Discrepancy between the tuberculin skin test and the whole-blood interferon gamma assay for the diagnosis of latent tuberculosis infection in an intermediate tuberculosis-burden country.
BACKGROUND
A recently developed whole-blood interferon gamma (IFN-gamma) assay based on stimulation with the Mycobacterium tuberculosis-specific antigens early secreted antigenic target 6 and culture filtrate protein 10 shows promise for the diagnosis of latent tuberculosis (TB) infection.


OBJECTIVE
To compare the tuberculin skin test (TST) and the whole-blood IFN-gamma assay in the diagnosis of latent TB infection according to the intensity of exposure.


METHODS
A prospective comparison between the whole-blood IFN-gamma assay and the TST using a 2-TU dose of purified protein derivative RT23 in a population with intermediate TB burden was conducted sequentially between February 1, 2004, and February 28, 2005, in a Korean tertiary referral hospital.


METHODS
Of 273 participants, 220 (95.7%) had received BCG vaccine. Participants were grouped according to their risk of infection: group 1, no identifiable risk of M tuberculosis infection (n = 99); group 2, recent casual contacts (n = 72); group 3, recent close contacts (n = 48); group 4, bacteriologically or pathologically confirmed TB patients (n = 54).


METHODS
Levels of agreement between the TST and the IFN-gamma assay and the likelihood of infection in the various groups.


RESULTS
For the TST with a 10-mm induration cutoff, the positive response rate in group 1 was 51%; group 2, 60%; group 3, 71%, and group 4, 78%. For the IFN-gamma assay, the positive response rate in group 1 was 4%; group 2, 10%; group 3, 44%; and group 4, 81%. The overall agreement between the TST and the IFN-gamma assay in healthy volunteers was kappa = 0.16. The odds of a positive test result per unit increase in exposure across the 4 groups increased by a factor of 5.31 (95% confidence interval [CI], 3.62-7.79) for the IFN-gamma assay and by a factor of 1.52 (95% CI, 1.20-1.91) for the TST (P<.001). Using a 15-mm induration cutoff for the TST did not make a substantial difference to the test results.


CONCLUSIONS
The IFN-gamma assay is a better indicator of the risk of M tuberculosis infection than TST in a BCG-vaccinated population.
Kang
Young Ae
YA
Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine and Lung Institute, Seoul National University College of Medicine, Seoul, Republic of Korea.
Lee
Hye Won
HW
Yoon
Ho Il
HI
Cho
BeLong
B
Han
Sung Koo
SK
Shim
Young-Soo
YS
Yim
Jae-Joon
JJ
D000293:Adolescent; D000328:Adult; D000368:Aged; D000942:Antigens, Bacterial / Q000276:immunology; D001500:BCG Vaccine; D001426:Bacterial Proteins / Q000276:immunology; D005260:Female; D006801:Humans; D007371:Interferon-gamma / Q000276:immunology*; D007723:Korea; D008297:Male; D008875:Middle Aged; D009169:Mycobacterium tuberculosis / Q000276:immunology / Q000302:isolation & purification* / Q000502:physiology; D012306:Risk; D014374:Tuberculin Test; D014397:Tuberculosis, Pulmonary / Q000175:diagnosis* / Q000276:immunology; D017735:Virus Latency
D000942:Antigens, Bacterial; D001500:BCG Vaccine; D001426:Bacterial Proteins; C117940:CFP-10 protein, Mycobacterium tuberculosis; C093276:ESAT-6 protein, Mycobacterium tuberculosis; D007371:Interferon-gamma