Evaluation of GeneXpert MTB/Rif Ultra assay performance on formalin-fixed paraffin-embedded tissues for Mycobacterium tuberculosis detection

We evaluated the Xpert MTB/Rif Ultra assay performance for <i>Mycobacterium tuberculosis</i> (MTB) detection in formalin-fixed paraffin-embedded tissue (FFPET) compared to mycobacterial culture or laboratory-developed MTB PCR test (LDT). FFPET samples with histological features suggestive of tuberculosis from 2018 to 2023 were selected. Five hundred microlitres of tissue lysis buffer was added to FFPET scrolls and incubated at 75 °C for 5 min. After adding 50 µl of proteinase K and overnight incubation at 56 °C, sample aliquots were processed as per the manufacturer's instructions. MTB culture or LDT assay results were used as a reference for sensitivity and specificity calculations. Of 51 eligible FFPET, 32 were positive for MTB either by culture or LDT PCR on FFPET. Xpert MTB/Rif Ultra detected MTB in 23/32 positive specimens [71.9%, 95% confidence interval (CI) 54.6-84.4%]. Of nine discordant specimens, seven were MTB positive by culture and two were identified by LDT MTB PCR only, as no specimen was submitted for MTB culture. Of 19 negative samples, 100% specificity (95% CI 83.2-100.0%) was attained via Xpert MTB/Rif Ultra. Implementation of Xpert MTB/Rif Ultra on FFPET within clinical laboratories is promising, given its improved turnaround time compared to MTB culture and ability to detect MTB in cases where no tissue is available for culture.