Diagnostic Approach for Infectious Colitis

Initial Evaluation

The American Gastroenterological Association recommends comprehensive stool testing, including C. difficile assay, bacterial culture, and ova/parasites testing, along with assessment of stool inflammatory markers for all patients with suspected infectious colitis 1, 2
Microbiological testing should include C. difficile toxin assay and stool cultures for common bacterial pathogens in all patients presenting with suspected infectious colitis 1, 2, 3
Multiplex PCR stool pathogen panels are a reasonable alternative to traditional stool cultures, offering faster results 3
Ova and parasites testing should be performed based on travel history, shellfish consumption, or relevant epidemiologic risk factors 2, 3

A complete blood count (CBC) should be performed to assess for leukocytosis, anemia, and left shift 1, 2, 7
A comprehensive metabolic panel (CMP) including electrolytes, renal function, and liver function tests should be performed 1, 2, 6
Inflammatory markers, including C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR), should be measured, though they lack specificity to differentiate infectious from other causes of colitis 1, 2

Vital signs, including temperature, heart rate, and blood pressure, should be assessed to identify signs of septic shock or hemodynamic instability 2, 7
Abdominal examination should include assessment of tenderness, distension, and bowel sounds 2, 7
Rectal examination should be performed to assess for blood on digital exam and perianal inspection for fissures or abscesses 1, 2

A CT scan of the abdomen/pelvis should be performed for patients with severe colitis symptoms, including fever, significant abdominal pain, bleeding, or concern for complications 5, 6
Abdominal radiography is essential in severe cases to exclude colonic dilatation, toxic megacolon, or perforation 2

Flexible sigmoidoscopy or colonoscopy with biopsies should be performed in patients with positive stool inflammatory markers, severe symptoms, or when diagnosis remains uncertain 1, 3, 9
Sigmoidoscopy is preferred over colonoscopy in moderate-to-severe disease due to lower perforation risk 1, 2, 9

Severe infectious colitis is characterized by stool frequency >6 bowel movements/day above baseline, fever, rigors, hemodynamic instability, signs of peritonitis or ileus, WBC ≥15,000 cells/mL or marked left shift, creatinine elevation >50% above baseline, and elevated serum lactate 7, 10

Screening for hepatitis B, hepatitis C, HIV, and tuberculosis should be considered if immunosuppressive therapy may be needed 3, 5
Thyroid function tests should be performed in patients with persistent diarrhea not responding to treatment 5, 6
Fecal elastase and qualitative fecal fat should be measured if steatorrhea is present or pancreatic insufficiency is suspected 3
Tissue transglutaminase IgA and total IgA should be measured to exclude celiac disease in refractory cases 3