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Interferon-gamma release assay favored over skin test for pregnant women at risk for exposure to tuberculosis

Reuters Health • The Doctor's Channel Daily Newscast

NEW YORK (Reuters Health) – 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 in the June 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 9-month regimen of anti-TB medication,” Dr. Jennifer Lighter-Fisher from New York University School of Medicine, New York told Reuters Health in an email.

Dr. Lighter-Fisher and Dr. Ann-Marie Surette examined 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% of the women had at least one risk factor for exposure to M. tuberculosis.

“Current CDC diagnostic guidelines recommend IGRAs as the preferred
diagnostic for pregnant women with risk factors for exposure to Mycobacterium
tuberculosis (M. tb),” Dr. Lighter-Fisher explained. “These recommendations were suggested prior to any data on IGRAs performance during pregnancy. Validating IGRAs during pregnancy is important because an altered immune response occurs during pregnancy.”

All but 3 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 (5 of them had been given BCG vaccine).

The proportion of pregnant patients with a positive IGRA increased with the likelihood of exposure to M. tuberculosis, and none of the women and adolescents with no known risk factors for M. tuberculosis tested positive on either diagnostic 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 a negative to positive (or vice versa) result.

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. tb exposure,” Dr. Lighter-Fisher concluded. “The very women at risk for M. tb 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. tb infection.” ]

Dr. Lighter-Fisher added, “IGRAs cost between $30-60 a test, which is much more expensive than a skin test. Yet 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.”

“Only individuals with risk factors for exposure to M. tb 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.”

“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,” writes Dr. Howard Minkoff from SUNY Downstate, New York in a related editorial.

“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.”


Performance of an Interferon-Gamma Release Assay to Diagnose Latent Tuberculosis Infection During Pregnancy

The New-Generation Tuberculosis Test: Should It Be Used in Pregnancy?

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