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Capilia test for identification of Mycobacterium tuberculosis in MGIT™-positive cultures
N S Gomathi1, S M Devi, R Lakshmi
1National Institute for Research in Tuberculosis, Chennai, India.
Background:
The performance of the Capilia test for rapid identification of Mycobacterium tuberculosis complex (MTC) in Mycobacterium Growth Indicator Tube (MGIT) positive samples with contaminating organisms is not well documented.
Objective:
To assess the diagnostic yield of the Capilia test in the rapid identification of MTC in MGIT-positive cultures.
Design:
A total of 459 selected sputum samples were cultured using BACTEC™ MGIT™ 960. Tubes flagged positive by the MGIT instrument (MGIT-positive) were examined for acid-fast bacilli and cording in smears, spotted on blood agar (BA), subcultured for biochemical tests and tested using the Capilia test. Based on smear and growth on BA, MGIT-positive tubes were grouped into MGIT true-positive, MGIT-positive with contamination and MGIT contamination. Performance parameters of Capilia test such as sensitivity, specificity, efficiency, and positive and negative predictive values (PPV, NPV) for each of these groups were determined against biochemical tests as gold standard.
Results:
Of the 346 MGIT-positives, respectively 233, 73 and 40 were MGIT true-positive, MGIT-positive with contamination and MGIT contamination. For the three groups, the PPV and NPV of the Capilia test were respectively 97%, 96% and 100%, and 32%, 27% and 60%.
Conclusion:
In settings with high contamination of MGIT cultures, the performance of the Capilia test is diminished.
