Association of Xpert MTB/RIF Cycle Threshold Values with Tuberculosis Treatment Outcomes

Considering the current recommendation of the World Health Organization to replace sputum smear microscopy with Xpert MTB/RIF as an initial diagnostic test for tuberculosis (TB), and that culture takes time to provide results, the cycle threshold (C_T) of the Xpert test may be the only way to assess bacillary load. The objective of this study is to evaluate the association of bacillary load, measured by the Xpert C_T, with the TB treatment outcomes.

In cohort study, Xpert C_T values were evaluated in cured and non-cured (failure and death) patients. Multivariate analysis was performed to evaluate if C_T is independently associated with TB treatment outcomes.

During this study period, 155 patients (84 cured and 71 non-cured) met the inclusion and were included in the analysis. The mean C_T value for Xpert MTB/RIF test was 20.7 ± 5.6 in cured patients and 17.1 ± 5.6 in non-cured patients (p < 0.0001). Previous TB was more frequent in non-cured (28.2%) than in cured patients (7.1%) (p < 0.0001). Non-cured patients were younger than cured ones (37.1 ± 13.3 vs 43.6 ± 16.2; p = 0.006). HIV was more frequent in non-cured (28.2%) than in cured patients (15.5%), although this difference was not statistically significant (p = 0.054). In multivariate analysis, C_T values, age, previous TB, and HIV were independently associated with non-cure.

Lower Xpert MTB/RIF C_T values were independently associated with worse treatment outcomes. The information from even a single test performed before starting treatment proved to be a relatively good predictor of TB treatment outcome.