Association between laboratory-based frailty index and unfavorable treatment outcomes in older adults with drug-susceptible pulmonary tuberculosis in the Republic of Korea: a retrospective cohort study

Introduction Older adults with drug-susceptible pulmonary tuberculosis (DS-PTB) face disproportionately poor treatment outcomes, but practical tools for risk stratification around the time of treatment initiation remain limited. Methods This retrospective cohort study evaluated the association between the laboratory-based frailty index (FI-Lab) and unfavorable treatment outcomes in adults aged ≥65 years with DS-PTB, who were treated between January 1, 2015, and December 31, 2024, at a tertiary academic medical center in the Republic of Korea. The FI-Lab score was derived from routine blood tests collected within 60 days before to 7 days after treatment initiation, and the primary outcome was a composite of death during treatment, treatment failure, or treatment discontinuation. Results Among 627 patients, 119 (19%) experienced an unfavorable outcome. Those in the highest FI-Lab quartile had greater odds of an unfavorable outcome than those in the lowest (odds ratio 3.42, 95% CI 1.75–6.69; p < 0.001; p -for trend = 0.0002), after adjustment for age, sex, comorbidity burden, treatment delay, baseline medications, cavitary disease, and bacteriological status. Results were consistent across seven pre-specified sensitivity analyses and in the Cox proportional hazards model (hazard ratio 3.28, 95% CI 1.60–6.73; p = 0.001). Discussion Higher FI-Lab scores were independently associated with unfavorable treatment outcomes in older adults with DS-PTB. Derived from routinely available blood tests, FI-Lab may offer a practical approach to frailty quantification around the time of treatment initiation, and prospective studies are needed to evaluate whether frailty-targeted interventions improve outcomes.