Diagnostic Approach to Bloody Diarrhea in Adults

Infectious Causes

The most common infectious causes of bloody diarrhea in immunocompetent adults are bacterial pathogens, including Campylobacter, Salmonella, Shigella, and STEC (Shiga toxin-producing E. coli), according to the Infectious Diseases Society of America 1
Bacterial pathogens remain the leading infectious causes of bloody diarrhea in immunocompetent adults, with the most common being Campylobacter, Salmonella, Shigella, STEC, Yersinia enterocolitica, and Non-cholera Vibrio species 1, 2, 3, 4
Parasitic causes, such as Entamoeba histolytica, become more relevant with prolonged symptoms 1

Stool studies should include bacterial culture or culture-independent diagnostic testing for Salmonella, Shigella, Campylobacter, and STEC O157, as recommended by the Infectious Diseases Society of America 1, 4
C. difficile testing should be performed if there is any antibiotic exposure in the preceding 8-12 weeks, according to the Infectious Diseases Society of America 1
Fecal lactoferrin or leukocyte microscopy should be used to document inflammation 4
At 10 days duration, parasitic evaluation, including stool examination for parasites, specifically Entamoeba histolytica, should be considered 1

Empiric antibiotics are generally not recommended for most cases of bloody diarrhea in immunocompetent adults, as the risks often outweigh benefits, according to the Infectious Diseases Society of America 5
Exceptions warranting empiric therapy include severe illness with signs of sepsis, immunocompromised patients, and clinical suspicion of enteric fever 5, 1
Rehydration is the cornerstone of management, and oral rehydration therapy should be used for mild-moderate dehydration, while intravenous fluids should be used for severe dehydration or inability to tolerate oral intake 3

Fever pattern is diagnostically important, and high fever suggests bacterial etiology or E. histolytica, while absence of fever with severe abdominal pain and bloody stools suggests STEC 1, 4
Abdominal pain characteristics help narrow the differential, and severe abdominal pain with grossly bloody stools and minimal fever suggests STEC, Salmonella, Shigella, Campylobacter, or Yersinia 1, 4
Recent antibiotic exposure mandates testing for C. difficile, and travel history within 3 days prior to symptom onset should prompt consideration of endemic pathogens 1
Immunocompromise status dramatically expands the differential and warrants broader testing 1