Improvement of Mycobacterium tuberculosis detection by X pert MTB/RIF Ultra: A head-to head comparison on X pert-negative samples

PLoS One. 2018 Aug 13;13(8):e0201934

Contributed by Dr Kalpesh Sukhwani

This is a retrospective single center study (Italy) where authors DE-identified frozen samples collected over a 4-year period, which had previously resulted smear-negative, Xpert-negative but MTB culture-positive and analyzed with X pert Ultra to assess increase in sensitivity of MTB detection.

During the study period 382 MTB culture-positive samples were archived: 314 resulted X pert-positive and 68 X pertnegative. Thirty-one of the 68 X pert-negative samples resulted positive with Ultra, with an overall improvement in MTB detection of 45.6%.

Out of 36 X pert negative respiratory samples, 18 resulted Ultra-positive with the following semi-quantitative loads: “low”(n = 1), “very low”(n = 11), “trace”(n = 6), with an improvement in MTB detection of 50%. The best performance was achieved on BAL specimens (53.8%).

Out of 32 X pert-negative non-respiratory samples, 13 resulted Ultra-positive with the following semi-quantitative loads: “very low”(n = 7), “trace”(n = 6), with an improvement in MTB detection of 40.6%. The best performance was achieved on biopsies (55.6%) and lymph nodes (50%). Category “trace” detected 12 out of the 31 Ultra-positive MTB samples; in the remaining 19 samples RIF susceptibility was determined with 100% concordance with the phenotypic susceptibility test.

This study further confirms the better performance of Ultra compared to the previous version in both respiratory and non-respiratory smear-negative samples, with an overall improvement of 45.6%.