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WHO Deems IGRAs Acceptable Alternative to Skin Test for Latent TB Detection

NEW YORK (360Dx) – The World Health Organization has deemed interferon gamma-release assays (IGRAs) an appropriate alternative to the tuberculin skin test (TST) for determining which patients in regions with a high disease burden may progress to active tuberculosis.

WHO issued the recommendation this week as part of broader guidelines for the diagnosis, treatment, and management of latent TB. The new recommendation is a positive for Qiagen and Oxford Immunotec, both of which market IGRAs.

In 2011 the WHO issued a policy statement on the use of IGRAs in low- and middle-income countries, concluding that there was insufficient data and low-quality evidence on their performance in these countries.

The organization also concluded at that time that neither IGRAs nor TSTs could accurately predict the risk of infected individuals developing active TB disease. Further, it had noted that because IGRAs were more expensive and technically complex than TSTs with only comparable performance, they could not be recommended as a replacement for TSTs in resource-constrained settings.

The WHO noted this week that its guideline development group (GDG) made a "strong" recommendation, based on "very low evidence," that that IGRAs are an acceptable alternative to the decades-old TST for latent TB infection (LTBI) testing.

"The GDG concluded that the comparison of TST and IGRA in the same population does not provide strong evidence that one test should be preferred over the other for predicting progression to active TB disease," the guidelines state. In addition, "TST may require significantly fewer resources than IGRA and may be more familiar to practitioners in resource-constrained settings; however, recurrent global shortages and stock-outs of TST reduce its use in scaling up programmatic management of LTBI."

Further, the WHO GDG strongly recommended the two tests as equivalent options, with relatively similar advantages and disadvantages. The group also stressed that the global shortage of TST should be addressed urgently and called for more investment into research on novel tests for LTBI with better predictive value.

Finally, the group cautioned that "imperfect performance of these tests can lead to false-negative results, particularly for young children and immunocompromised individuals such as people living with HIV."

IGRAs use tuberculosis peptides to simulate the presence of a specific TB protein, then provide a visual readout immunoassay of whole blood to measure the immune response to these peptides. Qiagen's QuantiFeron-TB Gold Plus assay directly measures interferon gamma released from white blood cells, while Oxford Immunotec's T-Spot.TB test measures the number of IFN-g-producing cells.

"The latest WHO guidelines recognize the need for high-burden countries to implement outreach and treatment for the most vulnerable patients with latent TB infection, in addition to treating those with the active disease," Masae Kawamura, senior director of TB Medical and Scientific Affairs at Qiagen, said in a statement. "We are working closely with public health authorities around the world, as well as supporting efforts to simplify the LTBI screening cascade of care using diagnostic tests and risk-based testing to improve the safety of TB preventive treatment programs."