Prospective evaluation of a whole-blood test using Mycobacterium tuberculosis-specific antigens ESAT-6 and CFP-10 for diagnosis of active tuberculosis

Abstract

A new immunodiagnostic test based on the Mycobacterium tuberculosis-specific antigens CFP-10/ESAT-6(QFT-RD1) has been launched as an aid in the diagnosis of latent tuberculosis (TB) infection (LTBI). The aim of this study was to evaluate this test for the diagnosis of active TB. Eighty-two patients with suspicion of TB and 39 healthy BCG-vaccinated persons were enrolled. Forty-eight had active TB, 25 did not, and 9 were excluded. Sensitivity and specificity of the test for active TB were evaluated in a prospective blinded manner in patients suspected of TB. The sensitivity of the QFT-RD1 was 85% (40/48; confidence interval [CI], 75 to 96), and it was higher than the sensitivity of microscopy, 42% (20/48; CI, 27 to 56; P = 0.001), and culture, 59% (27/46; CI, 44 to 73; P = 0.009). Of patients with extrapulmonary TB, 92% (12/13) were QFT-RD1 positive, whereas only 31% (4/13) were positive by microscopy and 42% (5/12) by culture (P < 0.05), and 87% (13/15) of those who were negative by both microscopy and culture were QFT-RD1 positive. By combining microscopy and culture with the QFT-RD1 test, sensitivity increased to 96% (CI, 90 to 102). Ten of 25 (40%) non-TB patients were QFT-RD1 positive, resulting in a specificity of 60%. However, 80% (8/10) of these had risk-factors for TB, indicating latent infection in this group. In healthy controls, only 3% (1/39) were QFT-RD1 positive. In conclusion, the QFT-RD1 test is sensitive for diagnosis of TB, especially in patients with negative microscopy and culture. The accuracy of the QFT-RD1 test will vary with the prevalence of LTBI. We suggest that the QFT-RD1 test could be a very useful supplementary tool for the diagnosis of TB.

Figures

FIG. 1.

Individual IFN-γ response to PPD and either of the RD1 antigens ESAT-6 or CFP-10 (E6/C10) (for each patient the highest IFN-γ responses) are shown. In the left panel, the responses of the TB suspect population (n = 73); and in the right panel, the responses for the healthy control population (n = 39). The cutoff value for a positive response was set at 0.35 IU/ml for ESAT-6 and CFP-10 and arbitrarily at 1.5 IU/ml for PPD. The dotted lines represent the cutoff for the specific antigens and PPD.

FIG. 2.

Results of the QFT-RD1 test, clinical, and microbiological investigations of all 73 TB suspects are shown in this diagram. Patients with a response in the QFT-RD1 test above 0.35 IU/ml were considered positive. The 48 patients were classified as having TB if either culture was positive for M. tuberculosis or histology and clinical findings indicated mycobacterial infection (i.e., granulomatous necrosis or identification of acid-fast bacilli) or the physician decided that the patient suffered from TB based on clinical findings, patient history, and/or X-ray, and the patients responded appropriately to a full course of anti-TB treatment. In total, 33 TB patients were positive by microscopy and/or culture and 15 had negative microscopy and culture and were diagnosed by clinical criteria. The final classification of a non-TB patient was based on the fact that M. tuberculosis was not found by conventional screening and another diagnosis was found (n = 15) or no other diagnosis was found (n = 10), but the patient recovered without an anti-TB treatment and TB was not diagnosed during follow-up.