Very little longitudinal information is available regarding the performance of T cell-based tests for Mycobacterium tuberculosis infection. To address this deficiency, we conducted a longitudinal assessment of the enzyme-linked immunosorbent spot test (ELISPOT) test in comparison to the standard tuberculin skin test (TST).
Methods and Findings
In tuberculosis (TB) contacts we repeated ELISPOT tests 3 mo (n = 341) and 18 mo (n = 210) after recruitment and TSTs at 18 mo (n = 130). We evaluated factors for association with conversion and reversion and investigated suspected cases of TB. Of 207 ELISPOT-negative contacts, 51 (24.6%) had 3-mo ELISPOT conversion, which was associated with a positive recruitment TST (odds ratio [OR] 2.2, 95% confidence interval [CI] 1.0–5.0, p = 0.048) and negatively associated with bacillus Calmette-Guérin (BCG) vaccination (OR 0.5, 95% CI 0.2–1.0, p = 0.06). Of 134 contacts, 54 (40.2%) underwent 3-mo ELISPOT reversion, which was less likely in those with a positive recruitment TST (OR 0.3, 95% CI 0.1–0.8, p = 0.014). Between 3 and 18 mo, 35/132 (26.5%) contacts underwent ELISPOT conversion and 28/78 (35.9%) underwent ELISPOT reversion. Of the 210 contacts with complete results, 73 (34.8%) were ELISPOT negative at all three time points; 36 (17.1%) were positive at all three time points. Between recruitment and 18 mo, 20 (27%) contacts had ELISPOT conversion; 37 (50%) had TST conversion, which was associated with a positive recruitment ELISPOT (OR 7.2, 95% CI 1.4–37.1, p = 0.019); 18 (32.7%) underwent ELISPOT reversion; and five (8.9%) underwent TST reversion. Results in 13 contacts diagnosed as having TB were mixed, but suggested higher TST sensitivity.
Both ELISPOT conversion and reversion occur after M. tuberculosis exposure. Rapid ELISPOT reversion may reflect M. tuberculosis clearance or transition into dormancy and may contribute to the relatively low reported ELISPOT conversion rate. Therefore, a negative ELISPOT test for M. tuberculosis infection should be interpreted with caution.
Philip Hill and colleagues report that both ELISPOT conversion and reversion occur afterM. tuberculosis exposure in an endemic country and that the ELISPOT results agree poorly with results from the tuberculin skin test.
Tuberculosis is a contagious bacterial infection, usually of the lungs. People with active tuberculosis spread the causative bacterium (Mycobacterium tuberculosis) in airborne droplets whenever they cough or sneeze. Most people exposed to M. tuberculosis in this way never become ill—their immune system successfully contains the infection. However, the bacteria remain dormant in the body and can cause disease years later if host immunity declines because of, for example, infection with the human immunodeficiency virus (HIV). Consequently, to control the spread of tuberculosis, individuals who have been in contact with people with active tuberculosis need to be tested for infection with M. tuberculosis and treated with antituberculosis drugs if positive. The standard test for infection is the tuberculin skin test (TST). In this, bacterial antigens (proteins that the immune system recognize as foreign) are injected under the skin. The immune system of infected individuals attacks the antigen and produces a hard swelling at the injection site. Unfortunately, this test does not detect all M. tuberculosis infections and an alternative, laboratory-based test has recently been developed. During M. tuberculosis infections, immune system cells called T lymphocytes produce interferon gamma. This protein activates macrophages, immune system cells that kill bacteria. The ELISPOT (enzyme-linked immunosorbent spot) test measures interferon gamma production by T lymphocytes.
Why Was This Study Done?
Commercial ELISPOT tests are available for the diagnosis of M. tuberculosis infection, but little is known about how they perform when used in repeat tests in individuals or whether the TST or ELISPOT test is better at predicting later development of tuberculosis. In this study, the researchers investigated these questions in a longitudinal assessment of the ELISPOT test in Gambians exposed to active tuberculosis.
What Did the Researchers Do and Find?
The researchers recruited people who had been in contact with active tuberculosis, did ELISPOT tests and TSTs at recruitment, then repeated the ELISPOT test after three months and both tests in some participants after 18 months. They analyzed how often ELISPOT conversion (a change from a negative to a positive result indicating the development of an active immune response) and reversion (a change from a positive to a negative result reflecting clearance of the bacteria or its entry into a dormant state) occurred, whether the TST results mirrored these changes, and which characteristics of the participants were associated with conversion or reversion. A quarter of participants who initially had a negative ELISPOT result had a positive result at three months, a conversion that was associated with a positive TST at recruitment. ELISPOT reversion at three months, by contrast, was associated with an initially negative TST and occurred in nearly half the participants. However, about a third of the participants had negative ELISPOT results at all three time points and a fifth had positive results at all times. Overall, the two tests agreed in 73% and 60% of the participants at recruitment and at 18 months, respectively. Finally, among the 13 contacts who developed active tuberculosis, some were initially positive in both tests but others showed subsequent conversion in one, both or neither test.
What Do These Findings Mean?
These findings indicate that both ELISPOT conversion and reversion occur after initial screening for M. tuberculosis infection. In addition, they suggest that the immune system responses to M. tuberculosis detected by TST and the ELISPOT test occur over different time scales and so the two tests might differ in their ability to detect M. tuberculosis infections at different times after exposure to the bacteria. Because very few contacts developed active tuberculosis, the findings do not indicate which test best predicts disease development after M. tuberculosis infection. Further studies are needed to provide this information and to unravel the complexities of ELISPOT conversion and reversion after exposure to M. tuberculosis. Importantly, however, the high frequency of ELISPOT reversion seen in this study suggests that a negative ELISPOT result may not reflect a lack of infection after exposure to M. tuberculosis and must, therefore, be interpreted with caution.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0040192.
The US Centers for Disease Control and Prevention provide fact sheets from the Division of Tuberculosis Elimination about tuberculosis, its testing and diagnosis, and its treatment
MedlinePlus Encyclopedia contains information on tuberculosis and the tuberculin skin test (in English and Spanish)
The American Lung Association offers fact sheets on tuberculosis and on the tuberculin skin test