即便是用IGRA, 有時也要等大於三個月才能決定TB contact

接觸TB後,TST大抵上2-12週轉陽,IGRA大抵上6-10週,但IGRA可以長到3個月以上


TB Tests Can Have a Lengthy Window Period

Two studies suggest that interferon-{gamma} release assay conversion sometimes occurs more than 3 months after exposure to active tuberculosis.

Increasingly, interferon-{gamma} release assays (IGRAs) are being used for diagnosing latent tuberculosis (TB). Although the tuberculin skin test (TST) traditionally used for this purpose converts to positive within 2 to 12 weeks after an individual is exposed to someone with active TB, the interval for IGRA conversion has remained unclear. Current guidelines recommend repeating an initially negative IGRA after 6 to 10 weeks, but this recommendation is not based on controlled studies. Two recent investigations address this issue.

Anibarro and colleagues conducted a longitudinal prospective analysis among contacts of TB patients in Spain. The 152 contacts received a TST and an IGRA at first visit; for individuals with negative or conflicting results, testing was repeated 2 months later. Results were considered definitive after a diagnosis was made, taking all test results into account. Agreement between the TST and IGRA was better on the second round of testing ({kappa}=0.73, agreement 88%) than on the first ({kappa}=0.60, agreement 81%). The definitive results also showed improved concordance ({kappa}=0.73, agreement 87%). Concordance of such results was excellent for the 97 participants without bacille Calmette-Guérin vaccination ({kappa}=0.81, agreement 91%). Definitive positive TST and IGRA results were each correlated with duration of TB exposure and with size of the place of contact.

Lee and colleagues investigated the time interval for IGRA conversion during a TB outbreak among 32 soldiers in a Korean military platoon. In April 2009, active pulmonary TB was diagnosed in a soldier who had developed a productive cough in January. Within 7 days, the remaining platoon members were screened for TB. Four additional soldiers with active TB were quickly identified by chest x-ray or computed tomography. (Despite isolation of these 5 individuals as soon as active TB was suspected, the remaining 27 were exposed for up to 3 months.) IGRAs were performed during the screening and were repeated at 2, 4, 8, 14, 18, and 30 weeks for men who tested negative. IGRA results were positive at the initial investigation for 17 men and subsequently for 9 more (3 at 2 weeks, 3 at 4 weeks, 3 at 14 weeks). No conversion occurred during the 30-week observation period for the remaining soldier.

Comment: Although most IGRA conversions occur within 4 to 7 weeks after TB exposure, some may be delayed for >3 months; agreement between TST and IGRA results show better concordance after this window period. Such factors must be considered if an IGRA is used to screen contacts of a TB patient.

Thomas Glück, MD

Published in Journal Watch Infectious Diseases July 20, 2011

Citation(s):

Anibarro L et al. Tuberculin skin test and interferon-{gamma} release assay show better correlation after the tuberculin ‘window period' in tuberculosis contacts. Scand J Infect Dis 2011 Jul; 43:424.

Lee SW et al. Time interval to conversion of interferon-{gamma} release assay after exposure to tuberculosis. Eur Respir J 2011 Jun; 37:1447.


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