Assessment Of The Quantiferon Tb Gold In-Tube Test For The Diagnosis Of Pulmonary Tuberculosis In Namibian Patients

ABSTRACT

Namibia had a tuberculosis (TB) case notification rate of 589 per 100 000 population in 2010. TB diagnostic methods, especially the conventional microbial culture to detect Mycobacterium tuberculosis (M. tuberculosis), takes long to produce results and those with active TB can spread the infection and treatment is delayed. Immunodiagnosis of TB by interferon-gamma (IFN-ɣ) release assays is relatively very rapid, in comparison to culture, as results can be obtained within 24 hours. The in-vitro Quantiferon TB Gold In-Tube (QFT-IT) test is an enzyme-linked immunosorbent assay (ELISA)-based test employing specific antigens of M. tuberculosis. In this thesis, the usefulness of this blood based test was validated in Human Immunodeficiency Virus (HIV) positive and negative Namibian patients. Furthermore, the levels of IFN-ɣ in QFT-IT supernatants of the TB cases were evaluated longitudinally to ascertain if the test would be useful as a tool for monitoring TB treatment response. One hundred (100) individuals suspected of having TB disease who were ≥ 18 years old, HIV positive and negative, from Katutura State Hospital were recruited in this study. Sputum for direct microscopy and culture, and blood for QFT ELISA were collected. Follow-up samples were collected from individuals in whom pulmonary TB was confirmed at 2 months on treatment, and on completion of treatment (month 6). The QFT-IT test ascertained TB disease at recruitment with a sensitivity of 90.5%, [95% confidence interval (CI) 69.6 – 98.55%] and specificity of 57.8%, [95% CI 44.8 – 70.1%], and a positive predictive value (PPV) of 41% and a negative predictive value (NPV) of 95% when Mycobacteria Growth Indicator Tube (MGIT) culture was used as gold standard. A statistically significant

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difference was observed between QFT-IT and MGIT culture (chi-square = 18.4, p < 0.05); and also between QFT-IT test and HIV status of participants (chi-square = 12.2, p < 0.001). The ANOVA test showed a statistically significant difference, (p < 0.05) within the IFN-ɣ levels between recruitment (before initiation of treatment) and at completion of treatment. The QFT showed no agreement with MGIT culture test and it indicated a better test in ruling out pulmonary TB caused by M. tuberculosis in patients; and it is not influenced by the HIV status. Interferon-gamma levels in QFT-IT supernatants declined when compared between recruitment and to completion of treatment, suggesting a role in monitoring TB treatment.

Key words: tuberculosis, Mycobacterium tuberculosis, interferon gamma release assays, quantiferon, immunodiagnostics, pulmonary tuberculosis, latent tuberculosis, active tuberculosis, tuberculosis suspects, patients, immune response, cytokines, clinical diagnosis, laboratory diagnosis, microbiological tests, direct microscopy, Mycobacterium Growth Indicator Tube, treatment response