[Comparison of conventional tuberculin skin test and QFT-2G, a new method for diagnosis of tuberculosis infection, with the use of the contact score].

BACKGROUND: QuantiFERON TB-Gold (QFT) has recently been developed as a new method for diagnosing tuberculosis (TB) infection. To evaluate the usefulness of QFT, we analyzed the relationship between QFT and the closeness of contact with a source of infection, in comparison with that of the tuberculin skin test (TST). METHODS: Male (n=322) and female (n=340) subjects (4-75 years old) who had contact with an index case received QFT and TST. The diagnostic criterion for TB infection with TST was defined as a test with an erythema diameter of > or = 30 mm. The closeness of contact with an index case was quantified in the "contact score," based on the information obtained with a questionnaire. RESULTS: There was a significant positive correlation between the contact score and QFT-positive rate, while there was no such relationship for TST positivity. The odds ratios for positive QFT rate for the subjects in the 3rd and 4th quartile groups of contact score (taking the QFT-positive rate in the lowest score quartile as unity) were 3.40 (95% confidence interval: 1.07-10.76, p<0.05) and 7.62 (95% confidence interval: 2.60-22.37, p<0.01), respectively. These odds ratios were also significantly greater than unity after adjustment for age, sex, history of BCG vaccination and history of health care-related jobs. There was a wide difference in the QFT-positive rates between the 2nd and 3rd quartiles of contact score (3.5% vs. 11.9%). The borderline value of the contact score between these two quartiles corresponded to 200, which could be a cutoff value for defining a high-risk contact. CONCLUSION: The QFT-positive rates correlated well with closeness of contact, while TST showed a poor correlation. Thus, QFT is considered more useful than TST for diagnosing tuberculosis infection.