Comparison of Tuberculin Skin Test and Interferon Gamma Assay for the Diagnosis Latent Tuberculosis

Problem statement: After HIV infection, TB is the most common cause of mortality due to a single infectious agent. Until 1985, tuberculosis cases in the world were decreasing every year, but then with the prevalence of HIV infection, TB cases are increasing every year. Obviously, with this prevalence of tuberculosis, the best way to control it is to track the patient and identify the people with latent TB and treat them. One third of the world population is infected with latent tuberculosis micro bacterium and tuberculosis causes two million deaths a year. Identification of people with latent TB infection is the primary aim to eradicate TB in the community. The method to measure gamma interferon in the blood called QuantiFERON (R)-TB Test is a promising diagnostic test for diagnosing latent TB infection and is more beneficent than compared to PPD. The aim of this study is to compare IFN-delta test and PPD test in diagnosis of patients with latent TB. Approach: In a crosssectional descriptive-analytic study on 76 patients referred to the clinic or infectious diseases ward of Imam Reza or Sina hospitals or Tabriz University of Medical Sciences during 2007 and 2008, the patients whose CXR had fibrotic images or calcification compatible with latent TB infection were included in the study and the correlations between IFN-delta and PPD tests was evaluated. Results: Mean age of the patients in this study was 64.96 +/- 4.54 years with minimum 43 and maximum 81 of age. About 33.3% of people under study had a positive PPD test and 4% of the patients had a positive IFN-delta and the correlation coefficient between IFN-delta test and PPD test was estimated at 0.376 (p<01/0). Age and gender had no effect on positivity of the results of PPD test and IFN delta tests. Conclusion: Capability of IFN-delta test in diagnosis of latent tuberculosis infection is comparable to that of PPD test; however, the correlation coefficient between the two tests was low and also, unlike other studies, gender and age had no effect on positivity or negativity of PPD and IFN-delta tests.