What is the initial workup and treatment for a patient with suspected infectious colitis?

Workup for Infectious Colitis

Initial Diagnostic Approach

All patients with suspected infectious colitis should undergo comprehensive stool testing including C. difficile assay, bacterial culture (or multiplex PCR panel), and ova/parasites testing based on risk factors, along with assessment of stool inflammatory markers. 1

Essential Stool Studies

• Microbiological testing should include C. difficile toxin assay and stool cultures for common bacterial pathogens (Salmonella, Shigella, Campylobacter, E. coli O157:H7) in all patients presenting with suspected infectious colitis 1
• Multiplex PCR stool pathogen panels are a reasonable alternative to traditional stool cultures where available, offering faster results 1
• Ova and parasites testing should be performed based on travel history (recent international travel, parasite-endemic regions), shellfish consumption, or relevant epidemiologic risk factors 1, 2
• CMV testing should be considered in patients with treatment-refractory colitis, severe disease, or those on immunosuppressive therapy 1

Stool Inflammatory Markers

• Fecal lactoferrin or calprotectin testing helps stratify patients who need urgent endoscopic evaluation, with lactoferrin showing 90% sensitivity for histologic inflammation 1
• These markers are particularly useful for distinguishing inflammatory from non-inflammatory diarrhea and guiding decisions about endoscopy 1

Laboratory Blood Tests

Initial Blood Work

• Complete blood count (CBC) to assess for leukocytosis (WBC >15,000 cells/mL suggests severe disease), anemia, and left shift 1
• Comprehensive metabolic panel (CMP) including electrolytes, renal function (creatinine elevation >50% above baseline indicates severity), and liver function tests 1
• Inflammatory markers including C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR), though these lack specificity to differentiate infectious from other causes of colitis 1
• Serum lactate in severe cases, as levels >5.0 mmol/L indicate critical illness requiring potential surgical intervention 1

Important caveat: Blood tests have low specificity for infectious colitis and cannot reliably distinguish it from inflammatory bowel disease or other causes 1

Physical Examination Findings

Key clinical features to assess include:

• Vital signs: Temperature >38.5°C, tachycardia, hypotension (signs of septic shock or hemodynamic instability) 1
• Abdominal examination: Tenderness, distension, decreased bowel sounds (suggesting ileus), rebound tenderness or guarding (peritonitis) 1
• Rectal examination: Blood on digital exam, perianal inspection for fissures or abscesses 1
• General assessment: Weight, hydration status, signs of malnutrition 1

Imaging Studies

When to Image

• CT scan of abdomen/pelvis should be performed for patients with severe colitis symptoms including fever, significant abdominal pain, bleeding, or concern for complications 1
• Abdominal radiography is essential in severe cases to exclude colonic dilatation, toxic megacolon, or perforation 1

CT Findings in Infectious Colitis

CT may demonstrate mesenteric vessel engorgement, bowel wall thickening (particularly low-attenuation mural thickening), fluid-filled colonic distention, pericolonic fat stranding, or ascites 1

Endoscopic Evaluation

Indications for Endoscopy

• Flexible sigmoidoscopy or colonoscopy with biopsies should be performed in patients with positive stool inflammatory markers, severe symptoms, or when diagnosis remains uncertain 1
• Sigmoidoscopy is preferred over colonoscopy in moderate-to-severe disease due to lower perforation risk 1
• Early endoscopy correlates with improved outcomes and helps predict response to treatment 1

What Endoscopy Reveals

Endoscopic findings help differentiate infectious colitis from inflammatory bowel disease and assess severity. Look for:

• Distribution pattern (infectious colitis may show segmental involvement; C. difficile often affects left colon) 1
• Pseudomembranes (pathognomonic for C. difficile) 1
• Ulceration depth and extent (deep ulcers predict steroid-refractory course) 1
• Mucosal inflammation characteristics 1

Biopsies from at least one site are essential for histopathologic confirmation and to exclude alternative diagnoses 1

Special Considerations

Risk Stratification

Severe infectious colitis is characterized by:

• Stool frequency >6 bowel movements/day above baseline 1
• Fever, rigors, hemodynamic instability 1
• Signs of peritonitis or ileus 1
• WBC ≥15,000 cells/mL or marked left shift (>20% bands) 1
• Creatinine elevation >50% above baseline 1
• Elevated serum lactate 1
• Pseudomembranous colitis on endoscopy 1

Additional Testing in Specific Contexts

• Screening for hepatitis B, hepatitis C, HIV, and tuberculosis should be considered if immunosuppressive therapy (infliximab, vedolizumab) may be needed 1
• Thyroid function tests in patients with persistent diarrhea not responding to treatment 1
• Fecal elastase and qualitative fecal fat if steatorrhea present or pancreatic insufficiency suspected 1
• Tissue transglutaminase IgA and total IgA to exclude celiac disease in refractory cases 1

Common Pitfalls to Avoid

• Do not rely on single negative stool tests - C. difficile testing may require repeat testing if clinical suspicion remains high, though this practice is being reconsidered with newer molecular tests 1
• Avoid colonoscopy in severe acute colitis due to perforation risk; flexible sigmoidoscopy is safer 1
• Do not use antiperistaltic agents or opiates in suspected infectious colitis, particularly C. difficile, as they increase complications 1, 3
• Consider medication review - recent antibiotics, proton pump inhibitors, and NSAIDs can predispose to or worsen infectious colitis 1