Performance of loop-mediated isothermal amplification assay for diagnosis of extrapulmonary tuberculosis and...

Extra-pulmonary TB(EPTB) accounts for 15-20% of total TB cases in India. Many cases remain undiagnosed due to poor sensitivity/long turn-around-time of conventional diagnostic tests. Molecular tests offer rapidity, improved sensitivity and exquisite specificity, but are expensive, require skilled manpower and enhanced laboratory infrastructure. Loop-mediated isothermal amplification (LAMP) assay is a unique, temperature-independent DNA amplification test facilitated by visual optic-readout. WHO has recommended use of LAMP for pulmonary TB diagnosis in 2016. For END-TB strategy to succeed, its necessary to capture all forms of TB. The aim of the study was to determine the sensitivity and specificity of LAMP assay against culture, Xpert MTB/RIF assay and Composite Reference Standard(CRS) for diagnosis of EPTB.

In a cross-sectional study hundred consecutive EPTB specimens were processed for microscopy, culture, Xpert and LAMP assay. Standard formulae of sensitivity and specificity and McNemar chi square test of significance was applied.

Hundred specimens included in the study were fluids(65), pus(19) and tissue(16). TB was detected in 38 specimens by any of the four methods. Positivity of microscopy-5%, culture-28%, Xpert-25% and LAMP-32%. Sensitivity and specificity of LAMP against culture was 85.71% and 88.89%; against Xpert was 88% and 86.67%; against CRS was 80% and 88.6% respectively. LAMP detected TB in 32 patients of which 28 were put on anti-TB treatment (ATT). Of the 62 patients with negative results in all the tests, 22 were put on ATT.

LAMP has good sensitivity for EPTB diagnosis. Further studies are required to establish utility of LAMP as EPTB diagnostic tool.