Comparison of GeneXpert and line probe assay for detection of Mycobacterium tuberculosis and rifampicin-mono resistance at the National Tuberculosis Reference Laboratory, Kenya.

BACKGROUND: The dual challenge of low diagnostic sensitivity of microscopy test and technical challenge of performing a TB culture test poses a problem for case detection and initiation of Tuberculosis (TB) second-line treatment. There is thus need for a rapid, reliable and easily accessible assay. This comparative analysis was performed to assess diagnostic performance characteristics of GeneXpert MTB/RIF and Line Probe Assay (LPA). METHODS: Three hundred twenty nine sputum samples of patients across the 47 counties in Kenya suspected to have drug resistant TB were picked and subjected to GeneXpert, LPA and Culture MGIT at the National TB Reference Laboratory. Sensitivity, specificity and predictive values were then determined to assess the performance characteristics of the various assays. RESULTS: Against culture MGIT as the gold standard for TB diagnosis, GeneXpert had a sensitivity, specificity, positive predictive value, and negative predictive value of 78.5, 64.9, 59.4 and 82.2% respectively while LPA had 98.4, 66.0, 65.4 and 98.4%. For diagnosis of rifampicin mono-resistance GeneXpert had a moderate agreement (Kappa 0.59, P < 0.01) (sensitivity 62.50%, specificity 96.50%) while LPA that had almost perfect agreement (Kappa = 0.89, p < 0.01) with a (sensitivity 90.0% and specificity 99.1%). CONCLUSION: LPA has a better performance characteristic to GeneXpert and an alternative to culture with regards to detection of RIF's mono-resistance.

Biocidal effect of bleach on Mycobacterium tuberculosis: a safety measure.

BACKGROUND: Although treatment of sputum using bleach has shown increased sensitivity in smear microscopy, the safety aspect of sodium hypochlorite (NaOCl) has not been addressed. OBJECTIVE: To determine the biocidal effect of NaOCl on Mycobacterium tuberculosis when used as a safety measure in direct sputum smear microscopy for the diagnosis of tuberculosis (TB). METHOD: A total of 156 smear- and culture-positive pooled sputum specimens were assessed for the viability of M. tuberculosis after treatment with NaOCl. Each specimen was divided into 11 equal portions. One portion was directly cultured. Each of five portions was treated with 3.5% NaOCl and the other five with 5% NaOCl. Specimens were then cultured at intervals of 15 min and 1, 3, 15 and 24 h. RESULTS: A total of 18 (11.5%) specimens showed growth after treatment with NaOCl. Of these, 5 (3.2%) showed growth upon treatment with 3.5% NaOCl at 15 min and 15 and 24 h. Ten (6.4%) showed growth after treatment with 5% NaOCl between 15 min and 3 h, but none showed growth after 15 h. Three (1.9%) specimens showed growth in both concentrations. CONCLUSION: Our preliminary findings show that exposure of sputum to 5% NaOCl for at least 15 h may be a safety procedure in smear microscopy for the diagnosis of TB. However, further studies are warranted.

Improved diagnosis of Ziehl-Neelsen smear negative tuberculosis using sodium hypochlorite sedimentation method.

BACKGROUND: Bacteriological diagnosis of tuberculosis (TB) is largely dependent on Ziehl-Neelsen (ZN) microscopy. This method has a low sensitivity. Although concentration of sputum with sodium hypochlorite (NaOC1) followed by sedimentation increases the sensitivity of direct smear microscopy, no study has focused on the effect of NaOC1 on smear negative sputum specimens. OBJECTIVE: To establish whether 3.5% NaOC1 sedimentation method specifically improves the diagnosis of Ziehl-Neelsen smear negative tuberculosis. DESIGN: A prospective study. SETTING: Mbagathi District Hospital and Center for Respiratory Diseases Research, Kenya Medical Research Institute. SUBJECTS: Two hundred and thirty confirmed direct ZN smear negative sputum specimens from new TB suspects were analysed. RESULTS: Seventy (30.4%) specimens were culture positive. Of these, 19 were ZN smear positive. The ZN sensitivity, specificity, positive and negative predictive values were 27.1%, 99%, 95% and 76%, respectively, after sedimentation with 3.5% NaOC1. CONCLUSION: Overnight sedimentation using 3.5% NaOC1 significantly improves diagnosis of ZN smear negative TB. This technique has potential to improve diagnosis in TB diagnostic services especially in settings with high burden of dual TB/HIV infection.