Usefulness of the GeneXpert MTB/RIF Ct for predicting tuberculosis infectiousness

The GeneXpert® MTB/RIF test is faster and more sensitive than smear microscopy (SM) for the diagnosis of pulmonary tuberculosis (pTB). We evaluated the contribution of the Ct ("Cycle threshold") of the GeneXpert MTB/RIF test to estimate infectiousness.

We performed a multicenter retrospective analysis of patients with pTB confirmed by positivity of culture and GeneXpert. The median Ct was calculated on Day 0 (D0). We studied the following correlations: between the Ct and SM on D0, Ct and Time-To-Detection (TTD) of the culture in liquid medium on D0 and then the correlation between the Ct on D0 and the culture on D15, Month 1 (M1), and M2.

A total of 426 patients were included. SM was negative for 130 (30.6 %) patients. The global median Ct was 20.1. The Ct was significantly correlated with the SM grade at diagnosis (Xpert MTB/RIF or Xpert MTB/RIF Ultra, r = -0.77, p < 0.001 and r = -0.69, p < 0.0001, respectively). D0 Ct was significantly correlated with TTD (r = 0.71, p < 0.0001 and r = 0.71, p < 0.0001, respectively) and with a positive culture only on D15 (p < 0.0001 and p < 0.0001, respectively).

The Ct value correlated well with the smear grade and with the bacillary load at diagnosis. Smear microscopy is less sensitive for estimating the risk of transmission. The GeneXpert test (and its Ct) could therefore replace smear microscopy for diagnosis but also for estimating the infectiousness of patients, particularly in low-resource countries where smear and culture are poorly available.