Comparison of Indigenous tuberculin skin test (TST) antigen solution developed in India with Tubersol in diagnosing latent tuberculosis

Background: In India, the prevalence of Latent Tuberculosis Infection(LTBI) is estimated to be around 40%(Chadha VK et al. Int J Tuberc Lung Dis.2005;9:1072-82). Various formulations of PPD(Purified protein derivative) are available for the Tuberculin skin test(TST), for the diagnosis of LTBI, which may give variable responses in different ethnicities. The Indigenous PPD marketed in India, although widely used has not been compared with an International standard PPD. Objectives: To compare the bio-equivalency of Indigenous PPD(Arkray) with the Internationally accepted standard PPD(Tubersol) and to compare their results with QuantiFERON-TB Gold Plus(QFT-Plus). Methods: The subjects were recruited from the Christian Medical College, Vellore and JIPMER, Pondicherry, India. Tubersol and Arkray 5 TU dosage were administered in the volar aspects of the right and the left forearms and the reactions were read after 48 hours. A blood sample was collected for the QFT-Plus test. Results: 512 subjects with a median age of 22 yrs participated. Of these, 139(27%) subjects tested positive(>10mm induration) with Arkray and 203(39.6%) with Tubersol. The agreement between the two TSTs was good, moderate, and fair, at cut-offs of 5mm(kappa-0.77), 10mm(kappa-0.58), 15mm induration(kappa-0.44), respectively. However, there was poor agreement between both the TSTs and QFT-Plus(kappa-0.200 for Arkray & 0.17 for Tubersol), at the cut-off of 10mm. Conclusion: Indigenous PPD(Arkray) has shown good agreement with the internationally accepted PPD (Tubersol). We conclude that PPD(Arkray) can be used for the diagnosis of LTBI. Agreement between TST and QFT appears to be poor.