Improved diagnostic evaluation of suspected tuberculosis

Abstract

Background: The role of new T-cell-based blood tests for tuberculosis in the diagnosis of active tuberculosis is unclear.

Objective: To compare the performance of 2 interferon-gamma assays and tuberculin skin testing in adults with suspected tuberculosis.

Design: Prospective study conducted in routine practice.

Setting: 2 urban hospitals in the United Kingdom.

Patients: 389 adults, predominantly of South Asian and black ethnicity, with moderate to high clinical suspicion of active tuberculosis.

Intervention: Tuberculin skin testing, the enzyme-linked immunospot assay (ELISpot) incorporating early secretory antigenic target-6 and culture filtrate protein-10 (standard ELISpot), and ELISpot incorporating a novel antigen, Rv3879c (ELISpot(PLUS)) were performed during diagnostic assessment by independent persons who were blinded to results of the other test.

Measurements: Sensitivity, specificity, predictive values, and likelihood ratios.

Results: 194 patients had a final diagnosis of active tuberculosis, of which 79% were culture-confirmed. Sensitivity for culture confirmed and highly probable tuberculosis was 89% (95% CI, 84% to 93%) with ELISpot(PLUS), 85% (CI, 79% to 90%) with standard ELISpot, 79% (CI, 72% to 85%) with 15-mm threshold tuberculin skin testing, and 83% (CI, 77% to 89%) with stratified thresholds of 15 and 10 mm in vaccinated and unvaccinated patients, respectively. The ELISpot(PLUS) assay was more sensitive than tuberculin skin testing with 15-mm cutoff points (P = 0.01) but not with stratified cutoff points (P = 0.10). The ELISpot(PLUS) assay had 4% higher diagnostic sensitivity than standard ELISpot (P = 0.02). Combined sensitivity of ELISpot(PLUS) and tuberculin skin testing was 99% (CI, 95% to 100%), conferring a negative likelihood ratio of 0.02 (CI, 0 to 0.06) when both test results were negative.

Limitations: Local standards for tuberculin skin testing differed from others used internationally. The study sample included few immunosuppressed patients.

Conclusion: The ELISpot(PLUS) assay is more sensitive than standard ELISpot and, when used in combination with tuberculin skin testing, enables rapid exclusion of active infection in patients with moderate to high pretest probability of tuberculosis.

Appendix Figure 1

Study flow diagram, stratified by final diagnosis and then by test result. These are the same data shown in Figure 1 but displayed in a different manner to allow scrutiny of the entire raw data set. BCG = bacille Calmette–Guérin; ELISpot = enzyme-linked immunospot assay incorporating early secretory antigenic target-6 and culture filtrate protein-10; ELISpot^PLUS = enzyme-linked immunospot assay incorporating early secretory antigenic target-6, culture filtrate protein-10 and Rv3879c; TB = tuberculosis; TST= tuberculin skin testing. *Results not available because of history of previous TB (clinically contraindicated) (45 patients), patient did not return for reading (8 patients), result not recorded (8 patients), reason unknown (3 patients), patient death (1 patient), test performed elsewhere (1 patient), or patient declined consent (1 patient). †Results indeterminate because of no achievement of positive control (11 patients), high background (1 patient), peptide contamination (1 patient), inconclusive assay (1 patient), or defective ELISpot plate (1 patient). ‡Tuberculin skin test results were based on a 15-mm cutoff point and considered positive if induration was ≥15 mm on the Mantoux test or grade 3 to 4 on the Heaf test regardless of BCG vaccination status.

Appendix Figure 2

Likelihood ratios, sensitivities, and specificities of tests used in combination for diagnostic evaluation, using TST as the first test. Data are for patients in whom results on both tests were available (n = 265). Except where stated, numbers are likelihood ratios with 95% confidence intervals. Tuberculin skin test thresholds for positivity were induration ≥15 mm on the Mantoux test or grade 3 to 4 on the Heaf test. ELISpot = enzyme-linked immunospot incorporating early secretory antigenic target-6 and culture filtrate protein-10; ELISpot^PLUS = enzyme-linked immunospot incorporating early secretory antigenic target-6, culture filtrate protein-10 and Rv3879c; TB = tuberculosis; TST = tuberculin skin testing. Top. TST followed by ELISpot. *Combined sensitivity of 1 or more positive results from tests used in combination, 97% (CI, 93% to 99%). †Combined specificity for a double negative result from tests used in combination, 62% (CI, 53% to 71%). Bottom. TST followed by ELISpotPLUS. ‡Combined sensitivity of 1 or more positive results from tests used in combination, 99% (CI, 95% to 100%). §Combined specificity for a double negative result from tests used in combination, 61% (CI, 52% to 70%).

Figure 1

Study flow diagram. BCG = bacille Calmette–Guérin; ELISpot = enzyme-linked immunospot assay incorporating early secretory antigenic target-6 and culture filtrate protein-10; ELISpot^PLUS = enzyme-linked immunospot assay incorporating early secretory antigenic target-6, culture filtrate protein-10 and Rv3879c; TB = tuberculosis; TST= tuberculin skin testing. *Results indeterminate because of no achievement of positive control (11 patients), high background (1 patient), peptide contamination (1 patient), inconclusive assay (1 patient), or defective ELISpot plate (1 patient). †Results not available because of history of previous TB (clinically contraindicated) (45 patients), patient did not return for reading (8 patients), result not recorded (8 patients), reason unknown (3 patients), patient death (1 patient), test performed elsewhere (1 patient), or patient declined consent (1 patient). ‡Tuberculin skin test results were based on a 15-mm cutoff point and considered positive if induration was ≥15 mm on the Mantoux test or grade 3 to 4 on the Heaf test regardless of BCG vaccination status.

Figure 2

Likelihood ratios, sensitivities, and specificities of tests used in combination, using ELISpot or ELISpot^PLUS first. Data are for patients in whom results on both tests were available (n = 265). Except where stated, values are likelihood ratios with 95% confidence intervals. Tuberculin skin test thresholds for positivity were induration ≥15 mm on the Mantoux test or grade 3 to 4 on the Heaf test. ELISpot = enzyme-linked immunospot incorporating early secretory antigenic target-6 and culture filtrate protein-10; ELISpot^PLUS = enzyme-linked immunospot incorporating early secretory antigenic target-6, culture filtrate protein-10 and Rv3879c; TB = tuberculosis; TST = tuberculin skin testing. Top. ELISpot followed by TST. *Combined sensitivity of 1 or more positive results from tests used in combination, 97% (CI, 93% to 99%). †Combined specificity for a double negative result from tests used in combination, 62% (CI, 53% to 71%). Bottom. ELISpot^PLUS followed by TST. ‡Combined sensitivity of 1 or more positive results from tests used in combination, 99% (CI, 95% to 100%). §Combined specificity for a double negative result from tests used in combination, 61% (CI, 52% to 70%).