Diarrhea is defined as an increase in daily stool weight above 200 g with increased liquidity and frequency of more than three times per day.
Diarrhea classification can be based on four factors: duration of the illness, mechanism, severity, and stool characteristics.
Diarrhea lasting less than 4 weeks is defined as acute, and diarrhea persisting for more than 4 weeks is termed chronic diarrhea.
The major mechanisms of diarrhea are secretory and osmotic.
The laboratory tests used to differentiate these mechanisms are fecal electrolytes (fecal sodium, fecal potassium), fecal osmolality, and stool pH.
Osmotic gap = 290 – [2 (fecal sodium + fecal potassium)]
Secretory Diarrhea | Osmotic Diarrhea | |
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Causative Agent | bacterial, viral and protozoan infections, drugs, hormones, inflammatory bowel disease, endocrine disorders, neoplasms and collagen vascular disease | disaccharidase deficiency, malabsorption, laxatives, magnesium-containing antacids, amebiasis and antibiotic administration |
Osmotic Gap | <50 mOsm/kg | >50 mOsm/kg |
Electrolytes | increased electrolytes | negligible electrolytes |
Fecal Fluid pH | >5.6 | <5.6 |
Diagnostic Tests | stool cultures, ova and parasite examinations, rotavirus immunoassay, fecal leukocytes | fecal fat examination, muscle fiber detection, trypsin screening, Clinitest, D-xylose tolerance test, lactose tolerance test, fecal electrolytes, stool pH, fecal osmolality |
Early Dumping | Late Dumping |
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occurs 10 to 30 minutes after eating | occurs 2 to 3 hours after eating |
symptoms of nausea, vomiting, bloating, cramping, diarrhea, dizziness and fatigue | symptoms of weakness, sweating and dizziness |
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Color | Clinical Significance |
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brown | normal (intestinal oxidation of stercobilinogen to urobilin) |
pale yellow, white, gray | blockage of the bile duct, barium sulfate |
red | blood from the lower gastrointestinal tract, rifampin, beets, food coloring |
black, tarry | blood from the esophagus, stomach or duodenum (it takes approximately 3 days to appear in the stool; during this time, hemoglobin is degraded), iron, charcoal, bismuth (antacids) |
green | oral antibiotics (oxidation of fecal bilirubin to biliverdin), green vegetables or food coloring |
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Appearance | Clinical Significance |
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watery | diarrhea |
small, hard | constipation |
slender, ribbon-like | obstruction in the intestines |
bulky, frothy, greasy, foul odor, may float | biliary obstruction, steatorrhea |
mucus-coated | intestinal inflammation (pathologic colitis), irritation (excessive straining during excretion) |
if blood-streaked, damage to the intestinal walls, possibly caused by bacterial or amebic dysentery or malignancy |
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Leukocytes, primarily neutrophils, are seen if the intestinal mucosa is affected.
Specimens can be examined as wet preparations stained with methylene blue or as dried smears stained with Wright’s or Gram stain.
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It can be helpful in the diagnosis and monitoring of patients with pancreatic insufficiency.
It is frequently ordered in conjunction with microscopic examinations for fecal fats.
Slides for muscle fiber detection are prepared by emulsifying a small amount of stool in 10% alcoholic eosin, which enhances the muscle fiber striations.
To produce a representative sample, patients should be instructed to include red meat in their diet prior to collecting the specimen.
Specimens should be examined within 24 hours of collection.
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Specimens from suspected cases of steatorrhea can be screened microscopically for the presence of excess fecal fat.
The procedure can also be used to monitor patients undergoing treatment for malabsorption disorders.
In general, correlation between the qualitative and quantitative fecal fat procedures is good; however, additional unstained phospholipids and cholesterol esters are measured by the quantitative procedure.
Lipids included in the microscopic examination of feces are neutral fats (triglycerides), fatty acid salts (soaps), fatty acids, and cholesterol.
Neutral fats are readily stained by Sudan III and appear as large orange-red droplets, often located near the edge of the coverslip.
Observation of more than 60 droplets/highpower field can be indicative of steatorrhea; however, the split fat stain representing total fat content can provide a better indication.
The breakdown of neutral fats by bacterial lipase and the spontaneous hydrolysis of neutral fats may lower the neutral fat count.
This also precludes the comparison of the two slide tests to determine whether maldigestion or malabsorption is causing steatorrhea.
To perform neutral fat staining:
Soaps and fatty acids do not stain directly with Sudan III; therefore, a second slide must be examined after the specimen has been mixed with acetic acid and heated.
Examination of this slide reveals stained droplets that represent not only the free fatty acids but also the fatty acids produced by hydrolysis of the soaps and the neutral fats.
In an examination of this split fat slide, both the number and size of the fat droplets must be considered.
Normal specimens may contain as many as 100 small droplets, less than 4 μm in size, per high-power field.
To perform split fat staining:
Cholesterol is stained by Sudan III after heating and as the specimen cools forms crystals that can be identified microscopically.
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Any bleeding in excess of 2.5 mL/150 g of stool is considered pathologically significant but is not visible, so fecal occult blood testing (FOBT) is necessary.
Annual testing for occult blood has a high positive predictive value for detection of colorectal cancer in the early stages and is recommended for persons older than age 50.
The most frequently encountered screening tests for occult blood are based on detection of the pseudoperoxidase activity of hemoglobin, reacting with hydrogen peroxide to oxidize a colorless compound to a colored compound:
The immunochemical fecal occult blood test (iFOBT), is specific for the globin portion of human hemoglobin and uses anti-human hemoglobin antibodies, so it does not require dietary or drug restrictions.
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It is used as a confirmatory test for steatorrhea.
It requires the collection of at least a 3-day specimen and a regulated intake of fat (100 g/d) in the diet prior to and during the collection period.
The method routinely used for fecal fat measurement is the Van de Kamer titration, although gravimetric methods are available.
Another test, the acid steatocrit, is a more rapid and more convenient test to estimate the amount of fat excretion.
Near-infrared reflectance spectroscopy (NIRA) is a rapid procedure for fecal fat that requires less stool handling by laboratory personnel.
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It distinguishes between fetal hemoglobin from hemoglobin A, and between maternal hemoglobins AS, CS, and SS, and fetal hemoglobin.
The material to be tested is emulsified in water to release hemoglobin (Hb) and, after centrifugation, 1% sodium hydroxide is added to the pink hemoglobincontaining supernatant.
The presence of maternal thalassemia major would produce erroneous results owing to the high concentration of hemoglobin F.
Stool specimens should be tested when fresh.
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