Comparative Evaluation of Genexpert MTB/RIF with AFB Smear Microscopy Methods in the Diagnosis of Tuberculosis

Abstract

In recent times, emphasis has shifted from older phenotypically and biochemical methods of diagnosis to molecular methods especially Polymerase Chain Reaction (PCR) technique.  This study aims at assessing the performance of Ziehl-Neelson (ZN) acid fast bacilli (AFB) smear microscopy in comparison with GeneXpert Mycobacterium tuberculosis/rifampicin assay (MTB Rif assay), a real time PCR method in the diagnosis of Mycbacterium tuberculosis (MTB) and to observe the diagnostic value of AFB smear at a tertiary hospital (University of Nigeria Teaching Hospital, Enugu) in Nigeria, a resource limited setting.Bacteriologic diagnosis and identification of MTB was made using ZN AFB smear microscopy and Cepheid GeneXpert MTB/RIF system. This research was a descriptive study based on retrospective data from a total of six hundred and fourteen (614) samples collected from presumptive TB patients who attended UNTH- ART TB laboratory. Frequency and percentages were used to summarize categorical variables while means and standard deviations were obtained for continuous variables. Associations between categorical variables were done using chi square while accuracy of the test was done using receiver operating characteristics. P value less than 0.05  were  regarded as significant A total of 614 adults and paediatric patients were enrolled into the study, 381 (62%) were females while 233 (38%) were males. The mean age was 42.90yrs with standard deviation of 16.09yrs. Sixty-five (65) patients had pulmonary TB (PTB). Among the patients diagnosed with PTB, 51 (78.5%) were correctly classified positive with AFB smear result while 65 (100%) had MTB detected by Cepheid GeneXpert method We evaluated the outcome of the performance of GeneXpert with AFB smear microscopy methods in the diagnosis of PTB and observed 78% concordance between GeneXpert and smear result. Diagnostic value of ZN AFB smear method when compared to GeneXpert is significantly high and it is therefore an indispensable tool for the diagnosis of TB in Nigeria, a resource limited setting.

Keywords: Mycobacterium tuberculosis (MTB), GeneXpert MTB/RIF assay, Ziehl- Neelson Acid Fast Bacilli (ZN-AFB), Polymerase Chain Reaction (PCR). Human Immunodeficiency Virus (HIV).