June 02, 2015
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Poor specificity of TB skin test may lead to overtreatment in children

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Tuberculin skin testing resulted in poorer specificity when compared with a commercial interferon-gamma release assay in identifying latent tuberculosis among a cohort of internationally adopted children.

The majority of positive results from tuberculin skin testing (TST) were associated with a history of TB exposure. Moreover, there was an increased likelihood for a positive TST result on univariate testing with increasing age and if the child was born in Africa or Eastern Europe vs. Asia.

“Infections with Mycobacterium tuberculosis is a global public health issue,” Kevin B. Spicer, MD, PhD, MPH, of the College of Medicine at The Ohio State University, and colleagues wrote. “It is estimated that one of three of the world’s population is infected with M. tuberculosis, and TB is the leading cause of mortality because of an individual infectious organism. Accurate diagnosis of active TB is sometimes difficult, especially among children who often have paucibacillary disease.”

Spicer and colleagues sought to assess correspondence between TST and the interferon-gamma release assay, T-SPOT.TB (Oxford Immunotec); identify the factors associated with positive results on both; and to examine the effect of age on test performance.

The study cohort included 109 children aged 4 months to 16 years from the International Adoption Clinic of Nationwide Children’s Hospital in Columbus, Ohio, between November 2008 and January 2011. Children were evaluated within 1 month of arrival to the United States.

Of 107 TSTs, a positive result occurred in 23.4% vs. 4.6% of the 109 T-SPOT.TB tests. There was a 71% overall agreement between TST and T-SPOT.TB, with a prevalence-adjusted, bias-adjusted kappa coefficient of 0.68, according to the researchers.

Children with a history of M. tuberculosis exposure were more likely to have a positive test result for both TST (OR = 25.4; 95% CI, 4.8-261.6) and T-SPOT.TB (OR = 78.9; 95% CI, 9.7-∞). The five children with a positive T-SPOT.TB achieved a TST result induration of at least 15 mm.

Neither bacille Calmette-Guérin vaccination nor scar were associated with a positive TST. None of the 17 children aged younger than 1 year had a positive result for either diagnostic assay.

“Results suggest that in this population, latent TB may be overestimated by TST because of poor specificity,” the researchers concluded. “Longitudinal follow-up and repeat T-SPOT.TB on nontreated children may help to clarify this issue.” – by Jennifer Southall

Disclosure: The researchers report no relevant financial disclosures.