Evaluation of Genxpert Real Time PCR POC in the Diagnosis of Multidrug Resistant TB Among Patients Attending NTBLTC Saye-Zaria Nigeria

Rationale: Xpert MTB/RIF is a novel automated molecular diagnostic platform recently endorsed by the World Health Organization. However, performance-related data from high HIV prevalence settings are limited. Objectives: To evaluate of GeneXpert MTB/RIF assay for the rapid diagnosis of pulmonary and multidrug resistant tuberculosis in patients and to compare its sensitivity, specificity, positive and negative predictive values against those of the conventional methods. Methods: Xpert MTB/RIF was evaluated using single sputum samples from 400 routine microscopy and culture laboratory investigations for ambulant patients with suspected TB. Measurements and Main Results: The performance outcomes of GeneXpert MTB/RIF for the detection of Mycobacterium tuberculosis compared with direct and concentrated TB smear microscopy using culture as gold standard was found to be of statistically high significant value. The overall sensitivity of the assay was 86.32% (77.74 to 92.51) and specificity of 91.18% (81.16 to 94.24%) at 95% CI respectively. In direct smear-positive culture-positive cases the sensitivity was 80.23% (70.25% - 88.04%), whereas in smear-positive culture-positive cases the sensitivity was concentrated smear-negative culture-positive cases it was 81.947% (71.11%- 90.02%). The overall specificity for the diagnosis of TB using both culture negative groups (probable and non-TB) was 83.97% (79.42 - 87.87% - 94.26%) at 95% CI. Using only the non-TB group, Xpert MTB/RIF specificity was 91.18% (87.16% - 94.26%) at 95%CI. The assay was negative in all twenty four cases that were culture-positive for non-TB mycobacteria, and negative in 10 cases with sputum isolates that were contaminated by bacterial over growth while only7 (1.75%) of 400 evaluated samples yielded an indeterminate Xpert MTB/RIF result. Xpert-MTB positive predictive value was 77.36 (68.21-84.92%) while Negative predictive value was 95.02(91.63-97.3%) using culture as a gold standard. A single Xpert MTB/RIF assay outperformed direct smear microscopy and showed an 9% relative increase in the rapid (potentially within 24 h) TB case-detection rate (36 additional cases) compared with 75 smear-positive cases and thus detected significantly more patients than smear microscopy (75% [18.8%] of 400 vs. 111 [27.87%] of 400). Xpert MTB/RIF is an accurate rapid diagnostic tool for tuberculosis (TB) and rifampicin resistance. The key advantage of Xpert MTB/RIF is that it diagnosed 20.7% of culture-negative TB cases, 27.7% of smear negative TB cases in a high HIV prevalence setting. Conclusion: XpertMTB/RIF outperformed smear microscopy, established a diagnosis in a significant proportion of patients with smear negative TB, detected many highly likely TB cases missed by culture, and accurately ruled out rifampicin-resistant TB. Sample-specific factors had limited impact on performance. Performance in patients infected with HIV, especially those with advanced immunosuppression, warrants further study.