Laboratory Testing for Acute Diarrhea and Nausea
For a patient presenting with acute diarrhea and nausea, the initial laboratory workup should focus on stool testing for infectious pathogens (stool culture or multiplex PCR panel, Shiga toxin testing, and C. difficile if indicated) along with basic blood work to assess hydration status (electrolytes, renal function, complete blood count). 1
Immediate Assessment and Basic Labs
Blood Work:
- Electrolytes, renal function (BUN/creatinine), and complete blood count should be ordered to evaluate for dehydration-related imbalances and guide rehydration therapy 1
- A peripheral white blood cell count may be useful clinically to assess severity, though it should not be used to establish etiology 2
- Vital signs including temperature, blood pressure, heart rate, and oxygen saturation help assess for bacterial etiology 1
Stool Testing for Acute Infectious Diarrhea
Recommended Stool Tests:
- Stool culture or multiplex PCR panel for bacterial pathogens (Salmonella, Shigella, Campylobacter) 1
- Shiga toxin testing (STEC detection) - critical because if positive, requires close monitoring for hemolytic uremic syndrome (HUS) 2, 1
- C. difficile testing if the patient has recent antibiotic exposure, healthcare exposure, or is hospitalized 1
Tests to AVOID in Acute Diarrhea:
- Fecal leukocyte examination and stool lactoferrin should NOT be used to establish the cause of acute infectious diarrhea 2, 1
- Stool ova and parasite testing is NOT indicated for acute diarrhea unless there is travel history to endemic areas or immigration from high-risk regions 2, 3
- Serologic tests should NOT be performed to establish etiology 2
Special Monitoring if STEC Confirmed
If Shiga toxin-producing E. coli (STEC) is identified:
- Frequent monitoring of hemoglobin, platelet counts, electrolytes, BUN, and creatinine is essential to detect early HUS manifestations 2
- Peripheral blood smear examination for red blood cell fragments is necessary when HUS is suspected 2
When Symptoms Persist Beyond 7-14 Days
If diarrhea continues beyond the typical viral gastroenteritis duration (>7-14 days), consider transitioning to a chronic diarrhea workup:
- Giardia antigen testing or PCR (>95% sensitivity and specificity) 2, 4
- IgA tissue transglutaminase (tTG) with total IgA to screen for celiac disease 4, 5
- Fecal calprotectin or fecal lactoferrin to screen for inflammatory bowel disease 4, 5
- Consider thyroid function tests and other non-infectious causes if symptoms persist beyond 2 weeks 1
Common Pitfalls to Avoid
- Do NOT order stool cultures or ova and parasite testing for nosocomial diarrhea (developing >72 hours after hospitalization) - these have extremely low yield and are not cost-effective 6, 7
- Do NOT use ESR or CRP to screen for IBD in acute presentations - these have poor diagnostic accuracy 2
- Avoid over-testing initially - the clinical history often provides more diagnostic value than extensive stool panels 4
- Prioritize rehydration as the first therapeutic intervention before waiting for test results - oral rehydration solution at 50-100 mL/kg over 2-4 hours for mild-to-moderate dehydration, or IV fluids for severe dehydration 1