Is the detection of Mycobacterium tuberculosis DNA by ligase chain reaction worth the cost:: experiences from an inner London teaching hospital

Aims-To evaluate the clinical usefulness and the costs of using a rapid, commercial ligase chain reaction test (LCx) to detect Mycobacterium tuberculosis directly from clinical samples. Methods-A prospective study of 2120 routine clinical specimens from 1161 patients over a 13 month period. Investigations for mycobacterial disease by microscopy, culture, and the Abbott LCx assay were performed. Sequential LCx assays were monitored in a cohort of patients undergoing treatment. The costs of the assay were calculated using the WELCAN system. Sensitivity, specificity, and positive and negative predictive values of the LCx assay were compared with conventional tests. The performance of the assay in patients undergoing treatment and cost in terms of WELCAN units converted to pounds/annum was studied. Results-The assay was 85%/88% sensitive and 98%/100% specific in culture confirmed/clinically confirmed cases of tuberculosis, respectively. The assay was not useful for the measurement of treatment outcomes. The test cost approximately pound 42 500/annum. Conclusions-The assay is a rapid, sensitive, and specific adjunct to clinical diagnosis, especially in differentiating non-tuberculous mycobacteria. However, it does not differentiate old and treated tuberculosis from reactivated disease, it is not useful to monitor adherence to treatment, and it is expensive.