Multicenter Evaluation of the Abbott LCx Mycobacterium tuberculosis Ligase Chain Reaction Assay

Abstract

Four Australian hospital laboratories evaluated the performance of the Abbott LCx Mycobacterium tuberculosis assay with 2,347 specimens (2,083 respiratory and 264 nonrespiratory specimens) obtained from 1,411 patients. A total of 152 specimens (6.5%) were culture positive for Mycobacterium tuberculosis complex (MTBC); of these, 79 (52%) were smear positive. After resolution of discrepant data, the overall sensitivity, specificity, and positive and negative predictive values for the LCx assay were 69.7, 99.9, 99.1, and 97.7% respectively. For smear-positive respiratory specimens that were culture positive for MTBC, the values were 98.5, 100, 100, and 98.4%, respectively, while the values for smear-negative respiratory specimens were 41.5, 99.9, 96.4, and 98%, respectively. Relative operating characteristic curves were constructed to demonstrate the relationship between sensitivity and specificity for a range of possible cutoff values in the LCx assay. These graphs suggested that the assay sensitivity for respiratory samples could be increased from 70.2 to 78.6%, while the specificity would be reduced from 99.9 to 99.4% by inclusion of a grey zone (i.e., LCx assay values of between 0.2 and 0.99). An algorithm is presented for the handling of specimens with LCx assay values within this grey zone.

A peritoneal dialysis patient with fatal culture‐negative peritonitis

“…35 Polymerase chain reaction may be useful for detecting as little as 10 fg of mycobacterial DNA (equivalent to two mycobacteria), although the sensitivity of the test is greater in smear-positive clinical samples (70%) than those which are smear-negative (40%). 36 Although tuberculous peritonitis usually reflects reactivation of latent infection, the chest X-ray is generally normal in only half the cases and contributes to a diagnosis of tuberculosis in only approximately 20% of cases. 28 Fifty per cent of patients are anergic on Mantoux testing, with only one-third having at least an intermediate reaction to a purified protein derivative.…”

“…Moreover, the diagnostic value is not improved by assaying for the ADA2 isoenzyme 34 or for γ‐interferon levels 35 . Polymerase chain reaction may be useful for detecting as little as 10 fg of mycobacterial DNA (equivalent to two mycobacteria), although the sensitivity of the test is greater in smear‐positive clinical samples (70%) than those which are smear‐negative (40%) 36 . Although tuberculous peritonitis usually reflects reactivation of latent infection, the chest X‐ray is generally normal in only half the cases and contributes to a diagnosis of tuberculosis in only approximately 20% of cases 28 .…”