Microbiology Laboratory, Department of Pathology, United Christian Hospital, Hong Kong

Recently, a new commercial molecular diagnostic technique uses LCR amplication technology for the direct detection of DNA from the Mycobacterium tuberculosis (MTB) in respiratory specimens is available in UCH Microbiology laboratory. This LCX assay uses primers that are complementary to the MTB target nucleic acid sequence which is found within the single copy chromosomal gene encoding for protein antigen b. This gene sequence was found to be specific to the MTB complex only. The primers are also designed such that when they are hybridized to the target sequence, a gap of several nucleotides separates the adjacent primers. The gap is filled using thermostable DNA polymerase and nucleotides, and only the nucleotides required to fill gap are provided in the reagent. This design greatly increases the specificity of the reaction since the gap must be filled correctly for the primers to be joined by the ligase. Hence, this technique is best described as "gapped LCR". The performance of LCX assay is evaluated by testing with clinical respiratory specimens and comparing to traditional smear and culture methods. A total of 121 specimens were tested and LCX showed a sensitivity and specificity of 73.3% and 95.8% respectively when comparing with culture as gold standard.