Acute diarrhea laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:

Overview

An elevated/reduced concentration of serum/blood/urinary/CSF/other [lab test] is diagnostic of [disease name].

OR

Laboratory findings consistent with the diagnosis of [disease name] include [abnormal test 1], [abnormal test 2], and [abnormal test 3].

OR

[Test] is usually normal among patients with [disease name].

OR

Some patients with [disease name] may have elevated/reduced concentration of [test], which is usually suggestive of [progression/complication].

OR

There are no diagnostic laboratory findings associated with [disease name].

Laboratory Findings

Laboratory investigations performed in the workup of patients with acute diarrhea include Complete blood count (CBC), glucose levels, white blood cells (WBC) detection, urine analysis, calcium levels, Thyroid stimulating hormone (TSH) levels, Liver function tests (LFTs), Blood urea nitrogen (BUN) / creatinine levels, Hepatitis serologies and stool examination.

• Stool examination includes the following:
  • Stool culture
  • Stool electrolytes
  • Stool osmolality
  • Ova and parasites
  • Fecal lactoferrin
  • Fecal leukocytes
  • Test for C. difficile

Laboratory Evaluation of Acute Diarrhea

Spot Stool Analysis

• Stool spot analysis is preferred over 24 hour stool collection as it is less cumbersome.

Occult Blood

• A positive test result suggests the presence of inflammatory bowel disease and acute causes of bloody diarrhea.
• Fecal occult blood positivity may also be associated with diarrhea due to idiopathic secretory diarrhea, laxative abuse, and microscopic colitis.

White Blood Cells

• Wright's staining and microscopy is the standard method for the detection of white blood cells (WBCs) present in stool.
• Neutrophils in the stool present in patients having acute infectious diarrhea may be detected by latex agglutination test.

Stool Culture

• In immunocompetent patients with acute diarrhea, stool culture is not routinely performed.
• If the patient has a history of swimming in streams or ponds and consuming untreated water from wells, stool culture may be performed to evaluate the patient for Aeromonas or Pleisomonas species.
• In immunocompromised patients, stool culture is included as a part of routine investigations to rule out infection due to bacteria such as Samonella, Campylobacter, protozoa and fungi.

• Protozoa may detected by the use of fecal enzyme-linked immunosorbent assay (ELISA).

Quantitative Stool Analysis

• A 48 or 72-hour quantitative stool collection may sometimes may be useful in the evaluation of acute diarrhea in patients.
• Full analysis of the collection includes measurement of:
  • Stool weight
  • Stool fat content
  • Stool osmolality
  • Stool electrolyte concentrations
  • Magnesium concentration and output
  • Stool pH
  • Stool occult blood
  • Fecal chymotrypsin
  • Fecal elastase activity
  • Prior to collection period, patient should eat a regular diet containing adequate amounts of calories and fat. All medications, especially antidiarrheal medications should be avoided.

Fecal Weight

• Stool weight is also a useful index in the diagnosis of acute diarrhea.
• Stool weight of >200g/day is considered diarrheal.
• Low stool weight with increased frequency of stools may be indicative of incontinence or pain.
• Cessation of diarrhea with fasting is indicative of osmotic diarrhea caused by nonabsorbable substances or secretory diarrhea due to laxatives.

Stool Osmotic Gap

• The stool osmotic gap is calculated from electrolyte concentrations in stool water by the following formula : 290 - 2([Na+] + [K+]).
• The osmolality of stool present in the distal intestine is preferred over fecal fluid, as measured osmolality of fecal fluid increases with bacterial fermentation of carbohydrates to osmotically active organic acids.
• Stool osmotic gap in cases of osmotic diarrhea is characterized by osmotic gaps >125 mOsm/kg and in secretory diarrheas, osmotic gap is <50 mOsm/kg.
• In mixed cases, the osmotic gap lies between 50-125mOsm/kg.

Fecal pH

• A fecal pH of < 5.3 indicates that carbohydrate malabsorption (such as that associated with lactulose or sorbitol ingestion) is a major cause of diarrhea.
• A pH of > 5.6 argues against carbohydrate malabsorption as the only cause and malabsorption syndrome that involves fecal loss of amino acids and fatty acids in addition to carbohydrate, have a higher fecal pH.

Fecal Fat Concentration and Output

• The upper limit of fecal fat output measured in normal subjects (without diarrhea) ingesting normal amounts of dietary fat is approximately 7 g/day (9% of dietary fat intake)and values more than this signify the presence of steatorrhea.
• A fecal fat concentration of <9.5 g/100 g of stool more likely to be seen in small intestinal malabsorptive syndromes because of the diluting effects of coexisting fluid malabsorption.
• A fecal fat concentrations of ≥9.5 g/100 g of stool were seen in pancreatic and biliary steatorrhea, in which fluid absorption in the small bowel is intact.

Analysis for Laxatives

Analysis for laxatives should be done early in the evaluation of diarrhea of unknown etiology. The simplest test for a laxative is alkalinization of 3 mL of stool supernatant or urine with one drop of concentrated sodium hydroxide and a pink or red color is a positive result. Stool water can be analyzed specifically for phenolphthalein, emetine and bisacodyl and its metabolites, using chromatographic or chemical tests. Urine can be analyzed for anthraquinone derivatives.

• If stool electrolyte analysis suggests secretory diarrhea (osmotic gap <50), the patient may have ingested a laxative capable of causing secretory diarrhea, such as sodium sulfate or sodium phosphate ingestion.
• If stool electrolyte analysis suggests osmotic diarrhea (osmotic gap >125 mOsm/kg), magnesium (Mg2+) laxatives may have been ingested.
• If fecal osmolality is significantly less than 290 mOsm/kg (the osmolality of plasma), water or hypotonic urine has been added to the stool.
• If the osmolality is far above that of plasma, hypertonic urine may have been added to stool. Urinary contamination can be confirmed by a finding of high monovalent cation concentration (e.g., [Na+] + [K+] > 165, physiologically impossible in stool water) and a high concentration of urea or creatinine in stool water.

Tests for Bacterial Overgrowth

• The gold standard for diagnosis of bacterial overgrowth has been quantitative culture of an aspirate of luminal fluid and specifically a positive jejunal culture (>106 organisms/mL) in chronic diarrhea patients can be considered evidence of clinically significant bacterial overgrowth in the upper small intestine.

• A breath test using [14C] glycocholate is used to test bacterial overgrowth. The radiolabeled conjugated bile acid is deconjugated by the bacteria, and the 14C in the side chain is metabolized to 14CO2, which is exhaled.

• Another 14C-breath test using [14C] xylose, nonradioactive glucose and nonradioactive lactulose have been developed to test bacterial overgrowth.

• An elevated concentration of hydrogen in breath after overnight fasting also has been proposed as an insensitive but specific marker of small intestinal bacterial overgrowth. This elevated hydrogen concentration may also be seen in patients with malabsorption syndrome.

• An abnormal Schilling II test result (radiolabeled B12 given with intrinsic factor) that normalizes after therapy with broad-spectrum antibiotics has also been considered as a test for small intestinal bacterial overgrowth (the so-called Schilling III test).