Diarrhea Tests  

The cause of diarrhea can be determined from diarrhea symptoms, or by below diarrhea tests:

• stool tests;
• blood tests;
• urine tests;
• neurological tests;
• food allergy and intolerance tests; 
• absorption tests;
• abdominal ultrasound, endoscopic ultrasound, CT, X-ray with barium, plain abdominal X-ray, abdominal arteriography, HIDA scan, ERCP and MRCP;
• colonoscopy, EGD, capsule endoscopy, double baloon enteroscopy;
• Octreoscan.

Stool Tests In Diarrhea

Stool tests are done in chronic or heavy acute diarrhea and may include:

• Stool culture can show bacteria, causing diarrhea. Clostridium difficile toxin test is recommended in antibiotic associated diarrhea. For diagnosis of some rare bacteria, like Vibrio cholerae, separate tests may be required. Ova and parasites (O&P) test is recommended when diarrhea lasts more than a week. Specific antigen tests for giardia, cryptosporidium, E. histolytica, Helycobacter pylori, or Rotavirus also exist. False positive results of stool culture are from contamination of stool sample with urine or blood, or due to more than an hour passed between sample taking and testing. False negative results may be from antibiotics, laxatives, antidiarrheal drugs, or recent barium or x-ray investigation.
• Fecal occult blood test (FOBT) should be done when intestinal bleeding is suspected (after hemorrhoids, anal fissure, menstruation, and urinary bleeding are excluded). Bloody diarrhea, or diarrhea with a hidden blood may appear in heavy bacterial or parasitic infection, inflammatory bowel disease, and in diverticulitis. Other causes of bloody diarrhea are rare. Stool guaiac test (e.g. Hemoccult) - may detect daily blood loss of about 10 ml (1). Positive test confirms presence of the blood in the stool, but not its origin. Negative result means, that no blood was in the stool during the testing period, so additional samples at different occasions may be required. Three days before the test, red meat, and food which contains blood have to be avoided to prevent false positive results, and radishes, horseradish, turnips, cauliflower, uncooked broccoli, figs, melon, citrus fruits, and vit C supplements have to be avoided to prevent false negative results. Test should not be done in bleeding hemorrhoids, urinary bleeding, or during the menstruation. Five samples from each of three consecutive stools have to be taken. Immunochemical test - iFOBT (e.g. HemSelect) uses antibodies which react with human hemoglobin. The test may detects smaller amount of blood (50 µg/g stool) than guaiac test, but only from lower GI tract, not from the stomach or above (32). No dietary or drug restrictions are needed before the test; one sample from one or two stools is required. Both guaiac and immunochemical test kits are available in pharmacies without prescription, but testing by a lab or doctor is more reliable.
• WBC stool test. White blood cells in the stool may be found in severe bacterial infection, Crohn’s disease or ulcerative colitis.
• Fecal proteins. Lactoferrin is released from leukocytes, and it can be detected in stool in 5 minutes. Increased lactoferrin is a sign of inflammation, so it is used in acute diarrhea to distuinguish between mild viral or parasitic infection (negative) and invasive bacteria (positive), and in chronic diarrhea to distinguish between IBS (negative) and IBD (positive). In later case lactoferrin test was found to be 86% sensitive and 100% specific (26,27). Calprotein is another leukocyte protein, excreted in stool, which correlates with activity of IBD (29). Fecal A1AT test detects serum protein alpha1-antitrypsin in the stool in the protein losing enteropathy (PLE). Radionuclide labeled proteins, injected into a vein, and then appearing in the stool, also speak for PLE. Tests for proteins, excreted in colorectal cancer, are still in the research phase. Protein CD23 was found in the stool of patients with a food allergy (31). 
• Fecal IgA antibodies may be found in certain food allergies (to milk, egg, soy). In one clinical trial, IgA antigliadin antibodies in the stool were found in only 30% of patients with celiac disease, so they were considered as useless for diagnosis (2).