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Randomised Clinical Trial Investigating the Specificity of a Novel Skin Test (C-Tb) for Diagnosis of M. tuberculosis Infection

Background Tuberculin skin testing is simple and relatively inexpensive, but the specificity of PPD is affected by BCG vaccination. Objective Determine optimal dose and specificity of recombinant ESAT-6 and CFP-10 (C-Tb) produced in Lactococcus lactis for diagnosis of M. tuberculosis infection. Methods In a dose finding phase I trial 0.01 or 0.1 µg preserved and unpreserved C-Tb was injected by Mantoux technique in 38 patients with active tuberculosis and induration responses measured. In a phase II specificity trial in 151 uninfected, BCG vaccinated participants 0.1 µg C-Tb was compared to 2 TU PPD. Results 0.1 µg C-Tb gave a median induration of 15 mm after 2 days. Phenol preservation did not affect the response. The specificity of C-Tb was 99.3% (95% CI 96–100%) regarding indurations ≥5 mm as a positive outcome. This was higher than the specificity of PPD (63% using a cut-off of 5 mm or 92% using a cut-off of 15 mm to adjust for non-specific BCG responses). Local adverse reactions following C-Tb injection included transient itching and discomfort as expected components of the immune response. Conclusion C-Tb offers a simple and convenient skin test to diagnose M. tuberculosis infection using a single, universal cut-off unaffected by BCG vaccination. Trial Registration ClinicalTrials.gov NCT01033929 and NCT01241188.