What investigations should be sent for a adult patient with unknown past medical history presenting with persistent vomiting and diarrhea for 5 days?

Investigations for Persistent Vomiting and Diarrhea (5 Days)

For an adult with 5 days of vomiting and diarrhea, immediately assess hydration status clinically, then send basic blood work (electrolytes, renal function, CBC) and stool studies (culture/PCR for bacterial pathogens including STEC with Shiga toxin, C. difficile, and parasites), while prioritizing rehydration as the first therapeutic intervention. 1, 2

Immediate Clinical Assessment (Before Laboratory Testing)

Hydration Status Evaluation

• Check for signs of dehydration: dry mucous membranes, decreased skin turgor, tachycardia, orthostatic vital signs, decreased urination, or altered mental status 2
• Measure vital signs: temperature (≥38.5°C suggests bacterial etiology), blood pressure (lying and standing), heart rate, oxygen saturation 1, 2
• Document stool characteristics: presence of blood or mucus, frequency, and volume 2

Critical History Elements

• Recent antibiotic use within 8-12 weeks (raises concern for C. difficile) 2
• International travel history (increases likelihood of parasitic infections) 2
• Food exposures: raw/undercooked foods, unpasteurized dairy 2
• Immunocompromised status: HIV/AIDS, immunosuppressive therapy, chronic illness 2
• Occupational exposures: daycare, healthcare, or food service work 2

Laboratory Investigations

Blood Tests (Send Immediately)

• Electrolytes (sodium, potassium, chloride, bicarbonate): to assess for dehydration-related imbalances and guide rehydration 1, 2
• Renal function (BUN, creatinine): to detect early renal impairment from dehydration 1
• Complete blood count with differential: hemoglobin and platelet monitoring is critical if STEC suspected (to detect early hemolytic uremic syndrome) 1
• Peripheral blood smear: if HUS suspected based on bloody diarrhea (look for red blood cell fragments) 1

Note: While CBC and differential should not be used to establish diarrhea etiology, they are clinically useful for assessing complications 1

Stool Studies (Indicated in This Case)

Send stool testing because the patient has persistent symptoms beyond typical viral gastroenteritis duration (>3-5 days). 2, 3

Bacterial Pathogens

• Stool culture or multiplex PCR panel for: Salmonella, Shigella, Campylobacter, Yersinia 1, 2
• STEC testing with Shiga toxin detection (including E. coli O157:H7 and non-O157 STEC) 1, 2
• C. difficile testing given the duration of symptoms and if any recent antibiotic exposure 2

Parasitic Testing

• Ova and parasites examination including Giardia, Cryptosporidium, Cyclospora, Entamoeba histolytica 1, 2
• This is particularly important given the 5-day duration, as parasitic infections cause prolonged symptoms 2

Tests NOT Recommended

• Fecal leukocyte examination and stool lactoferrin: should NOT be used to establish infectious diarrhea etiology 1
• Serologic tests: NOT recommended for establishing etiology of infectious diarrhea 1
• Routine fecal calprotectin: insufficient data for acute infectious diarrhea (reserved for chronic diarrhea >4 weeks to exclude IBD) 1

Additional Investigations if Symptoms Persist Beyond 14 Days

If diarrhea continues beyond 2 weeks despite initial workup and treatment, consider non-infectious causes:

• Celiac serology (IgA tissue transglutaminase) 1
• Thyroid function tests (TSH) 1
• Fecal calprotectin to screen for inflammatory bowel disease 1
• SeHCAT scan or serum 7α-hydroxy-4-cholesten-3-one for bile acid diarrhea 1
• Colonoscopy with biopsies (right and left colon, not rectal) to exclude microscopic colitis 1

Critical Pitfalls to Avoid

• Never delay rehydration while waiting for test results — dehydration causes the morbidity and mortality, not the diarrhea itself 2, 3
• Do NOT give empiric antibiotics for simple watery diarrhea — this promotes resistance and may worsen STEC outcomes (increases HUS risk) 1, 2, 3
• Do NOT use antimotility agents (loperamide) if bloody diarrhea, high fever, or suspected STEC — this can precipitate toxic megacolon or HUS 3
• If STEC is confirmed, monitor closely for HUS with frequent hemoglobin, platelet, electrolyte, and renal function checks 1

Management Priority

Rehydration is the first priority — initiate oral rehydration solution (ORS) at 50-100 mL/kg over 2-4 hours for mild-to-moderate dehydration, or IV fluids (Ringer's lactate or normal saline) for severe dehydration 2, 3