Contribution of smear-negative, culture-positive pulmonary tuberculosis to nosocomial transmission

A study investigates the role of smear-negative (SmN), culture-positive tuberculosis (TB) patients who undergo aerosol-generating procedures (AGPs) in hospital-based TB transmission. The central question is whether these SmN patients contribute to nosocomial transmission at a rate comparable to smear-positive (SmP) TB cases. Conducted from 2016 to 2019 at a university-affiliated medical center in northern Taiwan, the study employed a prospective cohort design, enrolling hospitalized TB index cases and their in-hospital contacts. SmN and SmP cases were included only if they were not airborne-isolated prior to treatment and had non-HCW contacts.

The methodology was robust, featuring a two-year follow-up and the use of QuantiFERON-TB Gold In-Tube (QFT) testing, a reliable interferon-gamma release assay (IGRA) for detecting latent TB infection (LTBI). This prospective approach allowed for temporal analysis of transmission events and assessment of risk factors. Importantly, cases of multidrug-resistant TB and healthcare workers were excluded to avoid confounding from differing exposure and infection control practices.

Among 953 identified close contacts, 504 underwent QFT testing—393 exposed to SmP and 111 to SmN index cases who received AGPs. LTBI was found in 16.0% of SmP contacts and 15.3% of SmN contacts. SmN index cases accounted for 21.3% of total nosocomial TB transmission events. None of the contacts who received LTBI treatment developed active TB, whereas four untreated or untested contacts did, including one exposed to a SmN case.

The analysis identified several transmission risk factors, including age under 65, male sex, smoking, diabetes, chronic obstructive pulmonary disease (COPD), and undergoing AGPs—especially sputum suction. Conversely, being smear-negative was associated with a significantly lower transmission risk. Nonetheless, AGPs independently increased transmission risk, suggesting these procedures elevate exposure risks regardless of smear status.

The study concludes that SmN TB patients who receive AGPs have a similar risk of transmitting TB as SmP patients. This finding supports the expansion of infection control measures to include SmN cases undergoing AGPs. Hospitals should maintain a high level of suspicion for TB in high-risk patients, apply rapid diagnostic testing, and ensure that close contacts of all TB cases—SmN or SmP—receive appropriate LTBI screening and treatment.

Source: Yang, Y.J., Pan, S.C., Lee, M.R., Chung, C.L., Ku, C.P., Liao, C.Y., Tsai, T.Y., Wang, J.Y., Fang, C.T. and Chen, Y.C., 2024. Quantifying the contribution of smear-negative, culture-positive pulmonary tuberculosis to nosocomial transmission. American Journal of Infection Control, 52(7), pp.807-812.