Fecal Analysis

Stool analysis works by subtraction: the laboratory measures what digestion failed to remove. Undigested striated muscle points at impaired proteolysis, orange-red fat droplets at impaired fat handling, neutrophils at an invaded or inflamed intestinal wall, and occult blood at bleeding too slow to see. Each finding gains meaning only after the specimen itself is verified as timely, uncontaminated, and appropriate for the test requested.

Stool formation and normal composition

Roughly 100 to 200 g of stool is passed daily. It consists of undigested foodstuff (cellulose), sloughed intestinal epithelium, bacteria, gastrointestinal secretions, bile pigments, electrolytes, and water; transit through the colon takes 18 to 24 hours.¹ Roughly 9,000 mL of fluid enters the gastrointestinal tract daily from food, water, saliva, and gastric, biliary, pancreatic, and intestinal secretions. Only 500 to 1,500 mL reaches the large intestine, and about 100 to 150 mL leaves in stool. The colon can reabsorb up to 3,000 to 4,000 mL of water. Diarrhea develops when the fluid load exceeds that reabsorptive capacity, water absorption is inhibited, or transit is too fast for full absorption. Slowed transit permits extra water reabsorption and produces constipation, with small, hard, spherical stools (scybala).

Normal brown color comes from bile pigment. Conjugated bilirubin secreted into the small intestine is hydrolyzed and then reduced by intestinal bacteria to colorless tetrapyrroles (urobilinogen, stercobilinogen, mesobilinogen), which spontaneously oxidize to the orange-brown urobilins that color normal stool. Bacterial fermentation of undigested carbohydrate produces normal flatus (about 400 to 700 mL/day). Poorly digested foods such as certain legumes, or a disaccharidase deficiency such as lactose intolerance, can produce excess gas that makes stool foamy or causes it to float. Either appearance can be a normal variant.

Diarrhea mechanisms and classification

Diarrhea is defined by increased stool weight (above 200 g/day), increased liquidity, and frequency exceeding three stools per day, relative to the individual’s own baseline pattern. Three mechanisms, alone or combined, produce it.¹

Acute diarrhea begins suddenly and usually resolves within 1 to 2 weeks, though it can last up to 4 weeks. Symptoms and a small panel of stool tests classify it as noninflammatory or inflammatory.

Chronic diarrhea (longer than 4 weeks, often 8 or more) may be inflammatory or noninflammatory. Inflammatory causes include ulcerative colitis, Crohn disease, food allergy, and radiation enteritis with bloody diarrhea, and celiac disease, tropical sprue, and microscopic colitis with watery diarrhea. Noninflammatory diarrhea is further classified by its response to fasting. Small-to-normal-volume diarrhea that ceases with fasting implicates maldigestion or malabsorption (pancreatic disease, lactose or other sugar intolerance, bacterial overgrowth, or irritable bowel syndrome’s motility and secretory mix). Large-volume diarrhea (1 to 3 L/day or more) that persists with fasting implicates a secretory process such as a hormone-secreting tumor or laxative abuse.

Stool electrolytes and the osmotic gap

Secretory and osmotic diarrhea are evaluated with fecal sodium and potassium.² Calculate the stool osmotic gap with the standard assumed baseline:

stool osmotic gap (mOsm/kg) = 290 − 2 × (stool sodium + stool potassium)

The 290 mOsm/kg value is an assumed plasma-like baseline. Measured fecal osmolality has a separate role: it detects dilution, contamination, or storage artifacts, because fermentation after collection changes the measured value, so it is not substituted into this calculation.² A gap below 50 mOsm/kg supports secretory diarrhea; a gap above 75 mOsm/kg supports osmotic diarrhea, with values in between remaining indeterminate. Interpret either pattern with stool pH, fasting response, volume, and clinical context. Delayed processing or added water can make the calculated gap misleading.

Stool pH adds a corroborating clue: bacterial fermentation of unabsorbed sugar produces lactic acid and drops pH below about 5.5, while secretory diarrheas stay neutral to alkaline (pH above 5.6). Diarrhea that stops with fasting points to osmotic disease, because the offending ingested or malabsorbed solute has been removed. Diarrhea that persists despite fasting, typically with a markedly increased 24-hour volume and nocturnal urgency, points to secretory disease, classically from hormone-secreting tumors or laxative abuse.

A stool specimen measures 30 mmol/L sodium and 50 mmol/L potassium. The osmotic gap is 290 − 2 × (30 + 50) = 130 mOsm/kg, a wide gap that supports osmotic diarrhea and directs the workup toward an unabsorbed solute.

Steatorrhea

Steatorrhea is fecal fat excretion above the method-specific upper reference limit, commonly 6 to 7 g/day.¹ It can result from maldigestion, including decreased pancreatic enzymes from pancreatitis, cystic fibrosis, or pancreatic cancer, or decreased bile-acid micelle formation from severe hepatocellular disease, bile duct obstruction, or bacterial deconjugation in stasis states. It can also result from malabsorption due to damaged mucosa in celiac disease or tropical sprue, abetalipoproteinemia, or lymphatic obstruction from lymphoma or Whipple disease.

The historical D-xylose absorption test uses a measured oral dose followed by blood and urine collection. A low result has limited specificity for mucosal disease because renal impairment, delayed gastric emptying, altered intestinal transit, bacterial overgrowth, age, and collection errors can also lower recovery. A normal result with ongoing steatorrhea can support maldigestion. The test is now infrequently used; direct evaluation of pancreatic function, celiac disease, or another suspected cause is usually preferred.

Stool in steatorrhea is characteristically pale, greasy, bulky, spongy or pasty, and markedly foul-smelling. It may float or appear foamy from trapped gas, a nonspecific feature also seen in normal stool. Macroscopic findings can be misleading when diarrhea and steatorrhea coexist, so confirm steatorrhea with a fecal fat determination.

Specimen collection

Random specimens in clean plastic or glass screw-top containers suffice for occult blood screening and qualitative microscopy (leukocytes, muscle fibers, fat).¹ Quantitative tests, especially fecal fat, require a timed 2- to 3-day collection in a large, preweighed container that also accommodates homogenization. Daily excretion correlates poorly with intake on the same day, so only a multi-day pool is representative. Specimens must never be contaminated with urine, which is toxic to protozoan trophozoites and chemically interfering, or with toilet water or disinfectant. Containers holding an ova-and-parasite preservative are never reused for other tests. Timed collections accumulate gas from ongoing fermentation, so open containers slowly and cover them with a disposable tissue while opening to control spatter. Reported noncompliance with occult blood collection ranges from 50 to 90% and can lead to missed disease. Clear verbal and written instructions, in the patient’s own language where possible, improve collection and diagnostic yield.

Macroscopic examination

Color and consistency are frequently the first evidence of gastrointestinal disease, though pigmented food and medication can mimic pathology and must be excluded first.¹

Visible mucus is not a typical normal finding. Small amounts may accompany benign straining or constipation, whereas larger amounts occur with villous adenoma, and mucus may also accompany colitis, intestinal tuberculosis, ulcerative diverticulitis, bacillary dysentery, neoplasm, and rectal inflammation. Report mucus when seen.

Microscopic examination

Fecal leukocytes, lactoferrin, and calprotectin. Fecal neutrophils appear when the intestinal wall is invaded or inflamed, as in ulcerative colitis, Crohn disease, bacillary dysentery, pseudomembranous C. difficile colitis, ulcerative diverticulitis, intestinal tuberculosis, and abscess or fistula.¹ They are absent in noninvasive disease, including amebic colitis, viral gastroenteritis, and most malabsorption. Celiac disease and microscopic colitis instead produce mononuclear cells. One to three neutrophils per high-power field is considered abnormal on a fresh wet mount (two drops Löffler methylene blue, mixed, read after 2 to 3 minutes) or a permanent Wright or Gram-stained smear; Gram stain also shows bacterial morphotypes directly. Detection of any neutrophil under oil immersion has roughly 70% sensitivity for invasive bacterial disease.

Two immunochemical markers avoid reliance on fresh-cell morphology. Fecal lactoferrin is a granulocyte secondary-granule protein stable when refrigerated or frozen and detected by latex agglutination, including in C. difficile-associated disease. Fecal calprotectin is a neutrophil cytoplasmic protein stable at room temperature for days. Calprotectin rises in inflammatory bowel disease and infectious colitis but remains low in irritable bowel syndrome, which makes it a noninvasive discriminator; gastroenterology guidelines use values around 50 µg/g when applying calprotectin to inflammatory bowel disease management, and laboratories establish method-specific intervals.³

Muscle fibers. Undigested striated fibers signal defective proteolysis and, with fecal fat testing, help diagnose and monitor pancreatic insufficiency (especially cystic fibrosis), biliary obstruction, malabsorption, and gastrocolic fistula.¹ The patient eats red meat before collection to provide substrate, and the specimen is examined within 24 hours. Emulsify a small stool aliquot in 10% eosin in alcohol to sharpen striations, cover it, hold it for 3 minutes, and scan under high power for exactly 5 minutes. Count only fibers with well-preserved cross-striations. The striation pattern grades digestion: undigested fibers have striations in both directions, partially digested fibers in one direction only, and digested fibers have none. Report more than ten undigested fibers as increased (creatorrhea), which correlates with impaired digestion and rapid transit.

Qualitative fecal fat. This two-slide screen uses Sudan III, Sudan IV, or Oil Red O, which stain neutral fat, fatty acids, soaps, and cholesterol orange-red.¹ For slide 1 (neutral fat), mix a drop of a 1-part-stool-to-2-parts-water emulsion with 95% ethanol and saturated alcoholic Sudan III. Triglycerides stain immediately as large orange-red droplets; more than 60 droplets per high-power field suggests steatorrhea. The split-fat slide better reflects total fat, because bacterial lipase and spontaneous hydrolysis reduce the neutral-fat fraction independently of the total load. For slide 2 (split fat), mix the same emulsion with acetic acid and heat gently to near boiling. This liberates fatty acids from their salts (soaps) and hydrolyzes remaining triglycerides so that every lipid class stains. Cholesterol crystallizes into visible plates as the slide cools. Fewer than about 100 small droplets (4 µm or smaller) per high-power field is normal; about 100 droplets in the 6 to 75 µm range is the classic steatorrhea pattern. The two slides localize the defect. Increased neutral fat with normal split fat means digestion did not occur (pancreatic or biliary maldigestion). Increased split fat with normal neutral fat means digestion occurred while absorption failed (mucosal malabsorption). Increases on both slides indicate combined, more severe disease.

Ova and parasite examination is ordered for diarrhea with a suggestive history (travel, daycare exposure, immunosuppression, waterborne outbreak, or persistence despite a negative bacterial workup).¹ Collection technique controls yield. Use a clean, dry, watertight container without urine or toilet-water contamination. Collect three specimens over no more than 10 days, ideally on alternating days, because shedding is intermittent. Deliver soft or watery stool promptly, because fragile trophozoites degenerate within hours at room temperature; formed stool, which is more likely to contain hardier cysts, may be refrigerated briefly. Avoid barium, antacids, bismuth, mineral oil, and antimicrobials because they interfere with detection. Direct saline wet mounts, with iodine as needed, examine soft or bloody stool promptly for motile trophozoites. Formed stool is concentrated, typically by formalin-ethyl acetate sedimentation, to recover cysts, oocysts, eggs, and larvae, followed by a permanently stained smear (trichrome or iron hematoxylin) for identification. Organism identification and reporting belong to microbiology procedures; the laboratory performing the fecal examination supplies the collection rules and the wet-mount and concentration screening above.

Fecal occult blood testing

Occult blood testing is the most frequently ordered fecal test, used to screen for colorectal disease and to evaluate unexplained anemia. Interpretation depends on method principle, diet and drug interferences for the guaiac method, and the fact that a positive screen requires diagnostic follow-up.¹

Bleeding above about 2.5 mL per 150 g of stool (roughly 2 mg hemoglobin/g stool) is pathologically significant but grossly invisible. The following three methods increase in sensitivity and specificity in the order listed.

• Guaiac-based (gFOBT) exploits hemoglobin’s pseudoperoxidase activity: heme catalyzes peroxide-driven oxidation of colorless guaiac to blue. Its comparatively low sensitivity reduces false-positive reactions from physiologic blood, dietary pseudoperoxidases, and intestinal bacterial peroxidases. For the Hemoccult II SENSA product, collect two thin smears from different areas of each of three consecutive bowel movements, develop with two drops of developer per smear, and read any trace of blue within 60 seconds.⁴ Develop no sooner than 3 days after sample application (dietary peroxidases degrade on the card), or wait 3 to 5 minutes after application for immediate testing; slides remain valid for up to 14 days at room temperature. Rehydration is not recommended because it increases false positives. False positives: red meat and fish, certain raw vegetables (turnip, broccoli, cauliflower, horseradish) and fruits (cantaloupe, banana, pear, plum), aspirin and nonsteroidal anti-inflammatory drugs (which also cause occult GI bleeding directly), and menstrual or hemorrhoidal contamination; patient instructions restrict red meat and vitamin C for 3 days before and during collection, and nonsteroidal anti-inflammatory drugs for 7 days. False negatives: ascorbic acid above 250 mg/day, which reduces the oxidized chromogen back to colorless; a smear applied too thickly, which traps reagent; an expired developer or card; or a specimen or prepared slide held too long before testing.
• Immunochemical (FIT) uses antibodies against the globin portion of intact human hemoglobin. No dietary or medication restriction is needed, and specificity is higher. Because digestive and bacterial enzymes degrade globin during transit from the upper GI tract, FIT has low sensitivity for upper GI bleeding; a bleeding gastric ulcer may therefore be missed by a test optimized for colorectal screening.⁵ Detection is manual (visual test and control lines) or automated (photometric). Current screening guidance offers annual FIT or annual high-sensitivity guaiac testing as stool-based options for average-risk adults 45 to 75 years old.
• Porphyrin-based (fluorometric, HemoQuant type) converts heme, and the heme-derived porphyrins produced by bacterial degradation during transit, to fluorescent porphyrins and quantifies both together. Because it measures the degradation products as well as intact heme, it is the most sensitive method for upper GI bleeding and the only one of the three offering true quantitation. Red meat (nonhuman heme) still causes false positives, so a 3-day meat restriction remains necessary. The assay is labor-intensive and confined to reference laboratories.⁶

Quantitative fecal fat

Quantitative fecal fat is the definitive steatorrhea test, and the underlying cause requires the separate maldigestion and malabsorption evaluation described above.¹ The patient maintains a regulated 100 to 150 g/day fat intake for 2 to 3 days before and throughout a minimum 3-day collection. Gather the specimen in a large preweighed container, weigh and homogenize it on receipt, and refrigerate it to stop ongoing bacterial lipid degradation. Avoid laxatives, synthetic fat substitutes, and mineral-oil or lubricant contamination. The Van de Kamer titration is the classical reference method. It extracts fecal lipid, converts it to fatty acids, and titrates to a neutral endpoint with sodium hydroxide; the titration captures about 80% of total fecal fat. Gravimetric weighing captures essentially all fecal fat, but either method requires flammable, corrosive solvents. Report results as grams of fat per 24 hours or as the coefficient of fat retention:

coefficient of fat retention (%) = [(dietary fat − fecal fat) ÷ dietary fat] × 100

At a 100 g/day fat intake, normal excretion is roughly 1 to 6 g/day with retention of 95% or more. Children 3 years and older and adults are both expected to retain at least about 95% of ingested fat, and retention below that threshold indicates steatorrhea.

The acid steatocrit adapts the microhematocrit method for rapid screening or pediatric monitoring where repeated 72-hour collections are impractical: dilute 0.5 g stool 1:4 with deionized water, vortex, mix with 5N perchloric acid at 20% of the homogenate volume, confirm pH below 1, load into a 75-µL hematocrit capillary, seal, and spin at 13,000 rpm for 15 minutes. The spin separates a fat layer over solid residue:

acid steatocrit (%) = fat layer length ÷ (fat layer length + solid layer length) × 100

Adult fecal fat (g/24 hr) is approximately 0.45 × (acid steatocrit %) − 0.43, with values below about 31% considered normal. Pediatric fecal fat (up to age 15) is approximately 0.1939 × (acid steatocrit %) − 0.2174. Values in infants over 6 months are normally below 10%, with 10 to 20% equivocal and above 20% abnormal; younger infants show physiologically high, widely variable values regardless. Near-infrared reflectance spectroscopy (scanning a homogenized 48- to 72-hour collection at 1,400 to 2,600 nm) and nuclear magnetic resonance spectroscopy of microwave-dried specimen are rapid alternatives that use few reagents and correlate well with gravimetry.

APT test

The APT test distinguishes a neonate’s own gastrointestinal hemorrhage from swallowed maternal blood based on fetal hemoglobin’s alkali resistance.¹ Use a fresh, visibly bloody specimen; black or tarry stool indicates that hemoglobin has already degraded to hematin. Emulsify the specimen in water and centrifuge it. Split the pink supernatant into a reference tube and a test tube, then add 1% sodium hydroxide to the test tube. Fetal hemoglobin resists alkali denaturation and stays pink for at least 2 minutes; adult hemoglobin denatures to yellow-brown within that window. Run cord-blood and adult-blood controls in parallel. Maternal thalassemia major invalidates the test because the mother’s HbF fraction is too high to serve as an adult reference.

Fecal enzymes

Pancreatic insufficiency (chronic pancreatitis, cystic fibrosis) leaves undigested protein, carbohydrate, and fat in stool and can be screened by measuring pancreatic proteases directly.¹ Chymotrypsin survives intestinal transit better than trypsin and remains stable in stool for up to 10 days at room temperature, which makes it a more sensitive spectrophotometric marker of mild insufficiency. Elastase I is more stable in feces than trypsin or chymotrypsin. This pancreatic isoenzyme is concentrated roughly fivefold in feces relative to pancreatic juice and is measured by ELISA with a monoclonal antibody specific to human pancreatic elastase-1. Results are less affected by motility disorders, mucosal disease, or enzyme-replacement therapy, so a random specimen can help distinguish pancreatic from nonpancreatic steatorrhea. The historical trypsin screen, an x-ray-film gelatin-digestion spot test, is obsolete because of false negatives from trypsin inhibitors or intestinal degradation and false positives from bacterial proteases in old specimens.

Reducing substances and carbohydrate intolerance

Unabsorbed carbohydrate draws water osmotically and ferments, and the fermentation produces diarrhea, acid, and gas. Idiopathic lactase deficiency predominates, especially in populations of African, Asian, and southern European descent. Secondary disaccharidase deficiency follows mucosal injury (enteritis, celiac disease) or certain drugs.¹ Testing centers on infants. A copper-reduction (Clinitest) test on one part stool emulsified with two parts water detects reducing sugars; 0.5 g/dL or more indicates carbohydrate intolerance. Sucrose is not reducing and escapes detection. A concurrent stool pH below 5.5 supports bacterial fermentation. In premature infants, a positive result correlates with necrotizing enterocolitis. Positive screens require evaluation of the suspected sugar with a current validated method. Options include a hydrogen or methane breath test, a supervised dietary challenge, or direct disaccharidase assay on an intestinal biopsy.

For lactose breath testing, the North American Consensus uses 25 g lactose with measurement for at least 3 hours, and a hydrogen rise of 20 ppm or more above baseline supports malabsorption; record methane and symptoms, because malabsorption and intolerance are distinct.⁷ Apply the substrate, duration, and positive threshold specified by the validated method in use.

References

1. McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2022.
2. Schiller LR, Pardi DS, Sellin JH. Chronic diarrhea: diagnosis and management. Clin Gastroenterol Hepatol. 2017;15(2):182-193. doi:10.1016/j.cgh.2016.07.028.
3. American Gastroenterological Association Institute. AGA clinical practice guideline on the role of biomarkers for the management of Crohn's disease. Gastroenterology. 2023. Accessed August 31, 2026. gastrojournal.org.
4. Beckman Coulter. Hemoccult II SENSA: Product Instructions. 462489EF. Accessed August 31, 2026. HemoCue product instructions.
5. United States Preventive Services Task Force. Colorectal cancer: screening. Final recommendation statement. 2021. Accessed August 31, 2026. USPSTF recommendation.
6. Mayo Clinic Laboratories. HemoQuant, feces: test catalog. Generated May 20, 2026. Accessed August 31, 2026. Mayo Clinic test catalog.
7. Rezaie A, Buresi M, Lembo A, et al. Hydrogen and methane-based breath testing in gastrointestinal disorders: the North American Consensus. Am J Gastroenterol. 2017;112(5):775-784. doi:10.1038/ajg.2017.46.