LCx Mycobacterium tuberculosis assay is valuable with respiratory specimens, but provides little help in the diagnosis of extrapulmonary tuberculosis

BACKGROUND. Commercial nucleic tests, designed for the detection or Mycobacterium tuberculosis DNA/RNA in respiratory samples, are often applied also in nonrespiratory specimens in order to verify the diagnosis of extrapulmonary tuberculosis. AIM. To evaluate the value of the Abbott LCx Mycobacterium tuberculosis assay for the diagnosis of pulmonary and extrapulmonary tuberculosis based on routine clinical laboratory results. METHODS. The assay was used to analyse 350 respiratory and 826 nonrespiratory specimens from 961 patients, of whom 3.6% had culture-proven tuberculosis, The results obtained by the LCx assay were compared with the records on mycobacterial isolates of the national reference laboratory and, in the case of positive findings, with clinical data. RESULTS. In comparison with culture, the sensitivity, specificity and positive/negative predictive value of the assay on respiratory specimens were 87.5%, 99.7%, 93.3% and 99.4%, respectively. With nonrespiratory specimens. the overall sensitivity, specificity and positive/negative predictive value of the LCx assay were 73.3%, 98.0%, 40.7% and 99.5%, respectively. When clinical and histological data were also included, the positive predictive value of LCx with nonrespiratory specimens was 45.8%. CONCLUSION. Critical interpretation of the nucleic acid amplification results obtained from nonrespiratory specimens is necessary in both laboratory and clinical settings.