Evaluation of the Abbott LCx Mycobacterium tuberculosis assay for direct detection of Mycobacterium tuberculosis complex in human samples

Seven hundred thirty-seven clinical samples from 460 patients were processe d for direct detection of Mycobacterium tuberculosis complex by a semiautom ated ligase chain reaction commercial assay, the LCx Mycobacterium tubercul osis Assay (LCx assay) from Abbott Laboratories. Results were compared to t hose of direct microscopy and standard microbiological culture. Of 26 patie nts (5.7%) with a culture positive for M. tuberculosis, 22 (84.6%) were fou nd positive by the LCx assay. The sensitivity of the LCx assay was 98% for smear-positive samples and 27% for smear-negative samples. With an overall culture positivity rate for M. tuberculosis of 8.3% (61 of 737 samples) and after resolution of discrepant results according to clinical data, the sen sitivity, specificity, and positive and negative predictive values of the L Cx assay were 78, 100, 95, and 98%, respectively, compared to 85, 100, 100, and 98%, respectively, for culture and 67, 99, 87, and 97%, respectively, for acid-fast staining. In conclusion, the LCx assay proved satisfactory an d appears to be an easy-to-use 1-day test which must be used with standard culture methods but can considerably reduce diagnosis time versus culture. However, its clinical interest appears to be limited in our population with low mycobacterial prevalence because of its cost considering the small gai n in sensitivity versus direct microscopy.