Division of Infectious Diseases

Do you really need to test the poo? Diagnostic stewardship for (outpatient) diarrheal illness

Rapid molecular testing has changed the landscape of diagnostic approaches to many infectious disease syndromes, including diarrheal illnesses. These panels typically have the capacity to diagnose multiple organisms in one test. The FilmArray gastrointestinal pathogen panel (BioFire) tests 22 stool pathogens. Despite the impact of improved clinical efficiency, these tests are often expensive, especially in the outpatient setting, but the convenience of a comprehensive testing panel can lead to unnecessary testing.

Recent IDSA guidelines on management of diarrheal illness only recommends rapid diagnostic testing in patients who are immunocompromised or those with fever, severe diarrhea, abdominal pain, bloody stools. Clark et al. sought to validate these recommendations in the outpatient setting by evaluating the yield of the molecular testing, and clinical outcomes.  In this retrospective study, 629 patients were included in the analysis, and over two-thirds of patients had a duration of diarrhea greater than 14 days.

Given the possibility of asynchronous testing without face-to-face encounter, physical exam findings were not included in assessment, only documentation of patient-reported abdominal pain. Only 20% of the specimens actually had pathogens detected, and were clinically relevant in only 5% of patients (19/107 in immunocompromised patients vs 14/522 immunocompetent patients, p<0.001).

The authors validated that the IDSA guideline-based criteria had a sensitivity of 97% (95% CI 84.2-99.9), specificity of 33.9% (95% CI: 30.1-37.8), negative predictive value of 99.5% (95% CI: 97.3-100.0), and a positive predictive value of 7.5% (95% CI: 5.2-10.4). They found that application of these testing criteria would have avoided testing in 32% of patients, and avoided unnecessary antibiotics) in 23% of patients.

The authors concluded that duration—based criteria for testing stool is not warranted, however given the higher prevalence of clinically relevant pathogens in immunocompromised patients (who may demonstrate fewer systemic symptoms), broadly testing in this patient population is reasonable. C. difficile was not included in this study, and guidelines recommend two-step testing for C. difficile infections (rather than single-step PCR, as would be the case for this panel).

The retrospective nature and absence of clear physical exam findings limit the findings. Nevertheless, this study emphasizes the opportunity for diagnostic stewardship to decrease inappropriate testing without significant clinical penalty, and identifies immunocompromised patients as a valuable subgroup where less restricted use of this test for diarrhea is reasonable.

Citation: Stephen D Clark, Michael Sidlak, Amy J Mathers, Melinda Poulter, James A Platts-Mills, Clinical yield of a molecular diagnostic panel for enteric pathogens in adult outpatients with diarrhea and validation of guidelines-based criteria for testing, Open Forum Infectious Diseases, ofz162, https://doi.org/10.1093/ofid/ofz162



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