Caitlin Dugdale, MD, a physician-investigator in the Division of Infectious Diseases at Massachusetts General Hospital and an Assistant Professor of Medicine at Harvard Medical School, is the lead author of a new study in Open Forum Infectious Diseases, Second Time’s the Charm? Assessing the Sensitivity and Yield of Inpatient Diagnostic Algorithms for Pulmonary Tuberculosis in a Low-Prevalence Setting.

What Question Were You Investigating in this Study?

CDC guidelines currently recommend collecting three respiratory specimens 8-24 hours apart for acid fast bacilli (AFB) smear and culture in addition to one nucleic acid amplification test (NAAT) for evaluation of persons with suspected pulmonary tuberculosis, but data supporting this practice are limited.

We sought to understand the sensitivity and yield of one, two, or three AFB smears with or without NAAT to diagnosis pulmonary tuberculosis.

What Methods or Approach Did You Use?

A retrospective cohort study.

What Were the Results?

We found that tuberculosis diagnostic testing with two AFB smears offered the same yield as three AFB smears while potentially reducing laboratory burden and patient-time spent in airborne isolation precautions. Use of one or two NAATs increased sensitivity to detect pulmonary tuberculosis when added to AFB smear-based diagnostic testing alone.

What are the Clinical Implications?

Tuberculosis incidence is on the rise in the US. It is important to promptly recognize individuals with possible tuberculosis entering healthcare settings and to initiate airborne isolation precautions to prevent spread of TB to other patients and healthcare personnel.

When the TB workup is complete and negative, it is also important to promptly remove airborne isolation precautions, as airborne infection isolation rooms in hospitals are a limited resource.

However, the CDC-recommended approach to TB diagnosis with 3 acid fast bacilli (AFB) smears on respiratory specimens collected 8-24 hours apart will miss a significant proportion of individuals with TB, and premature discontinuation of airborne isolation when individuals have falsely negative tests could increase risks of hospital-based transmission of TB.

We found that among individuals with culture-confirmed pulmonary tuberculosis, 50% of them did not have a positive AFB smear. Undergoing the CDC-recommended three AFB smears did not lead to any additional diagnoses of tuberculosis compared to two AFB smears.

However, adding one or more nucleic acid amplification tests (NAATs) substantially increased the sensitivity to detect tuberculosis when compared with AFB smears alone.

Our findings support reduced use of third AFB smears in favor of increased prioritization of NAAT in the evaluation of suspected pulmonary tuberculosis in low prevalence settings.

Such a strategy should be considered in future updates to national guidelines on evaluation of persons with suspected pulmonary tuberculosis in low prevalence settings.

What are the Next Steps?

We plan to use the results of this analysis to inform the development of a clinical decision support system to aid clinicians in the evaluation of hospitalized individuals with suspected TB.