Objective To study the role of Interferon gamma release assay (IGRA) (QuantiFERON–TB Gold In Tube) in blood in the diagnostic work up of active tuberculosis (TB) in a high TB prevalence region.
Design Prospective, comparative group study.
Setting Subjects presenting to the services of the Pulmonary Medicine Department of a large tertiary care teaching hospital in northern India.
Methods We prospectively enrolled, 30 cases of smear or histopathology proven newly diagnosed tuberculosis (18 pulmonary (PTB) and 12 extra-pulmonary (EPTB)) patients controls along with 30 healthy controls. All cases and controls underwent Tubercular Skin Test (TST) using 0.1 mL (1 tuberculin units) of purified protein derivative RT23 and IGRA using QuantiFERON-TB-Gold In Tube assay (QFT) in blood. For TST an induration ≥10 mm was taken as positive. QFT testing was performed and interpreted as per manufacturer's (Cellestis) instructions.
Results We studied 30 patients of active tuberculosis (18 PTB and 12 EPTB) and 30 healthy controls (14 men and 13 women, mean age 35.03±13.23 years). TST positivity had a sensitivity of 83.33% and 66.67% and specificity of 60% for both categories for the diagnosis of active PTB and EPTB respectively. In contrast QFT positivity had a sensitivity of 61.11% and 58.33% and specificity of 50% for the diagnosis of active PTB and EPTB respectively.
Conclusions In this study the QFT-IGRA had a limited overall usefulness in the diagnosis of active pulmonary and extrapulmonary TB. QFT, thus can neither be taken as rule in nor rule out test in a high TB prevalence region like India. TST with all its limitation probably will continue to serve a useful purpose. Larger prospective studies are needed in high incidence countries to determine the sensitivity and specificity of IGRAs, their ability to serve as useful rule-out test for active TB.
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