Abstract The relationship between age and isoniazid acetylation phenotype remains insufficiently characterized, particularly in elderly populations, where pharmacokinetic variability and susceptibility to toxicity may be increased. This study aimed to characterize isoniazid acetylation phenotypes in elderly Tunisian patients using a phenotypic index and to assess its clinical implications. A retrospective study was conducted on 476 Tunisian tuberculosis patients treated with isoniazid between 2019 and 2024. Plasma concentrations at 3 h post‐dose (C 3 ) were used to determine the acetylation index (I 3 ) and classify patients as rapid acetylators (RAs) or slow acetylators (SAs). Associations between acetylation phenotype, C 3, dose adjustments, adverse events, and clinical variables were assessed using non‐parametric and categorical statistical tests. Slow acetylators represented 68.1% of the cohort and were associated with significantly higher isoniazid exposure and a greater proportion of supra‐therapeutic concentrations. Overall, 66.0% of patients exhibited supra‐therapeutic concentrations. Acetylator status significantly influenced dose adjustment requirements, with 89.2% of SAs requiring modification, mainly dose reductions. Adverse events occurred in 34.9% of patients and were significantly more frequent in SAs compared with RAs (44.1% vs 15.1%, p <.001). No significant association was observed between age and acetylation phenotype or plasma exposure, although age was associated with adverse events. Renal function showed no significant correlation with acetylation status or exposure. The high interindividual variability observed supports the use of phenotypic assessment (I 3 ) to guide individualized dosing, particularly in settings where NAT2 genotyping is not routinely available. Prospective studies integrating pharmacogenetic and pharmacokinetic approaches are warranted to refine precision therapy in this high‐risk population.
Khefacha et al. (Mon,) studied this question.