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Preventive therapy key for children aged younger than 2 years, regardless of TB skin test results
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Data support “the use of preventive therapy regardless of the tuberculin skin test results” in children aged younger than 2 years, according to researchers from various institutions in Turkey and the United Kingdom.
The researchers assessed the effect of age and bacille Calmette-Guérin vaccination on the sensitivity and specificity of tuberculin skin test results of 979 children with TB. The researchers used results from an enzyme-linked immunospot assay as a reference standard.
“We computed the optimal tuberculin skin test cutoff points using receiver operating characteristic curves,” the researchers wrote.
Results indicated that with a tuberculin skin test cutoff of 10 mm or greater, tuberculin skin test sensitivity was 66% for children aged younger than 2 years; sensitivity was lower among older children (P=.006).
Furthermore, specificity was 75% for BCG–vaccinated children vs. 92% for unvaccinated children (P=.001). Sensitivity of tuberculin skin test remained less than 70% for children aged younger than 2 and remained less than 87% for BCG–vaccinated children aged 2 years and older. Overall accuracy was low for children with more than one BCG scar.
“In children aged >2 years, the accuracy of the tuberculin skin test can be improved by adjustment of cutoff points for BCG–vaccinated children but remains poor for children with >1 BCG scar,” the researchers wrote. “This methodology can define optimal tuberculin skin test cutoff points for diagnosis of TB infection tailored to target populations.”
Clin Infect Dis. 2009;48:302-312.
Because the characteristics of TB differ from country to country, recommendations on testing and treatment also vary by country. On the global level, WHO recommends practitioners consider isoniazid treatment for household contacts of patients who have contagious forms of TB, without reference to the TB skin test (TST) results.
However, for the United States, the CDC recommends the TST as a first step in diagnosing TB infection and determining course of treatment. To date, the CDC has recommended that one commercial version of the interferon-gamma release assays, QuantiFERON-TB Gold, can be used in place of TST as an aid in the diagnosis of TB infection, but with precautions about interpreting the results when the test is used for children. At this point, the use of interferon-gamma release assays like the immunospot assay has not been studied extensively in children in the United States, and the meaning of difference in results between the immunospot assay and TST is not known.
The CDC is currently working on updated guidelines for the use of interferon-gamma release assays for diagnosing TB infection in the United States. Research on this topic is ongoing to continue informing recommendations on the national level.
– Kevin Fenton, MD, PhD
Infectious Disease News Editorial Board member