What are the initial laboratory tests and treatment approaches for a patient presenting with diarrhea?

Initial Laboratory Evaluation and Treatment for Acute Diarrhea

For patients presenting with acute diarrhea, begin immediate oral rehydration therapy while selectively pursuing laboratory investigations only when clinical features suggest severe, inflammatory, or complicated disease—specifically targeting patients with fever, bloody stools, severe dehydration, immunocompromise, or symptoms lasting >3 days.

Clinical Assessment to Guide Laboratory Testing

History and Physical Examination Priorities

Obtain these specific clinical features to stratify need for testing 1:

• Stool characteristics: watery vs. bloody vs. mucoid/purulent 1
• Volume depletion signs: thirst, tachycardia, orthostatic vital signs, decreased skin turgor, dry mucous membranes, decreased urination 1
• Inflammatory features: fever (document temperature), abdominal pain, tenesmus 1
• Frequency and duration: onset timing, number of bowel movements, duration of illness 1
• Epidemiologic risk factors: recent travel to developing areas, antibiotic use, day-care exposure, consumption of raw/undercooked foods, known sick contacts, immunosuppression (AIDS, chemotherapy, extremes of age) 1

Selective Laboratory Testing Algorithm

When to Order Laboratory Tests

Do NOT routinely test patients with mild, watery diarrhea of short duration 1.

DO order tests for 1:

• Diarrhea lasting ≥3 days (some sources suggest ≥1 day with fever/blood) 1
• Fever documented in medical setting 1
• Bloody stools or dysentery 1
• Severe dehydration (dry mucous membranes, decreased urination, tachycardia, orthostasis, lethargy) 1
• Recent antibiotic use 1
• Hospitalization ≥3 days prior to symptom onset 1
• Immunocompromised state 1
• Day-care attendance/employment 1

Initial Laboratory Panel

When testing is indicated, order 1:

Blood tests:

• Complete blood count with differential (assess for leukocytosis suggesting infection, or neutropenia in cancer patients) 1
• Electrolytes (sodium, potassium, calcium, magnesium) to guide rehydration fluid composition 1
• Renal function (creatinine, BUN) to detect secondary renal impairment and assess dehydration severity 1
• Hemoglobin (assess blood loss or hemoconcentration) 1

For suspected STEC/E. coli O157 infection specifically (especially with bloody diarrhea), monitor 1:

• Hemoglobin and platelet counts (frequent monitoring to detect early HUS) 1
• Peripheral blood smear for red blood cell fragments when HUS suspected 1

Stool Studies—Selective Approach

Microbiological testing is indicated for patients meeting criteria above 1:

• Stool culture for Salmonella, Shigella, Campylobacter 1
• C. difficile testing (two-step approach with glutamate dehydrogenase EIA plus toxin detection) for patients with recent/current antibiotic use or healthcare-associated diarrhea 1
• STEC testing (including E. coli O157:H7 and Shiga toxin detection) for bloody diarrhea 1
• Ova and parasites (Giardia, Cryptosporidium, Entamoeba histolytica) for persistent diarrhea >7 days, travel history, or waterborne exposure 1

Do NOT routinely order 1:

• Fecal leukocyte examination (low predictive value) 1
• Fecal lactoferrin (not recommended for establishing etiology) 1
• Serologic tests (not useful for acute diagnosis) 1
• Peripheral WBC count solely to establish etiology (may be clinically useful but doesn't identify pathogen) 1

Special Populations

Cancer patients on chemotherapy 1:

• Lower threshold for testing in immunocompromised/neutropenic patients 1
• Blood cultures (minimum two sets, including from IV catheters) if febrile, especially with neutropenia 1
• Coagulation tests, CRP, procalcitonin to assess infection/inflammation 1

Traveler's diarrhea: Empiric treatment often preferred over testing (see treatment section) 1

Initial Treatment Approach

Rehydration—The Cornerstone

Oral rehydration therapy is superior to IV fluids for patients able to take oral fluids—it is safer, less costly, less painful, and equally effective 1.

Use WHO-formulated oral rehydration solutions (Ceralyte, Pedialyte, or generic equivalents with Na 90 mM, K 20 mM, Cl 80 mM, HCO₃ 30 mM, glucose 111 mM) 1. The patient's thirst naturally decreases with rehydration, protecting against overhydration 1.

IV fluids reserved only for patients unable to tolerate oral intake or with severe dehydration 1.

Antimotility Agents—Use With Caution

Loperamide can reduce bowel movements and fluid loss in watery diarrhea 2, but:

• Do NOT use in bloody diarrhea, high fever, or suspected invasive/inflammatory diarrhea (risk of toxic megacolon, worsening of invasive infections) 3
• Do NOT exceed recommended dosing (maximum 16 mg/day) due to cardiac risks including QT prolongation, Torsades de Pointes, and death 3
• Avoid in elderly taking QT-prolonging drugs (Class IA/III antiarrhythmics) 3
• Caution with CYP3A4/CYP2C8/P-glycoprotein inhibitors (itraconazole, gemfibrozil, quinidine, ritonavir) which increase loperamide levels 2-13 fold 3

Empiric Antibiotic Therapy—Highly Selective

Do NOT give empiric antibiotics to immunocompetent patients with bloody diarrhea while awaiting test results 1, EXCEPT for:

1. Infants <3 months with suspected bacterial etiology 1
2. Bacillary dysentery (fever, bloody stools, abdominal pain, tenesmus) presumed Shigella 1
3. International travelers with temperature ≥38.5°C or signs of sepsis 1

Empiric regimens when indicated 1:

• Adults: Fluoroquinolone (ciprofloxacin) OR azithromycin (based on local resistance patterns and travel history) 1
• Children: Third-generation cephalosporin (infants <3 months or neurologic involvement) OR azithromycin 1
• Immunocompromised with severe bloody diarrhea: Consider empiric treatment 1

Traveler's diarrhea exception: Empiric fluoroquinolone or TMP-SMZ (children) without stool testing can reduce illness from 3-5 days to 1-2 days 1.

Critical Pitfalls to Avoid

• Never give antibiotics for suspected STEC/E. coli O157 infection—quinolones may induce Shiga toxin production and precipitate HUS 1
• Avoid antimotility agents in inflammatory/bloody diarrhea—risk of toxic megacolon and prolonged pathogen shedding 3
• Don't over-test mild cases—stool culture yield is <5% in unselected patients, even lower with drug-induced diarrhea 1
• Monitor closely for HUS development in confirmed STEC cases with serial CBC, renal function, and blood smears 1

Reassessment Triggers

Re-evaluate if 1:

• No improvement after 48 hours of initial therapy 1
• Development of fever, blood in stool, or abdominal distention 3
• Symptoms persist ≥14 days (consider non-infectious causes: IBD, IBS, lactose intolerance, celiac disease) 1