammation markers.

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1. Specimen Collection and Normal Composition

• Container: A clean, dry, leak-proof, wide-mouthed container. The specimen must not be contaminated with urine (which harms trophozoites and dilutes chemistry) or toilet water (which can add free-living organisms).
• Timing for microscopy: Examine liquid stool within 30 minutes and soft stool within 1 hour of passage, because trophozoites lose motility and degenerate. Formed stool, which contains cysts and eggs, can wait up to 24 hours if refrigerated.
• Quantitative tests: Fecal fat requires a timed collection (usually 72 hours) in a pre-weighed container while the patient eats about 100 g of fat per day.
• Normal composition: An adult passes about 100 to 200 g of stool per day, roughly three-quarters water. The rest is bacteria, undigested cellulose, sloughed epithelial cells, bile pigments, and electrolytes.

2. Gross Examination, Occult Blood, Fecal Fat, and Fecal Leukocytes

Routine fecalysis evaluates digestive function, gastrointestinal bleeding, malabsorption, and intestinal infections.

Gross Stool Characteristics

• Color: Normal brown color is generated by stercobilin and urobilin, oxidized products of the urobilinogens (stercobilinogen) that colonic bacteria form from conjugated bilirubin.
  • Acholic (Clay-colored / Pale Gray): Post-hepatic biliary obstruction (absence of bile pigments entering the duodenum) or barium sulfate ingestion.
  • Melena (Black, Tarry Stool): Upper gastrointestinal bleeding (> 50 to 100 mL of blood, e.g., bleeding peptic ulcer); hemoglobin is enzymatically digested into dark acid hematin.
  • Hematochezia (Bright Red Stool): Lower gastrointestinal bleeding (e.g., hemorrhoids, diverticular disease, ulcerative colitis, colorectal adenocarcinoma).
  • Ribbon-like / Pencil-shaped Stool: Distal colorectal lumen obstruction (annular carcinoma, stricture).
  • Rice-Water Stool: Watery fluid with epithelial flecks and mucus; classic for Vibrio cholerae enterotoxin infection.

Fecal Occult Blood Testing (FOBT)

Occult blood represents hidden blood not visually apparent upon gross inspection, utilized in population screening for colorectal carcinoma.

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1. Guaiac-based FOBT (gFOBT, e.g., Hemoccult):

   • Chemical Principle: Relies on the pseudoperoxidase activity of heme in hemoglobin. Heme catalyzes the transfer of oxygen from hydrogen peroxide (H2O2) to phenolic gum guaiac (a colorless leuco-dye), oxidizing it into a brilliant blue-colored quinone compound.
   • Pre-Analytical Restrictions (3 Days Prior):
     • False-Positive Causes: Ingestion of red meat (bovine hemoglobin/myoglobin); foods rich in plant peroxidases (raw horseradish, turnips, broccoli, cantaloupe); aspirin and NSAIDs (inducing gastric micro-bleeding).
     • False-Negative Causes: Ascorbic acid (Vitamin C > 250 mg/day), a powerful reducing agent that scavenges peroxide intermediates, preventing guaiac oxidation even in the presence of massive occult blood.

2. Immunochemical Fecal Occult Blood Test (iFOBT / Fecal Immunochemical Test [FIT]):

   • Analytical Principle: Utilizes monoclonal or polyclonal antibodies directed against the protein globin portion of intact human hemoglobin.
   • Advantages:
     • No dietary restrictions: Antibodies do not cross-react with animal hemoglobin, plant peroxidases, or dietary antioxidants like vitamin C.
     • High Specificity for Lower GI Bleeding: Globin released from upper gastrointestinal bleeding is digested and destroyed by gastric pepsin and pancreatic proteases before reaching the colon, rendering it non-reactive with FIT antibodies. Thus, FIT specifically flags colorectal pathology without interference from upper tract gastritis.

Fecal Fat Screening (Steatorrhea)

Normal adult fecal fat excretion is up to about 6 g/day on a standardized 100-g fat diet; higher outputs (commonly quoted as > 6 to 7 g/day) define steatorrhea, resulting from either pancreatic exocrine insufficiency (maldigestion) or small intestinal mucosal disease (malabsorption).

• Qualitative Sudan III / Oil Red O Staining:
  • Neutral Fats (Triglycerides): Stool is emulsified with 95% ethanol and mixed directly with Sudan III dye. Neutral fats appear as large, spherical, refractile orange-red lipid droplets. Normal stool contains < 60 small droplets per high-power field (HPF). Presence of > 60 large droplets/HPF indicates neutral fat maldigestion (pancreatic lipase deficiency, as in chronic pancreatitis or cystic fibrosis).
  • Split Fats (Free Fatty Acids and Soaps): Stool is mixed with 36% glacial acetic acid and Sudan III, then heated to boiling. Acetic acid hydrolyzes fatty acid soaps into free fatty acids that coalesce into orange-red droplets upon cooling. Abundant split fats accompanied by normal neutral fats point to small intestinal mucosal malabsorption (e.g., celiac sprue, Crohn's disease) rather than pancreatic failure.
• Quantitative Fecal Fat: Gold standard consists of a 72-hour stool collection with dietary fat standardization (100 g/day fat intake) analyzed via Van de Kamer titration.

Fecal Leukocytes

Evaluated on wet mount preparations stained with Loeffler's methylene blue or Gram stain.

• Normal stool contains no leukocytes.
• Positive Fecal Leukocytes (> 3 PMNs/HPF): Indicates mucosal-invasive or inflammatory bacterial colitis: Shigella, Salmonella enterica, Campylobacter jejuni, enteroinvasive E. coli (EIEC), Clostridioides difficile, and active ulcerative colitis.
• Negative Fecal Leukocytes: Characteristic of toxin-mediated, non-invasive secretory diarrheas: enterotoxigenic E. coli (ETEC), Vibrio cholerae, Staphylococcus aureus enterotoxin, rotavirus, norovirus, and Giardia duodenalis.

Consistency and Mucus

• Watery stool reflects secretory or osmotic diarrhea; bulky, frothy, foul-smelling, greasy stool that floats suggests steatorrhea.
• Translucent mucus coating a formed stool suggests straining or spastic constipation. Mucus mixed with blood and pus suggests inflammatory or invasive colitis (shigellosis, amebic dysentery, ulcerative colitis).

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3. Microscopic Examination for Digestion Products

• Muscle (meat) fibers: A stool emulsion is stained with 10% eosin in alcohol, and undigested fibers show striations in both directions. More than 10 undigested fibers in a standard examination after a meat-containing diet indicates impaired protein digestion (creatorrhea), as in pancreatic insufficiency or cystic fibrosis.
• Fat droplets: Sudan III screening for neutral fat and split fat is described above. Pancreatic insufficiency increases neutral fat; malabsorption increases split fat with normal neutral fat.
• Starch granules (stained blue-black with iodine) suggest rapid transit or amylase deficiency, and vegetable cells and fibers are normal dietary residue.

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4. Chemical Tests Beyond Occult Blood

Fecal Carbohydrates

• A copper reduction test (Clinitest tablet) on a 1:2 dilution of liquid stool detects reducing sugars. More than about 0.5 g/dL suggests carbohydrate malabsorption or intolerance, such as lactase deficiency. Sucrose is not a reducing sugar and is missed unless the specimen is first hydrolyzed.
• Stool pH below about 5.5 suggests carbohydrate malabsorption, because colonic bacteria ferment unabsorbed sugars to organic acids.
• The D-xylose absorption test separates small-bowel malabsorption (low blood and urine xylose) from pancreatic maldigestion (normal xylose, because xylose needs no pancreatic enzymes).

APT (Alkali-Denaturation) Test for Swallowed Maternal Blood

• Used on bloody stool or vomitus of a newborn to decide whether the blood is the infant's (possible gastrointestinal bleeding) or maternal blood swallowed during delivery or breastfeeding.
• The specimen is mixed with water and centrifuged, and the pink supernatant is mixed with 1% sodium hydroxide.
• Fetal hemoglobin (HbF) resists alkali denaturation and stays pink. Adult hemoglobin (HbA) from the mother is denatured to alkaline hematin and turns yellow-brown within about 2 minutes.
• The test requires fresh, pink (oxygenated) blood; black, tarry stool already contains denatured hemoglobin and cannot be tested.

Fecal Enzyme and Inflammation Markers

• Fecal elastase-1: Measured by immunoassay. It is stable during intestinal transit, and low values (commonly below 200 µg/g of stool) indicate exocrine pancreatic insufficiency.
• Fecal trypsin and chymotrypsin: Older screens for pancreatic insufficiency. The classic gelatin-film test detected trypsin by its digestion of the gelatin on x-ray film.
• Fecal calprotectin and lactoferrin: Neutrophil proteins that rise in inflammatory bowel disease and invasive infections, but not in irritable bowel syndrome.

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5. Fecal Analysis Summary