What diagnostic tests should be ordered for a patient with acute bloody diarrhea two days after returning from travel to Mexico?

Diagnostic Testing for Acute Bloody Diarrhea After Travel to Mexico

Immediate Stool Testing Required

Order stool culture for Salmonella, Shigella, Campylobacter, Yersinia, and testing for Shiga toxin-producing E. coli (STEC) as the first-line diagnostic approach for bloody diarrhea following travel to Mexico. 1, 2

The presence of visible blood in stool after recent travel to a developing country mandates specific bacterial pathogen testing, as this clinical presentation strongly suggests invasive bacterial enterocolitis. 1, 3

Core Stool Studies

• Bacterial stool culture specifically requesting Salmonella, Shigella, Campylobacter, and Yersinia (note: Yersinia must be explicitly requested as it is not included in routine panels) 1, 2
• STEC testing using methods that detect Shiga toxin or genes encoding them, with differentiation of E. coli O157:H7 from other STEC serotypes 1, 2
• Stool examination for ova and parasites to detect Entamoeba histolytica, which can cause bloody diarrhea and is endemic in Mexico 1, 4
• C. difficile testing if the patient received antibiotics within the preceding 8-12 weeks 1, 4

Single Specimen Is Sufficient

A single diarrheal stool specimen is the optimal sample for laboratory diagnosis—multiple specimens are not necessary for acute bacterial diarrhea. 2 If a timely diarrheal stool cannot be collected, a rectal swab may be used for bacterial detection. 2

Supportive Laboratory Tests

Assess Severity and Complications

• Complete blood count (CBC) to evaluate for leukocytosis, anemia from blood loss, and thrombocytopenia (critical if STEC is suspected, as low platelets may herald hemolytic uremic syndrome) 2, 4
• Basic metabolic panel (BMP) to assess electrolyte abnormalities and renal function, particularly important with bloody diarrhea where dehydration risk is elevated 4
• Fecal lactoferrin or calprotectin to confirm inflammatory diarrhea, though the presence of visible blood already establishes this clinically 4

Critical Pathogen-Specific Considerations

STEC Requires Special Attention

When STEC is suspected (bloody diarrhea with minimal or no fever), laboratories should use sorbitol-MacConkey agar or chromogenic agar to screen specifically for O157:H7. 1, 2 Monitor hemoglobin and platelet counts closely if STEC is identified, as hemolytic uremic syndrome can develop. 4

Mexico-Specific Epidemiology

The most common bacterial pathogens causing bloody diarrhea in travelers to Mexico are enterotoxigenic E. coli (ETEC), Shigella, Campylobacter, and Salmonella. 1 Entamoeba histolytica should also be considered given the geographic exposure. 1

Molecular vs. Culture-Based Testing

Multiplex Molecular Panels

Multiplex gastrointestinal panels (such as FilmArray GI panel) can detect 15-22 enteropathogens simultaneously with high sensitivity and provide results within 1 hour. 1 However, these assays detect DNA rather than viable organisms, so clinical correlation is essential—positive results may represent colonization rather than active infection. 1, 4

When Culture Is Still Necessary

All specimens testing positive by molecular methods should be reflexed to culture if antimicrobial susceptibility testing would affect care or if public health reporting requires isolate submission. 1, 2 This is particularly important given increasing fluoroquinolone resistance in Campylobacter from Latin America. 1, 5

What NOT to Order for Acute Presentation

• Serology has no role in diagnosing acute bacterial diarrhea 2
• Multiple stool specimens are unnecessary for acute bacterial pathogens (though three specimens may be needed if parasites remain suspected after initial negative testing) 2
• Routine viral testing is not indicated for bloody diarrhea, as viruses (norovirus, rotavirus) cause non-bloody, watery diarrhea 1

When to Expand Testing

If symptoms persist beyond 14 days despite negative initial bacterial studies, expand evaluation to include Cryptosporidium, Cyclospora, Cystoisospora, and Giardia, as protozoal pathogens become more likely with persistent symptoms. 1, 4 Consider postinfectious irritable bowel syndrome or unmasking of inflammatory bowel disease if all infectious workup remains negative. 1, 6

Common Pitfalls to Avoid

• Do not assume simple traveler's diarrhea when blood is present—visible blood mandates full bacterial pathogen workup 1, 5
• Do not forget to specifically request Yersinia testing, as it is omitted from standard stool culture panels despite being a cause of bloody diarrhea 2
• Do not delay STEC testing—antibiotics may increase risk of hemolytic uremic syndrome in STEC infections, so identification before empiric treatment is critical 1
• Do not rely solely on molecular panels without culture backup when antimicrobial resistance patterns would influence treatment decisions 1