What lab work is recommended for a patient presenting with gastroenteritis?

Laboratory Work for Gastroenteritis

For uncomplicated acute gastroenteritis with mild symptoms, laboratory testing is generally not necessary and should be reserved for patients with specific clinical indications such as severe symptoms, bloody diarrhea, fever, signs of dehydration, or immunocompromised status. 1

When Laboratory Testing is NOT Needed

• Patients with mild symptoms that resolve within a week do not require microbial studies or routine blood work 2
• Viral gastroenteritis, the most common cause, can be managed clinically without laboratory confirmation in otherwise healthy patients 2, 3
• History and physical examination are sufficient for assessing mild cases, particularly when there is no decrease in oral intake or urine output and no vomiting 3

Indications for Laboratory Testing

Stool Studies Should Be Performed When:

• Fever, bloody or mucoid stools, severe abdominal cramping, or signs of sepsis are present 1
• Symptoms persist beyond 7-10 days 2
• Recent antibiotic exposure (test for Clostridioides difficile) 1, 2
• Immunocompromised patients 1
• Suspected outbreak situations 1

The recommended stool tests include:

• Multiplex antimicrobial testing (preferred over traditional stool cultures) 2
• Stool culture for Salmonella, Shigella, Campylobacter, Yersinia 1
• C. difficile toxin testing 1
• Shiga toxin testing or genomic assays for STEC (Shiga toxin-producing E. coli) 1
• Vibrio species if exposure to brackish water or raw shellfish 1

Blood Tests Should Be Performed When:

Complete blood count, electrolytes, and inflammatory markers are indicated for:

• Infants less than 3 months of age 1
• Signs of septicemia or systemic toxicity 1
• Moderate to severe dehydration 1
• Suspected inflammatory bowel disease (IBD) flare 1

Specific blood tests to consider:

• Full blood count (hemoglobin, leukocyte count, platelet count) 1
• Serum electrolytes (sodium, potassium, chloride, bicarbonate) 1
• Renal function (creatinine, BUN) 1
• C-reactive protein (CRP) 1
• Liver enzymes 1
• Blood cultures if fever with signs of sepsis or in high-risk patients 1

Special Populations and Circumstances

Suspected STEC Infection:

• Daily monitoring of complete blood count and platelet count is essential during days 1-14 to detect hemolytic uremic syndrome (HUS) 1
• A decreasing platelet count trend indicates higher risk of HUS development 1
• Increasing creatinine and blood pressure with volume overload require close monitoring 1

Inflammatory Bowel Disease Patients:

• Mandatory tests include: full blood count, electrolytes, liver enzymes, ESR, CRP, serum albumin, and fecal calprotectin when possible 1
• Must exclude infectious causes, especially C. difficile and Cytomegalovirus 1

Immunocompromised Patients:

• A broader differential diagnosis requires expanded testing for bacterial, viral, and parasitic agents 1
• Consider opportunistic infections including MAC and microsporidiosis 1

Common Pitfalls to Avoid

• Do not delay treatment while awaiting stool culture results - begin empiric therapy when clinically indicated 1
• Blood tests like CRP and white blood cell count have limited specificity for differentiating causes of acute abdominal pain and should not be used in isolation 1
• A normal CRP does not exclude inflammatory bowel disease - approximately 20% of patients with active Crohn's disease may have normal CRP levels 4
• Avoid over-testing in mild cases - the clinical history often provides more diagnostic value than extensive laboratory workup 5
• Stool cultures and microscopic examinations are no longer first-line tests - multiplex PCR panels are preferred 2