Pyrazinamide susceptibility testing in Mycobacterium tuberculosis: impact of false-positive BD MGIT 960 results and added value of sequencing for accurate diagnosis in a low-MDR-TB-incidence context.

INTRODUCTION: The reliability of phenotypic pyrazinamide (PZA) drug susceptibility test (DST) in Mycobacterium tuberculosis complex (MTBC) isolates is under growing scrutiny. The MGIT 960 System, the only WHO-endorsed method, has shown increasing false positive (FP) rates. In 2024, we reported a surge, and in 2025 Becton, Dickinson, and Company (BD) issued a global alert and suspended the PZA kit production. We assessed the extent of FP PZA resistance (PZA-R), its diagnostic impact, and the ways to distinguish FP from true positive (TP) results in our laboratory.

RESULTS: Between 2023 and 2025, 9% of the isolates were confirmed as TP PZA-R, mostly due to M. bovis. The MGIT 960 FP rate rose from 18% in 2023 to 33% in early 2025. Among 176 isolates, 47 (27%) were FP, 16 (9%) were TP, and 112 (64%) were true negatives (TN). The TP isolates had a longer TTP than the FP isolates (7.2 vs 5.7 days). The FP isolates showed MGIT intermediate growth index values between TP and TN (p < 0.005). Notably, the H37Rv reference strain had a 25% FP rate. The confirmatory testing increased the workload but prevented misreporting.

CONCLUSIONS: Our data reveal a high FP rate for PZA-R with the MGIT 960 System, with no reliable phenotypic alternative available. The sequencing of pncA has been crucial in preventing misreporting and ensuring accurate TB treatment decisions. There is an urgent need for better phenotypic tools and broader molecular approaches beyond pncA so that PZA-R can be detected more reliably and on a scale that supports everyday clinical practice.