THE PERFORMANCE OF NUCLEIC ACID AMPLIFICATION TESTS FOR THE DIAGNOSIS OF TUBERCULOSIS AND RESISTANCE TO RIFAMPICIN IN DOTS CENTRES OF A DISTRICT IN SOUTH INDIA

Rebecca Ann Jose, Dr. Jijo Oommen Roy, Dr. Anna Mathew*, Nisha M. and John Michael Raj

ABSTRACT

Title of Study: The Performance of Nucleic Acid Amplification Tests for the Diagnosis of Tuberculosis and resistance to rifampicin in DOTS centres of a district in South India. Background and Objectives: According to WHO the number of new tuberculosis (TB) cases reported in India in 2017 was 2740 and the rate of incidence of MDR-TB was 10/100,000 population. Early diagnosis facilitates treatment initiation and can limit the spread of the disease. The Revised National Tuberculosis Programme (RNTCP) has scaled up the use of the Cartridge Based Nucleic Acid Amplification Test (CBNAAT), also known as GeneXpert, and provides it free of cost to the patient. In addition to the existing 628 Machines, 507 machines have been deployed to cover all districts of the entire country. The CBNAAT is provided free of cost This study was undertaken to assess the performance of the CB-NAAT test in peripheral DOTS centres of this predominantly rural district. Methods: The diagnostic accuracy of the CB-NAAT test in diagnosing tuberculosis in patients presenting at the various DOTS centres of this district was assessed. After approval from ethics committee and written permission from the district tuberculosis officer, patients suspected to have tuberculosis, registered at the DOTS clinic for CB-NAAT 178 participants were serially recruited. Results: Of the 178 samples studied, 81 (45.5%) had a positive CBNAAT, while 71 (39.9%) had positive smear microscopy. The remaining 107 smear samples and 97 CBNAAT samples were negative. Of the 81 mycobacteria positive samples diagnosed by CBNAAT, 62 were also diagnosed by smear microscopy, giving a sensitivity of 87.3% while 19 cases identified by CBNAAT were sputum smear negative. Of the 107 smear microscopy negative samples, CBNAAT was negative in 91 subjects giving a specificity of 82.2%. The high positive predictive value (85.2%) indicates the strong probability that a person with a positive test has tuberculosis. The high negative predictive value (95.7%) shows that a negative test in effect rules out tuberculosis. The high likelihood ratio of nearly 5 with a confidence interval of 3.24 to 7.46 tells us that the test has a high probability to identify the presence of mycobacteria. Interpretation & Conclusions: The sensitivity of the CB-NAAT test obtained in our study was 87.3% and specificity was 82.20%. Rifampicin resistance was present in less than one-tenth of the samples (7.3%). The study showed that CB-NAAT is an equally sensitive and specific rapid molecular diagnostic test to detect TB and rifampicin resistance in biological specimens in patients suspected to have tuberculosis.

Keywords: Diagnosis of Tuberculosis, Gene expert, CB-NAAT, rifampicin resistance, sputum microscopy.