Non-actionable Results, Accuracy, and Effect of First- and Second-line Line Probe Assays for Diagnosing Drug-Resistant Tuberculosis, Including on Smear-Negative Specimens, in a High-Volume Laboratory

The MTBDRsl assay fails to generate a result in almost half of smear-negatives, resulting in substantial missed resistance, but has high rule-in value. Many rifampicin-resistant individuals would likely benefit from isoniazid. Programmatic direct testing was nevertheless associated with care cascade improvements. Background Rapid tuberculosis (TB) drug susceptibility testing (DST) is crucial. Genotype MTBDRsl is a widely deployed World Health Organization (WHO)-endorsed assay. Programmatic performance data, including non-actionable results from smear-negative sputum, are scarce. Methods Sputa from Xpert MTB/RIF individuals (n = 951) were routinely-tested using Genotype MTBDRplus and MTBDRsl (both version 2). Phenotypic DST was the second-line drug reference standard. Discrepant results underwent Sanger sequencing. Findings 89% (849 of 951) of individuals were culture-positive (56%, 476 of 849 smear-negative). MTBDRplus had at least 1 nonactionable result (control and/or TB-detection bands absent or invalid, precluding resistance reporting) in 19% (92 of 476) of smear-negatives; for MTBDRsl, 40% (171 of 427) were nonactionable (28%, 120 of 427 false-negative TB; 17%, 51 of 427 indeterminate). In smear-negatives, MTBDRsl sensitivity for fluoroquinolones was 84% (95% confidence interval, 67%-93), 81% (54%-95%) for second-line injectable drugs, and 57% (28%-82%) for both. Specificities were 93% (89%-98%), 88% (81%-93%), and 97% (91%-99%), respectively. Twenty-three percent (172 of 746) of Xpert rifampicin-resistant specimens were MTBDRplus isoniazid-susceptible. Days-to-second-line-susceptibility reporting with the programmatic advent of MTBDRsl improved (6 [5-7] vs 37 [35-46]; P < .001). Conclusions MTBDRsl did not generate a result in 4 of 10 smear-negatives, resulting in substantial missed resistance. However, if MTBDRsl generates an actionable result, that is accurate in ruling-in resistance. Isoniazid DST remains crucial. This study provides real-world, direct, second-line susceptibility testing performance data on non-actionable results (that, if unaccounted for, cause an overestimation of test utility), accuracy, and care cascade impact.

Automated summary

The MTBDRsl assay fails to generate a result in almost half of smear-negatives, resulting in substantial missed resistance, but has high rule-in value. The study found that MTBDRsl has high sensitivity for fluoroquinolones and second-line injectable drugs, but low specificity for both.