The Xpert MTB/RIF Cycle Threshold Value Predicts Mycobacterium tuberculosis Transmission to Close Contacts in a Brazilian Prospective Multicenter Cohort

Abstract

Background The Xpert MTB/RIF rapid molecular test provides a quantitative measure of Mycobacterium tuberculosis (Mtb) DNA in the form of cycle threshold (Ct) values. This information can be translated into mycobacterial load and used as a potential risk measure of bacterial spread for tuberculosis (TB) cases, which can impact infection control. However, the role of Ct values in assessing Mtb transmission to close contacts has not yet been demonstrated. Methods A prospective study was performed to investigate the association between Xpert MTB/RIF Ct values and Mtb transmission to close contacts of patients with culture-confirmed pulmonary TB in a multicenter Brazilian cohort. We evaluated clinical and laboratory data (age, sex, race, smoking habits, drug use, alcohol use, chest X-ray, Xpert MTB/RIF results) among pulmonary TB cases, and QuantiFERON-TB Gold Plus (QFT-Plus) results at baseline and after 6 months, for close contacts who had a negative result at baseline. Results A total of 1055 close contacts of 382 pulmonary TB cases were included in the study. The median Ct values from TB cases of QFT-Plus-positive (at baseline or 6 months) close contacts were lower compared with those who were QFT-Plus-negative. An adjusted logistic regression demonstrated that reduced Ct values from the index cases were independently associated with QFT-Plus conversion from negative to positive (odds ratio, 1.61; 95% confidence interval, 1.12-2.32) after adjusting for clinical characteristics. Conclusions Close contacts of pulmonary TB index cases who exhibit low Xpert MTB/RIF Ct values displayed higher rates of TB infection, reflecting Mtb transmission.