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Role of line probe assay in detection of extra-pulmonary tuberculosis/multidrug-resistant tuberculosis: The experience from Kerala state, India

Background: There are limited data on use of line probe assay (LPA) for detection of TB and Rifampicin resistance among extra-pulmonary tuberculosis (EPTB) patients under specific TB control programme settings. Our aim was to compare the positivity of LPA with Lowenstein–Jensen (LJ) and Bactec MGIT and to test its utility in faster testing and in patients on treatment. Methodology: The data of all 387 EPTB samples processed in two years by the state-level accredited laboratory in 2013–2014 were selected for the study. The laboratory used standard N-acetyl-L-cysteine – sodium hydroxide processing method for all samples except for cerebrospinal fluid and urine samples before conducting Ziehl-Neelsen smear microscopy, LPA and culture on both LJ and MGIT. Those samples with a negative LPA result if culture positive was subjected to a second LPA as per laboratory protocol. Doubtful Rifampicin resistant (RR) results were cleared by phenotypic testing by MGIT. Anti-TB treatment (ATT) duration was correlated with positivity of these tests later. Results and Discussion: LPA done on 321 processed samples out of 387 EPTB samples identified eight RR cases among 136 positive results within two to four days. Performing LPA on culture positive isolates identified an additional seven RR cases, thus reducing turn-around-time by two to eight weeks for the susceptibility results. It also demonstrated far better positivity in smear negative (15.2%) and in smear positive samples (74.3%) compared to culture methods as the test demonstrated clearly, better positivity among patients on ATT for more than two months. Conclusion: LPA in association with culture is an excellent combination with very good results in rapid identification of EPTB along with detection of Rifampicin resistance. LPA gives far superior positivity compared to MGIT and solid cultures among patients under ATT.