Blood Test for Tuberculosis Less Reliable During Pregnancy

Interferon gamma release assays (IGRA) are used to detect Mycobacterium tuberculosis infections in pregnant women.

The IGRA has replaced the traditional tuberculin skin test in some places as these assays are performed on blood samples in routine antenatal screening tests.

Scientists at the University of California (UCSF; San Francisco, CA, USA) studies the correlation between the skin test and the IGRA in detecting latent tuberculosis (TB). The team investigated the comparison of the two tests in 199 women, most of whom were Hispanic, and 65% were born in a country with a high TB prevalence. The cross-sectional study of pregnant women initiating prenatal care was carried out at a university-affiliated public hospital between January 5, 2009, and March 15, 2010. A Mantoux tuberculin skin test was performed after blood was taken for the QuantiFERON TB Gold In-Tube assay, which measures interferon gamma levels in response to presentation of synthetic peptides.

Of the women tested, 23% had positive tuberculin skin tests and 14% had positive IGRA results, for a 77.4% rate of agreement. There were no statistically significant predictors of discordant results. Those who were born in a high TB prevalence country were marginally associated with positive results on both tests with an odds ratio of 2.94. The QuantiFERON TB Gold In-Tube assay is a product of Cellestis (Valencia, CA, USA).

Deborah Cohan MD MPH, the senior author of the study said, "There was poor correlation between the traditional tuberculin skin test and the IGRA in detecting latent TB infection. This could represent either a false positive skin test or a false negative IGRA in pregnant women. Because the IGRA measures interferon gamma release and the normal immune changes in pregnancy may alter interferon gamma release, there is biological plausibility that IGRA do not function as well in pregnancy." The study was published in the December 2011 issue of the journal Obstetrics and Gynecology.

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