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Stool testing

17 min

Skills you'll build4 skills
  • Calculate a stool osmotic gap from stool electrolyte results
  • Tell fecal immunochemical testing (FIT) from guaiac testing by what each detects
  • Check the fat-controlled diet and timed collection before interpreting fecal fat
  • Recognize when watery stool falsely lowers a fecal elastase result

At the bench: Check specimen identity and suitability before analysis · Coordinate cell examination and chemistry for a body fluid specimen

Try first

Try first

A liquid stool has sodium 25 mmol/L and potassium 40 mmol/L. Its measured osmolality is 360 mOsm/kg. What is the stool osmotic gap?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

The stool osmotic gap

Stool osmotic gap (mOsm/kg) = 290 − 2 × [stool sodium (mmol/L) + stool potassium (mmol/L)]. The formula uses an assumed osmolality of 290 mOsm/kg because measured osmolality rises as bacteria keep metabolizing substrate in stored stool. A gap below 50 mOsm/kg supports a secretory diarrhea, where the bowel secretes salt and water. A gap above 75 mOsm/kg supports an osmotic diarrhea, where an unabsorbed solute holds water in the bowel. Values between the two are indeterminate. The measured osmolality is used only to check the specimen for added water, urine or a storage problem.1 Added water, urine, a delay in processing or an incomplete collection can all make the gap inaccurate.2

Two methods for fecal blood

Guaiac test (gFOBT)Fecal immunochemical test (FIT)
DetectsPeroxidase-like activity of hemeHuman globin, by antibodies
Reacts withHuman and dietary hemeHuman globin
Diet before collectionRed meat and vitamin C restrictedGenerally no restriction
Main limitationVitamin C can suppress the colorGlobin from upper-tract bleeding is digested before it passes

A positive result shows blood in the stool. It does not locate the bleeding, and it needs diagnostic follow-up.3,4

Quantitative fecal fat

Quantitative fecal fat is interpreted only when the patient eats 100 to 150 g of fat a day, for 3 days before and throughout a 48- or 72-hour collection. Excretion above 7 g/24 h on that diet suggests malabsorption. A short collection or a low fat intake lowers the result and can hide steatorrhea. Laxatives, mineral oil and diaper ointments can raise it.5 Check the diet record and the collection times before the result is interpreted.

Fecal elastase

Fecal elastase is tested on a semisolid or solid stool. Watery stool dilutes the enzyme and can give a falsely low value. A result below 100 µg/g is good evidence of exocrine pancreatic insufficiency in the right setting, and 100 to 200 µg/g is indeterminate. The usual assay detects human elastase, so enzyme replacement does not affect it.6

Follow the bench demonstration

Sources for this step
References
  1. Schiller LR, Pardi DS, Sellin JH. Chronic diarrhea: diagnosis and management. Clin Gastroenterol Hepatol. 2017;15(2):182-193.e3. doi:10.1016/j.cgh.2016.07.028
  2. McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2022.
  3. Beckman Coulter. Hemoccult SENSA and Hemoccult II SENSA Product Instructions. 462489EF. June 2015. Accessed September 27, 2026. https://www.beckmancoulter.com/download/file/wsr-116766/462489EF?type=pdf
  4. US Preventive Services Task Force. Colorectal cancer: screening. Final recommendation statement. May 18, 2021. Accessed September 27, 2026. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/colorectal-cancer-screening
  5. Mayo Clinic Laboratories. Fat, feces. Test catalog. Accessed September 27, 2026. https://www.mayocliniclabs.com/test-catalog/Overview/607701
  6. Whitcomb DC, Buchner AM, Forsmark CE. AGA clinical practice update on the epidemiology, evaluation, and management of exocrine pancreatic insufficiency: expert review. Gastroenterology. 2023;165(5):1292-1301. doi:10.1053/j.gastro.2023.07.007

Watch one

Read each decision and its explanation.

A liquid stool from a patient with 5 weeks of watery diarrhea is processed 30 minutes after passage.

What is the stool osmotic gap, and which pattern does it support?

TestResultReference intervalFlag
Stool sodium100 mmol/L
Stool potassium35 mmol/L
Stool osmolality, measured310 mOsm/kg

Specimen: Not measured on stool. Liquid stool, no urine or water seen; stool water tested

  1. Check the specimen: liquid, free of urine and water, and processed in 30 minutes.

    Added water, urine or a long delay would make the gap inaccurate.

  2. Use the measured 310 mOsm/kg only to check the specimen. It shows no evidence of dilution.

    Measured osmolality far below 290 mOsm/kg would indicate added water or dilute urine.

  3. Add and double: 2 × (100 + 35) = 270 mOsm/kg.

    Each cation travels with an anion, so the cations are doubled.

  4. Subtract: 290 − 270 = 20 mOsm/kg.

    The formula uses the assumed 290 mOsm/kg because measured osmolality keeps rising in stored stool.

  5. Compare: 20 mOsm/kg is below 50 mOsm/kg.

    The cutoffs are below 50 for secretory and above 75 for osmotic.

Stool osmotic gap = 290 − 2 × (100 + 35) = 20 mOsm/kg, which supports a secretory pattern.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

Which target distinguishes the fecal immunochemical test (FIT) from a guaiac fecal occult-blood test?

Incorrect. Heme pseudoperoxidase activity is the guaiac principle, and dietary heme reacts in it as well as human heme.

Incorrect. Porphyrin quantitation of the HemoQuant type measures heme-derived porphyrins. FIT uses antibodies to human globin.

Correct. Guaiac detects heme pseudoperoxidase activity from human or dietary heme. FIT's antibodies react with human globin, which upper-tract digestion can degrade before passage.

Hint
  1. FIT stands for fecal immunochemical test.
  2. Consider what an immunochemical test uses to detect its target.

Review Fecal occult blood methods

Problem 2 of 3 · Hints available

Fecal elastase is 80 µg/g in a watery stool specimen. Which limitation applies?

Incorrect. The usual assay detects human elastase, so testing can proceed during enzyme replacement.

Correct. Watery stool dilutes elastase, so a low value needs a semisolid or solid specimen before it supports exocrine pancreatic insufficiency.

Incorrect. The below-100 µg/g cutoff applies to a semisolid or solid specimen. Dilution makes this watery-stool value unreliable.

Hint
  1. Check the specimen before the result.
  2. Consider the effect of extra water on the concentration of an enzyme.

Review Pancreatic elastase

Problem 3 of 3 · On your own

A complete 72-hour fecal fat collection gives 5 g/24 h. The diet record shows the patient followed a low-fat diet and ate about 40 g of fat a day. What does the result show?

The 7 g/24 h limit applies on a diet of 100 to 150 g of fat a day. At 40 g a day, too little fat reached the bowel to show malabsorption.

Read fecal fat without checking diet and collection

Quantitative fecal fat is interpretable only after the fat-controlled diet and a complete timed collection. A short collection or low fat intake lowers measured excretion and can hide steatorrhea, and laxatives, mineral oil, or diaper ointments can raise it.

A low-fat diet lowers fecal fat. The result is lower than it would be on the required diet.

A low fat intake lowers measured excretion and can hide steatorrhea. The collection was complete, and the fat intake was inadequate.

Review Quantitative fecal fat and acid steatocrit

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Beatrix Lott, 61, MRN 2284567, has had watery diarrhea for 6 weeks.
  • One watery stool arrives 40 minutes after passage, free of urine and water.
  • The orders are stool sodium and potassium, fecal elastase, and a FIT collected with the kit's probe.
  • She mentions she ate steak the night before.
TestResultReference intervalFlag
Stool sodium30 mmol/L
Stool potassium50 mmol/L
Fecal elastase85 µg/g Low>200 µg/g>200Low

Specimen: Not measured on stool. Watery stool; FIT sampled with the kit's probe and buffer

Decision 1 of 3

What is the stool osmotic gap, and which pattern does it support?

This leaves out the factor of 2. The sodium and potassium sum is doubled before it is subtracted.

The calculation is correct. A gap above 75 mOsm/kg supports an osmotic pattern.

290 − 2 × (30 + 50) = 130 mOsm/kg, well above the 75 mOsm/kg cutoff for an osmotic pattern.

Review Stool electrolytes and the osmotic gap

Decision 2 of 3

What do you do with the elastase of 85 µg/g?

Dilution may explain the low value. The comment prevents it from being read as evidence of pancreatic insufficiency, and a semisolid or solid specimen gives a reliable result.

A value below 100 µg/g is read as evidence of pancreatic insufficiency. In a watery stool, dilution can produce that value.

Reported low elastase from watery stool without comment

Watery stool dilutes elastase, so a value below 100 µg/g from a liquid specimen can reflect dilution. Reporting it without a specimen comment presents it as evidence of exocrine pancreatic insufficiency. A reliable result needs a semisolid or solid specimen.

A repeat measures the same diluted stool and gives the same low value.

Review Pancreatic elastase

Decision 3 of 3

Does the steak affect the FIT?

Red meat restrictions apply to guaiac testing, which reacts with dietary heme. FIT antibodies detect human globin.

Applied guaiac dietary interference rules to FIT

Guaiac reacts with dietary heme and is suppressed by vitamin C, so its instructions restrict red meat and vitamin C. FIT detects human globin and generally needs no dietary restriction. Applying guaiac rules to FIT delays or rejects specimens without reason. FIT detects upper-tract bleeding poorly, because globin is degraded before passage.

FIT detects human globin with antibodies, so dietary meat does not react. It generally needs no dietary restriction.

The comment would describe a guaiac limitation. Meat does not react in FIT.

Review Fecal occult blood methods

Each test depends on what a watery stool does to it. Stool electrolytes are measured on watery stool, and they gave an osmotic gap of 130 mOsm/kg. Elastase needs a formed stool, so its low value is released with a specimen comment. FIT detects human globin, so the steak has no effect.

For reference

Sources checked