Laboratory Tests and Ancillaries
Lab studies are not usually needed but may be helpful in epidemics in etiology identification. Many diarrheal illnesses are viral or self-limiting and will resolve in <3 days; therefore, microbiological testing is not typically required in patients who present <24 hours after the onset of diarrhea. The exceptions to this are: Patients who present with blood or pus in stool, are febrile, dehydrated or immunocompromised, with severe abdominal tenderness or cramping, persistent diarrhea or nausea, suspected nosocomial infection, or with signs of sepsis.
Serum creatinine, blood urea nitrogen (BUN), and electrolyte levels should be taken in cases of dehydration or systemic toxicity. Other lab tests to be done if complications are suspected include a complete blood count (CBC), urinalysis, serum bicarbonate, total CO2 or arterial blood gas (ABG). Moderate to severe dehydration is indicated by a urine specific gravity of ≥1.020, urine osmolality of >800 mOsm/kg, serum osmolality of >300 mOsm/kg and BUN/creatinine ratio of >20 mg/dL; ABG with pH <7.35 and HCO3 <22 may be seen in severe dehydration.
Stool Exam
Diarrhea in Adults - Infectious_Diagnostics
Stools typical of cholera are watery, mucoid, and colorless with little food residue (rice water stools). Bloody mucoid diarrhea is characteristic of EIEC infection, while EHEC produces bloody diarrhea with hemorrhagic colitis and hemolytic uremic syndrome in 6-8% of cases.
Stool Microscopy
Stool microscopy is performed in cases of persistent or severe bloody diarrhea. In viral gastroenteritis, red and white cells are not normally found. Cholera may reveal bacteria with darting motility but no WBC or RBC. Y enterocolitica and C difficile produce heme-positive stool. No fecal white blood cells (WBC) is seen with EHEC infection. Fecal WBCs are present in 80-90% of diarrhea caused by Shigella or Salmonella and are less common with those caused by Campylobacter and Yersinia. Ova and parasite examination may use trichrome staining or modified acid-fast staining for Cyclospora, Cystoisospora and Cryptosporidia. This may be performed in patients who are immunocompromised or immunosuppressed, or those with a history of travel to developing countries with diarrhea >3-4 weeks.
Molecular Diagnostic Tests
Stool culture-independent molecular diagnostic tests are rapid tests that can identify difficult-to-diagnose viral, bacterial (eg E coli diarrheal pathovars [EAEC, EPEC, ETEC]) and protozoal infections. These help determine if empiric antimicrobial therapy could be beneficial. These are unable to preserve the organism for subsequent resistance testing or outbreak investigations.
Stool Culture and Sensitivity
Stool culture and sensitivity are not necessary for all cases of diarrhea unless a bacterial cause is suspected. This should be obtained before antibiotic therapy and performed in both non-endemic and endemic areas. During suspected diarrhea and shigella outbreaks, rapid diagnostic tests may be used, but it is still recommended to confirm results with stool culture.
Specific Indications
The following are specific indications for requesting a stool culture and sensitivity: Bloody stools or those that are positive for occult blood or leukocyte; prolonged diarrhea not previously treated with antibiotics, immunocompromised or immunosuppressed patients and epidemiologic purposes (eg history of travel to developing countries, cases involving food handlers). There is a need for early testing if the patient is >70 years old, or has severe abdominal pain, persistent vomiting, significant dehydration, or high fever and if C difficile-associated disease is suspected. Those with a high risk of complications (eg hemolytic-uremic syndrome, septic or non-septic arthritis, Reiter syndrome Guillain-Barre syndrome, post-infectious IBS, toxic megacolon, tropical sprue) and suspected outbreaks of enteric origin should also be tested.
Blood Cultures
Blood cultures are recommended for: Patients with signs of septic shock; suspicion of enteric fever; patients with signs and symptoms suggestive of systemic infections; immunocompromised patients; patients with high-risk conditions such as poorly controlled diabetes; patients with high-grade fever of unknown etiology; and patients with a history of travel or who had contact with travelers from enteric fever-endemic areas.
Biopsy
Intestinal biopsy specimens are indicated in patients with protracted diarrhea or who are immunocompromised individuals when an etiology cannot be identified through molecular testing or examination for ova and parasites. This can identify cytomegalovirus, histoplasmosis and protozoal infections.
Other Tests
Enzyme immunoassay may be used in patients with persistent diarrhea or a history of travel to identify Giardia or Cryptosporidia. Testing for C difficile toxin A/B may be performed in patients with previous antibiotic use, chemotherapy or hospitalization. Electron microscopy and polymerase chain reaction (PCR) may be used in viral gastroenteritis to diagnose adenovirus, astrovirus, norovirus, sapovirus and rotavirus.
Imaging
An abdominal series is indicated in patients with suspected bowel perforation, obstruction or toxic megacolon, while a computed tomography (CT) scan may be considered in older individuals with severe abdominal pain.