Does a patient with an 8-day history of diarrhea require laboratory tests?

Laboratory Testing for Patients with 8-Day Diarrhea

Laboratory testing is indicated for a patient with diarrhea persisting for 8 days, as this duration approaches persistent diarrhea status and warrants diagnostic evaluation to determine the etiology and guide appropriate treatment. 1, 2

Initial Laboratory Evaluation

• Stool testing should be performed for bacterial pathogens including Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and Shiga toxin-producing E. coli (STEC) 1, 3
• A single diarrheal stool specimen is the optimal sample for laboratory diagnosis of infectious diarrhea 3
• Complete blood count (CBC), basic metabolic panel (BMP) to assess for inflammation, dehydration, and electrolyte abnormalities 2
• Testing for Giardia is strongly recommended as it is a common cause of persistent diarrhea with excellent diagnostic tests available 1

Testing Based on Stool Characteristics

For Bloody or Mucoid Stools

• Test specifically for Salmonella, Shigella, Campylobacter, Yersinia, and STEC 3
• For STEC, use methods that detect Shiga toxin (or genes that encode them) and distinguish E. coli O157:H7 from other STEC serotypes 3
• Monitor hemoglobin and platelet counts if STEC is suspected to detect early signs of hemolytic uremic syndrome 1

For Watery Diarrhea

• Consider testing for C. difficile toxin, especially if there is history of antibiotic use within the preceding 8-12 weeks 2
• Testing for celiac disease with IgA tissue transglutaminase and a second test to detect celiac disease in the setting of IgA deficiency should be considered if diarrhea persists 1

Special Considerations

• If the patient has recently traveled internationally or immigrated from high-risk areas, stool examination for ova and parasites beyond Giardia is warranted 1
• For immunocompromised patients, perform a broader differential diagnosis with evaluation for Cryptosporidium, Cyclospora, Cystoisospora, and other opportunistic pathogens 3, 2
• Fecal leukocyte examination and stool lactoferrin detection should not be used to establish the cause of acute infectious diarrhea 1

When to Consider Additional Testing

• If symptoms persist beyond 14 days with negative initial testing, consider non-infectious causes including inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS) 1
• A clinical and laboratory reevaluation may be indicated in people who do not respond to an initial course of therapy 1
• Consider testing for bile acid diarrhea in patients with persistent watery diarrhea 1

Important Considerations

• All specimens that test positive by culture-independent diagnostic testing should be cultured if isolate submission is requested or required under public health reporting rules 3
• Serologic tests are not recommended to establish an etiology of infectious diarrhea 1
• Follow-up testing is not recommended in most people for case management following resolution of diarrhea 1

Clinical Approach Algorithm

1. Collect a single diarrheal stool specimen for bacterial culture and C. difficile testing 3
2. Order CBC, BMP, and test for Giardia 2
3. If bloody diarrhea is present, add specific testing for STEC 3
4. If watery diarrhea persists, consider testing for celiac disease and bile acid diarrhea 1
5. For patients with recent travel history, add ova and parasite examination 1
6. If all initial tests are negative and symptoms persist, consider non-infectious causes 1

This approach ensures appropriate diagnostic evaluation while avoiding unnecessary testing in patients with diarrhea approaching persistent status (≥14 days), where the likelihood of identifying a specific pathogen increases and the potential for complications is higher 4, 5.