New TB skin test safe, easy to use

A new skin test for latent M. tuberculosis, was safe and easy to use, and addresses the problem of false positive skin test results in patients who have received the BCG vaccine, according to data presented at the American Thoracic Society 2016 International Conference.

The novel test, C-Tb, developed by Statens Serum Intitut, in Denmark, is based on the antigens ESAT-6 and CFP10. It “combines the field friendliness of the PPD-based tuberculin skin test, with the high specificity of the interferon gamma release assays, or IGRAs,” Morten Ruhwald, MD, PhD, head of human immunology at Statens Serum Institut, and lead researcher, said in a press release.

The researchers presented data from two completed phase 3 trials, TESEC-05 and TESEC-06. TESEC-05 included 1,090 participants with TB symptoms and 100 endemic controls from Cape Town, South Africa. TESEC-06 included 979 participants from 13 clinical trial sites in Catalonia, Galicia and Basque Country, in Spain, with various risk profiles for M. tuberculosis infection.

In both trials, C-Tb and tuberculin skin tests were given to participants in a double-blinded fashion to the forearm. Researchers read skin indurations 2 to 3 days after. Blood for IGRA testing (Quantiferon, QFT-GIT) was drawn prior to skin testing.

According to the researchers, C-Tb’s safety profile was acceptable and comparable to tuberculin skin test. Researchers assessed test specificity in 212 presumed-unexposed Spanish controls, in which C-Tb had comparable specificity to QFT-GIT (both 97%, P = 1), with no impact of BCG vaccination. Previous BCG vaccination had subverted tuberculin skin test specificity (62% in BCG-vaccinated participants compared with 95% among those not vaccinated, P < .001), the researchers said. Sensitivity of C-Tb and QFT-GIT was comparable in participants with confirmed TB (77% vs. 81%; P = .08). In addition, there was a strong trend in increasing C-Tb test positivity with M. tuberculosis exposure. – by Jason Laday

Disclosure: Healio Family Medicine could not confirm the researchers’ relevant financial disclosures.

Reference:

Ruhwald M, et al. Diagnostic accuracy of the novel C-Tb skin test for latent M. tuberculosis infection: Results from two phase 3 clinical trials. Presented at the American Thoracic Society International Conference, May 13 to 18, 2016; San Francisco.