An improved whole-blood gamma interferon assay based on the CFP21-MPT64 fusion protein

Differentiation of latent tuberculosis infection (LTBI) from a healthy, unexposed population plays a vital role in the strategy of controlling and eliminating tuberculosis (TB). Both CFP21 and MPT64, antigens encoded by the RD2 region which are restricted in the Mycobacterium tuberculosis complex, are TB-specific diagnostic candidate antigens. In this study, we designed a fusion protein by linking both CFP21 and MPT64 with a 15-amino-acid peptide, (G(4)S(1))(3), and overexpressed the fusion protein in Escherichia coli. A new whole-blood gamma interferon assay based on the recombinant fusion protein, CFP21-MPT64 (rCM-WBIA), was developed and compared with the tuberculin skin test (TST) for screening of LTBI in household contacts of patients with sputum-positive TB. rCM-WBIA had a slightly higher sensitivity (66.7%; 24/36 contacts) than that of the TST (61.1%; 22/36 contacts) for household contacts. We found that rCM-WBIA had a very high sensitivity (90.9%) and specificity (71.4%) for LTBI detection compared with TST. The overall agreement between rCM-WBIA and TST was 83.3% (k = 0.64); rCM-WBIA positivity was associated with a larger TST induration. These results suggest that rCM-WBIA, based on the recombinant fusion protein CFP21-MPT64, is a promising alternative diagnostic tool for detection of LTBI.