IFN-gamma release assay favored for finding latent TB in pregnancy

The interferon-gamma release assay (IGRA) is more specific, at least as sensitive, and may better predict disease progression than the tuberculin skin test in pregnant women at risk of exposure to tuberculosis, according to a report this month in Obstetrics & Gynecology.

"If the IGRAs were to be used instead of the skin test during pregnancy, far fewer women (less than half in our study) will be identified as positive and therefore would have avoided a chest x-ray during pregnancy and a nine-month regimen of anti-TB medication," Dr. Jennifer Lighter-Fisher from New York University School of Medicine, New York told Reuters Health by email.

She noted that the Centers for Disease Control and Prevention currently recommend IGRAs over skin testing for pregnant women with risk factors for exposure to Mycobacterium tuberculosis. But the CDC recommendations were made without data on IGRA performance during pregnancy, "partially because the century-old tuberculin skin test is neither very sensitive nor specific," Dr. Lighter-Fisher and co-author Dr. Ann-Marie Surette wrote in their paper.

"Validating IGRAs during pregnancy is important because an altered immune response occurs during pregnancy," Dr. Lighter-Fisher told Reuters Health.

The two researchers analyzed the performance of the IGRA in 140 pregnant and 140 nonpregnant adolescents and women receiving care at public hospital outpatient clinics in New York City. About 90% had at least one risk factor for exposure to M. tuberculosis.

All but three of the 103 pregnant women and adolescents who had negative tuberculin skin test results also had negative IGRA, and the overall agreement between tuberculin skin test and IGRA results was 88% for all pregnant patients.

Only 12 of the 28 women (43%) with positive skin test results had positive IGRA results - and five of them had received the Bacillus Calmette-Guérin (BCG) vaccine.

The proportion of pregnant patients with a positive IGRA increased with the likelihood of exposure to M. tuberculosis, and no one without a known risk factor for M. tuberculosis had a positive result on either test.

Interferon-gamma levels did not differ significantly during any trimester of pregnancy.

None of the 25 women who underwent repeated IGRA testing during their pregnancy or postpartum converted from negative to positive (or vice versa).

IGRA results did not differ significantly between the pregnant and nonpregnant patients in the study.

"IGRAs could improve screening of pregnant women at risk of M. tuberculosis exposure," Dr. Lighter-Fisher said. "The very women at risk for M. tuberculosis exposure, the foreign born and urban poor, often seek medical treatment only during pregnancy. We are therefore missing an opportunity to appropriately identify and safely treat M. tuberculosis infection."

She added, "IGRAs cost between $30-60 a test, which is much more expensive than a skin test" - but she believes the higher cost would be offset by greater efficiency. "In our study we observed less than half of the women with positive skin tests had a positive IGRA. Therefore, the additional cost of IGRAs is offset by a decrease in the number of positive test results and the associated costs of evaluating and treating tuberculin skin test-positive patients," she wrote in her email.

"Only individuals with risk factors for exposure to M. tuberculosis should be given an IGRA," Dr. Lighter-Fisher said. "If there is no exposure risk, neither an IGRA nor a skin test should be performed."

In an editorial, Dr. Howard Minkoff from SUNY Downstate in Brooklyn, New York writes, "The IGRA will be most useful in women with BCG and in those with a low likelihood to return for a reading of their skin tests."

"Pending the results of future research," Dr. Minkoff notes, "obstetricians should avail themselves of the best tests available, depending on their patients' clinical circumstances and their institutional resources. Evidence from nonpregnant populations suggests that, in specific circumstances, the IGRA can be that test."

SOURCE: http://bit.ly/NKA7zB

Obstet Gynecol 2012;119:1088-1095,1085-1087.

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By Will Boggs MD

Published: June 21, 2012, 8 p.m.

Last updated: June 21, 2012, 8 p.m.

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