A comparative study of the diagnosis of pulmonary tuberculosis using conventional tools and polymerase chain reaction

The sensitivity of Polymerase Chain Reaction (PCR) makes it a potential diagnostic test for detection of M. tuberculosis in samples with low bacillary load. The aim was to assess the efficiency of PCR as compared to routine diagnostics in detection of M. tuberculosis from sputum samples of suspects referred to a tuberculosis clinic and those identified during a morbidity survey. Respiratory samples (sputum with or without saliva) from 144 individuals were examined by PCR using MPB64 primers culture and microscopy. 97 samples were from suspects referred to a tuberculosis clinic 26 were from suspects identified during a morbidity survey and 21 were from patients with diseases other than tuberculosis. Study was conducted blind. Total cases considered to be positive for tuberculosis by all criteria was 71. PCR detected 98% of culture positive 97% of smear positive culture positive and 100% of smear negative culture positive samples. PCR was also positive for 86% of smear negative samples from tuberculosis suspects diagnosed on the basis of other routine diagnostics and supporting clinical evidence. Seventeen samples were positive only by PCR but based on clinical parameters only 7 were considered as true positives. The sensitivity of PCR was 91.5% compared to 51% for smear microscopy and 68% for sputum culture. This was due to the fact that PCR could pick up bacterial DNA even from saliva mixed sputum specimens which are generally not considered appropriate for microbiology. The specificity of PCR (86%) was found to be lower than other diagnostic tests mainly due to lack of a suitable gold standard to assess its efficiency. This is an important limitation in evaluation of the test. PCR using MPB64 primers has potential and can be a useful adjunct to diagnose clinical tuberculosis particularly in smear negative paucibacillary cases. However the major limitation of PCR results from the absence of a suitable gold standard by which to evaluate the results. (authors)