Effectiveness of GeneXpert Technology in the Diagnosis of Smear-Negative Pulmonary Mycobacterium tuberculosis in HIV Positive Patients in Namibia

Tuberculosis (TB) remains a major public health problem accounting for more than 9.4 million incidents yearly of which most incidents are common in resource poor countries. The prevalence of Human Immu-nodeficiency Virus (HIV) in these countries is relatively high and TB/HIV co-infected patients often test negative for TB with direct microscopy, which poses diagnostic difficulties. The traditional diagnosis of HIV associated TB is complex, expensive, slow and technically demanding as it relies on conventional culture and drug susceptibility testing. The long delay required to obtain results has devastating consequences for patients who go undiagnosed or diagnosed too late. The purpose of the study was to determine the effectiveness of GeneXpert technology in the diagnosis of smear-negative pulmonary Mycobacterium tuberculosisin HIV positive patients. A retrospective cross sectional study was conducted on 400 patients suspected to have Mycobacterium tuberculosis and who were referred to the Namibia Institute of Pathology (NIP), Windhoek from November 2014 to May 2015. Patients with known HIV status who had their sputum tested by both the GeneXpert (Cepheid, California, United States of America) and direct microscopy (DM) for TB were included. Out of 400 sputum samples analysed for TB, 253 (63.3%) cases were positive using GeneXpert whereas the DM was positive only for 133 (33.3%) of the cases. Out of these 123 smear negative pulmonary tuberculosis (SN-PTB), 120 (98%) sputum samples were from HIV positive patients while 3 (2%) were from HIV negative patients. The sensitivity of the GeneXpert was calculated as 98% and the specificity was 52%. The majority of the patients who contributed to SN-PTB were found to be co-infected with HIV. The GeneXpert’s sensitivity was found to be high while the specificity was low. Even though this was the case, the GeneXpert as compared to DM could significantly reduce false negatives and the delay on treatment initiation can be significantly shortened, reducing premature death and ongoing transmission.