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COMPARISON OF WHOLE BLOOD INTERFERON-GAMMA ASSAY WITH TUBERCULIN SKIN TESTING IN HOUSEHOLD CONTACTS OF PULMONARY TUBERCULOSIS PATIENTS IN SALVADOR, BRAZIL

Fundacao Baiana para o Desenvolvimento das Ciencias, Salvador, Brazil

PURPOSE: QuantiFERON TB-Gold In Tube (QFT) has emerged as an alternative test to the tuberculin skin test (TST) to diagnose latent tuberculosis infection (LTBI). Household (HH) contacts of TB patients are at great risk for latent LTBI. We compared the performance of the whole blood IFN- test with TST in HH contacts of hospitalized TB patients in a Brazilian city with a high incidence of TB.

METHODS: Between November 2006-March 2007, HH contacts of bacteriologically confirmed pulmonary TB patients were recruited in Salvador, Brazil. All contacts underwent TST(2 UT), chest x-ray, and the QFT assay. Patients with a previous history of pulmonary TB were excluded from the study.

RESULTS: For 26 TB patients (15 males, 11 females with a median age 31 years), 119 HH contacts were interviewed (45 males 37.8%; median 4 contacts per household). However, 31 (26%) never returned for their TST reading and were lost to follow-up. Among the 88 contacts (median age 24; 31 males 35.2%,) 25 (28%) were children ; 15 years of age. With a TST cut-off point of 10 mm, 58 (65.8%) were positive. With a cut-off point of IFN- 0.35 IU/mL, 44 (50%) were QFT positive. Overall, 39 (44%) were positive by both tests (95% CI 3.4-31) 25 (28%) were negative by both tests and 24 (27%) had discordant results. Overall concordance between QFT and TST was high (73%) with a fair agreement (=0.46, standard error 0.09) between tests. The degree of sputum smear positivity of tuberculosis patients was not predictive of IFN- (p=0.15) or TST positivity (p=0.11) of the contacts.

CONCLUSION: TST and QFT assays produced comparable results among HH contacts. Due to the small sample size the agreement between tests was fair. There was a 26% loss to follow-up. This is an ongoing investigation and further assessment is needed.

CLINICAL IMPLICATIONS: Assessing TST in such a population is often difficult due to the need for the skin reaction to be examined afterwards. QFT could become an important screening tool.

DISCLOSURE: Almerio Machado Jr., None.