What investigations should be done for a patient presenting with acute gastroenteritis?

Investigations for Acute Gastroenteritis

For most patients with uncomplicated acute gastroenteritis, no laboratory or microbiological investigations are needed—focus on clinical assessment of hydration status and initiate oral rehydration therapy immediately. 1, 2

Clinical Assessment Takes Priority Over Testing

The physical examination is the most reliable method to evaluate hydration status and guide management. 2 Assess dehydration severity using specific clinical signs:

• Mild dehydration (3-5% fluid deficit): Slightly dry mucous membranes, normal skin turgor, alert mental status 1
• Moderate dehydration (6-9% fluid deficit): Dry mucous membranes, loss of skin turgor with tenting, decreased urine output 1
• Severe dehydration (≥10% fluid deficit): Severe lethargy or altered consciousness, prolonged skin tenting (>2 seconds), cool extremities with poor perfusion, rapid deep breathing 1

The most accurate assessment is acute weight change if premorbid weight is known; prolonged skin retraction time, abnormal capillary refill, and rapid deep breathing are the most reliable clinical predictors. 1

When to Order Investigations

Stool microbiological testing is reserved for specific clinical scenarios only: 3, 2, 4

Indications for Stool Studies:

• Bloody diarrhea with fever and systemic toxicity (suggests bacterial dysentery from Salmonella, Shigella, or enterohemorrhagic E. coli) 1
• Symptoms persisting beyond 7 days 3, 4
• Recent antibiotic exposure (test for Clostridioides difficile) 5, 4
• Recent foreign travel 6
• Immunocompromised patients (HIV, transplant recipients, immunosuppressive therapy, malignancy) 1, 3
• Infants <3-6 months with suspected bacterial gastroenteritis 3
• Institutionalized patients or outbreak settings 3
• Signs of severe systemic illness or sepsis 3

Recommended Stool Tests When Indicated:

• Multiplex antimicrobial testing is preferred over traditional stool cultures 4
• Clostridioides difficile testing using two-step approach with enzyme immunoassays or nucleic acid amplification tests 5
• Stool culture for Shigella, Salmonella, Campylobacter, and Shiga toxin-producing E. coli 5
• Ova and parasite examination if parasitic infection suspected based on travel history or prolonged symptoms 3

Laboratory Blood Tests

Blood tests are indicated only in patients with signs of severe disease or specific risk factors: 5, 3

Essential Laboratory Tests for Severe Cases:

• Complete blood count with differential to assess for leukocytosis (suggests bacterial infection or bowel ischemia), anemia, and eosinophilia 5
• Comprehensive metabolic panel including electrolytes, renal function, and liver function tests 5
• C-reactive protein as a marker of inflammation and infection severity 5
• Serum lactate in patients with suspected mesenteric ischemia or bowel necrosis (marker of tissue hypoperfusion) 5
• Procalcitonin correlates with intestinal necrotic damage and mortality in acute abdomen 5
• Blood cultures (minimum two sets) in febrile, toxic-appearing, or neutropenic patients 5

Imaging Studies

Imaging is not routinely indicated for uncomplicated acute gastroenteritis. 1

Indications for Imaging:

• Contrast-enhanced CT scan is the most reliable exam for immunocompromised patients with acute abdomen or when surgical complications are suspected 6
• Plain abdominal radiography during acute episodes to exclude bowel obstruction if severe abdominal pain is present 6
• CT angiography for hemodynamically stable patients with ongoing gastrointestinal bleeding after resuscitation 6

Common Pitfalls to Avoid

Do not delay rehydration therapy while awaiting diagnostic testing—rehydration should be initiated immediately based on clinical assessment. 1 The vast majority of acute gastroenteritis is viral and self-limited, requiring only supportive care with oral rehydration solution. 2, 4

Do not order routine stool studies for mild viral gastroenteritis—this leads to unnecessary costs, delays in treatment, and potential detection of microorganisms of uncertain significance that do not require treatment. 3, 4

Do not underestimate dehydration in elderly patients or immunocompromised individuals—they may not manifest classic signs and have higher mortality risk, warranting lower thresholds for investigation and admission. 1

Laboratory tests may not accurately reflect disease severity in immunocompromised patients—fever, leukocytosis, and peritonitis may be mild or absent even with serious intra-abdominal pathology. 6