Diagnosis of active pulmonary tuberculosis using the xpert MTB/RIF assay in smear negative tuberculosis suspects at Kenyatta National hospital and Mbagathi district hospital.

Abstract

Background: Pulmonary tuberculosis (PTB) is a major global health problem and currently it stands as the second leading cause of mortality from a single infectious disease worldwide according to the WHO world TB report 2012. Over the past century, diagnosis of PTB has relied almost exclusively on smear microscopy which has been shown to have a low sensitivity especially in patients co-infected with HIV. Novel diagnostic methods, such as the recently developed Xpert MTB/RIF assay are bound to be essential tools in the global fight against TB. Objectives: In this study, we sought to determine the yield of this novel assay in the microbiological diagnosis of PTB in patients suspected to have PTB who are sputum smear negative. Design: It was a cross sectional descriptive study Setting: The Kenyatta National Hospital (KNH) and Mbagathi District Hospital (MDH). Methods: Files of prospective patients seen at the Tb clinics of both hospitals and the Tb wards in MDH were perused. Those which met the inclusion criteria were recruited with consent into the study and the study questionnaire administered. Spot sputum samples from each patient which were of proper quality were tested for active mycobacterium tuberculosis and rifampicin resistance using the Xpert MTB/RIF assay. Data from the study questionnaire and results of the assay were entered and analyzed using SSPS version 17.0 software. Measured outcomes included the proportion of active PTB in smear negative PTB suspects; the prevalence of rifampicin resistance in this group and the association between the Xpert MTB/RIF assay result and patients presenting symptoms. Results: A total of 179 sputum samples were run from 179 eligible participants. The Xpert MTB/RIF assay yield was high at 19% (95%CI 13.5-25.5) with a 0% rifampicin resistance rate. Presence of night sweats on its own, or both night sweats and weight loss were weakly associated with Xpert MTB/RIF assay positivity (OR 10.1; 95%CI 1.3-76.7, p=0.007) and (OR 7.1; 95% CI 2.1-24.3, p<0.001) respectively. Conclusion: The additional yield of the Xpert MTB/RIF assay in patients who are sputum smear negative PTB suspects in our study is 19% which when taken in the context of a 34% notified smear negative rate in Kenya, is high. This has important public health implications wherein patients who can transmit Tb are not being identified by smear microscopy.