What is the diagnostic workup for white pus in stool?

Diagnostic Workup for White Pus in Stool

White pus in stool requires immediate stool testing for bacterial pathogens (Salmonella, Shigella, Campylobacter, STEC), C. difficile toxin, and parasites, combined with ileocolonoscopy with segmental biopsies to differentiate infectious colitis from inflammatory bowel disease. 1

Initial Stool Testing

The presence of visible pus (white blood cells and inflammatory exudate) indicates inflammatory diarrhea and mandates comprehensive stool evaluation:

Bacterial Pathogens

• Routine stool culture for Salmonella, Shigella, Campylobacter, and Shiga toxin-producing E. coli (STEC) 1, 2
• C. difficile testing if there is any history of antibiotic use within the preceding 8-12 weeks 1
• Consider Yersinia enterocolitica testing if there is persistent abdominal pain, especially right lower quadrant pain mimicking appendicitis 1
• Test for Vibrio species if there is exposure to brackish water or raw shellfish 1

Parasitic Evaluation

• Stool microscopy with ova and parasite (O&P) examination including permanent stained smears 3, 4
• Giardia lamblia and Entamoeba histolytica testing via EIA or NAAT, as these are common causes of inflammatory diarrhea with pus cells 3, 4
• Cryptosporidium testing via direct fluorescent immunoassay, EIA, or NAAT 1, 3
• Fresh stool specimens are critical, as delays degrade trophozoites and reduce diagnostic yield 3

Multiplex Molecular Testing

• Multiplex NAAT (gastrointestinal panels) can detect multiple bacterial, viral, and parasitic pathogens simultaneously 1, 3
• Remember these assays detect DNA, not necessarily viable organisms, so clinical correlation is essential 1
• Any positive culture-independent test should be followed by culture to enable antimicrobial susceptibility testing and outbreak detection 1, 3

Endoscopic Evaluation with Biopsies

Ileocolonoscopy with segmental biopsies is required to establish a definitive diagnosis when pus is present in stool, as no single reference standard exists for diagnosing inflammatory conditions. 1

Biopsy Protocol

• Multiple biopsies from both inflamed and uninflamed segments are essential, as lesions may be focal 1
• Segmental biopsies stored in separate containers to map the distribution of inflammation—this increases diagnostic accuracy from 66% to 92% 1, 5
• Rectal biopsies are mandatory to confirm or exclude rectal involvement 1
• Terminal ileum biopsies have the highest diagnostic value for Crohn's disease and can detect backwash ileitis in ulcerative colitis 1
• Obtain 2-3 tissue levels with multiple sections from each biopsy site to detect focal lesions 1

Endoscopic Features to Document

• Continuous vs. discontinuous involvement: Continuous confluent colonic inflammation with rectal involvement suggests ulcerative colitis, while discontinuous lesions suggest Crohn's disease 1
• Presence of ulcers, strictures, fistulae, or perianal involvement points toward Crohn's disease 1
• Pseudomembranes are diagnostic of C. difficile colitis, though endoscopy is only 51-55% sensitive for this diagnosis 6

Differential Diagnosis Considerations

Inflammatory Bowel Disease

• No single endoscopic or histologic feature is specific for Crohn's disease or ulcerative colitis 1
• Segmental biopsies are essential to differentiate IBD from infectious colitis—double-biopsy alone provides the correct diagnosis in only 66% of cases, while segmental biopsies change the diagnosis in 26% of cases 5
• Crypt architectural distortion and basal plasmacytosis on histology favor IBD over infectious colitis 1, 7

Infectious Colitis

• Gastrointestinal infections must always be excluded before diagnosing IBD 1
• Loose stools for more than 6 weeks usually discriminate IBD from most infectious diarrhea 1
• CMV colitis should be tested in immunosuppressant-resistant disease via PCR or immunohistochemistry of tissue biopsies 1

Microscopic Colitis

• This is a secretory diarrhea affecting older persons that requires histologic diagnosis, as endoscopy appears normal 8, 7
• Lymphocytic colitis shows increased surface intraepithelial lymphocytes 7
• Collagenous colitis shows thickened subepithelial collagen band 7

Additional Laboratory Testing

• Complete blood count with differential: Elevated white blood cell count and neutrophil predominance suggest bacterial infection; eosinophilia suggests parasitic infection with tissue phase 1
• Blood cultures if there are signs of septicemia, systemic manifestations, or suspected enteric fever 1, 2
• Fecal calprotectin correlates well with endoscopic disease activity in IBD (r > 0.8) and helps distinguish inflammatory from non-inflammatory diarrhea 1

Common Pitfalls to Avoid

• Do not test asymptomatic patients or perform follow-up testing after symptom resolution, except when required by public health authorities 1, 6
• Avoid sigmoidoscopy alone except in acute severe colitis, as it misses cases with atypical distribution (e.g., peri-appendiceal inflammation with left-sided colitis) 1
• Do not rely on serologic tests (pANCA, ASCA) to differentiate Crohn's disease from ulcerative colitis, as their accuracy is limited 1
• Fecal leukocyte examination and stool lactoferrin should not be used to establish the cause of acute infectious diarrhea 1

Immunocompromised Patients

If the patient is immunocompromised, expand testing to include:

• Cryptosporidium, Cyclospora, Cystoisospora, Microsporidia 1, 3
• Mycobacterium avium complex and CMV via endoscopic biopsy 1
• Strongyloides serology in addition to stool testing 1, 3