The ELISPOT has been used in research settings and in institutional outbreaks, and is being evaluated by the Food and Drug Administration for the diagnosis of LTBI. However, it has not previously been used in contact investigations in the United States or under routine program conditions. Our study is the first to address these critical issues. Because of the lack of a gold standard for LTBI, sensitivity and specificity of the ELISPOT and tuberculin skin tests cannot be calculated. Therefore, we derived a contact score that quantified the risk of acquiring recent tuberculosis infection on the basis of data from prospectively obtained, standardized interviews. We investigated the relationship between ELISPOT and tuberculin skin test results and this contact score, which served as a surrogate measure for likelihood of LTBI. ELISPOT results were more strongly associated with the contact score than were tuberculin skin test results in the entire study group and in all subpopulations evaluated, although the differences between these two tests were not statistically significant.
Persons born in the United States and those who were not vaccinated with BCG were at relatively low risk for preexisting LTBI, and their principal risk for LTBI was from recent exposure to a patient with tuberculosis. In this setting, higher contact scores were significantly associated with positive ELISPOT and tuberculin skin test results ( and ). The contact score appeared to relate more strongly to the ELISPOT than to the tuberculin skin test results, but this difference was not statistically significant. In these subpopulations, approximately 30% had positive skin tests, consistent with prior studies indicating that 30 to 40% of close contacts of patients with tuberculosis become infected with M. tuberculosis
). The frequencies of positive ELISPOT and tuberculosis skin tests were essentially identical in those in the lower quartiles of contact score. However, the ELISPOT was positive in 43% (20/47) of U.S.-born persons and 45% (24/53) of non–BCG-vaccinated persons in the highest contact score quartile. The tuberculin skin test was positive in 34% (16/47) and 38% (20/53) of these groups, respectively. This suggests that the ELISPOT may be more sensitive than the tuberculin skin test for detecting LTBI in contacts of patients with tuberculosis. Alternatively, persons who are transiently infected with M. tuberculosis
may mount a T-cell response that yields a positive ELISPOT but not a positive tuberculin skin test. Long-term follow-up studies of contacts with positive ELISPOT results but negative tuberculin skin tests are critical to separate these possibilities.
Among foreign-born and BCG-vaccinated persons (99% of whom were foreign-born), in the two lowest contact score quartiles, approximately 70% had positive tuberculin skin tests and 40% had positive ELISPOT results. These findings suggest a high prevalence of preexisting LTBI, which may explain the lack of a strong relationship between both ELISPOT and tuberculin skin test results and the contact score in these subpopulations ( and ), because neither test distinguishes recent from longstanding LTBI. Overall, in foreign-born and BCG-vaccinated persons, approximately two-thirds had positive tuberculin skin tests and half had positive ELISPOT results (p < 0.0001). Seventy-one percent of those with positive tuberculin skin tests and negative ELISPOT results were BCG-vaccinated, compared with only 33% of those with positive ELISPOT results and negative tuberculin skin tests (). These findings, prior publications (8
), and the fact that the CFP10 and ESAT-6 antigens used in the ELISPOT are not present in BCG support the contention that BCG vaccination can result in positive tuberculin skin tests but not positive ELISPOT results. The improved specificity of the ELISPOT suggests that using it to screen foreign-born and BCG-vaccinated contacts of patients with tuberculosis would reduce the number of persons incorrectly diagnosed with LTBI, limiting the cost and toxicity of inappropriately treating these individuals. However, positive ELISPOT results will identify persons with LTBI acquired either recently or in the distant past.
In evaluating contacts of patients with tuberculosis who are initially tuberculin-negative, skin testing is repeated 12 wk later, because the delayed-type hypersensitivity response to mycobacterial antigens may not be detectable until this time. We found that repeat ELISPOTs and tuberculin skin tests showed the same results as initial tests in 84 to 86% of cases. Persons with initial positive and subsequent negative ELISPOT results may have transient infection that causes temporary T-cell reactivity to mycobacterial antigens. Initial negative and subsequent positive ELISPOT results may reflect the fact that T-cell reactivity to mycobacterial antigens does not occur immediately after infection, as with contacts who develop skin test conversion after an initial negative test. Among persons born in the United States with a low prevalence of prior LTBI, seven of nine ELISPOT conversions and five of seven tuberculin skin test conversions occurred in persons in the two highest contact score quartiles (compare and Table E1). This suggests that conversions resulted from recent infection by the index case. The odds ratio for having a positive initial or repeat ELISPOT was 3.9 in the highest contact score quartile, compared with 2.7 when only the initial ELISPOT results were used. The odds ratio for having a positive initial or repeat tuberculin skin test was 2.4 in the highest contact score quartile, compared with 1.9 when only the initial tuberculin skin test was used. Therefore, as with the tuberculin skin test, a repeat ELISPOT probably permits detection of additional cases of recent tuberculosis infection.
Most studies comparing the results of new blood tests for LTBI with those of tuberculin skin tests have evaluated results in groups of persons with differing risk of LTBI, on the basis of broad risk factors such as household contact with patients with tuberculosis, country of birth, and intravenous drug use (20
). The current study has taken the evaluation of blood tests an important step further by developing a contact score on the basis of prospectively obtained interview data, which provides an estimate of an individual's likelihood for having LTBI after contact with a patient with tuberculosis. It will be important to evaluate the validity of the contact score in other populations. A potential shortcoming of this approach is the imprecision inherent in estimating exposure to tuberculosis. Information obtained regarding contact with the source case may not have been completely accurate, and we did not consider the duration of infectiousness of the index case, because this variable could not be precisely ascertained. However, we do not believe that this substantially affected our results because they were robust in sensitivity analyses that incorporated wide variations in the method to compute the contact score. Another possible problem is that our study population included a relatively high percentage of Hispanic subjects, and the results may differ in other ethnic groups.
We observed that the contact score may be more strongly associated with the ELISPOT than with the tuberculin skin test results in the entire population and in all subgroups evaluated, although these differences were not statistically significant. Ewer and colleagues (12
) reported similar findings in a tuberculosis outbreak at a school, except that the extent of exposure to tuberculosis correlated significantly better with ELISPOT than with tuberculin skin test results. The clearer superiority of the ELISPOT in Ewer and colleagues' study may have resulted from several factors. First, extent of exposure to tuberculosis in the outbreak was based largely on student classroom schedules and was probably more accurately quantified than in our contacts, who were evaluated under routine program conditions. Second, tuberculin skin testing in the outbreak was performed by the Heaf method, which is probably less accurate than the Mantoux technique that we used (2
). Finally, Ewer and colleagues evaluated a larger population of contacts at low risk for preexisting LTBI (459 students born in the United Kingdom), compared with 185 U.S.-born persons in the current report. If our findings are extrapolated to a larger population, 1,300 subjects are needed to have 80% power to show that the ELISPOT is superior to the tuberculin skin test. With 185 subjects, we had only 18% power to detect this difference.
Our findings contrast with those of a study in Gambia comparing the tuberculin skin test and the ELISPOT in 735 household contacts of patients with tuberculosis (20
). In this setting, the tuberculin skin test appeared to correlate more closely than the ELISPOT with extent of exposure to the index case, although the difference was not statistically significant. The authors suggested that the sensitivity of the ELISPOT was reduced because it uses only two antigens, ESAT-6 and CFP10. We believe that this is unlikely given our current findings and prior reports demonstrating that the ELISPOT is of equal or greater sensitivity than the tuberculin skin test for detection of LTBI (11
). The reasons for the apparently reduced sensitivity of the ELISPOT in the Gambian study remain speculative.
LTBI was recently diagnosed in 32 persons evaluated during a school tuberculosis outbreak by measuring IFN-γ concentrations in supernatants of whole blood incubated with ESAT-6 and CFP10 (8
), and this test (QuantiFeron-TB Gold) is available in Europe and the United States. Unlike this test, the ELISPOT measures the number of IFN-γ–producing cells. Until recently, ELISPOT required manual isolation of mononuclear cells from blood. However, it has now been simplified, using automated cell separation vacutainers, and is marketed as the T Spot-TB test, which has European regulatory approval and is being evaluated by the Food and Drug Administration.
We found that ELISPOT results were interpretable in more than 99% of cases. In contrast, in most clinical settings, a large proportion of tuberculin skin tests are not interpreted because the patient does not return for the second visit. Our results reflect optimal performance of the tuberculin skin test, which was performed by trained, experienced individuals. In many health care settings, tuberculin skin tests are performed by personnel with variable expertise, and the reliability of skin testing is likely to be substantially lower under these circumstances. As a result, the ELISPOT is logistically superior to the tuberculin skin test because it requires a single visit and is less operator-dependent.
In conclusion, in a large group of contacts of patients with tuberculosis evaluated in a routine clinical setting, the ELISPOT appeared to be at least as sensitive as the tuberculin skin test for detection of LTBI in U.S.-born persons, and was more specific than the skin test in BCG-vaccinated persons. The ELISPOT was technically feasible and robust, and is likely to be a more accurate means to diagnose LTBI than the tuberculin skin test. The improved specificity of the ELISPOT will reduce the number of false-positive diagnoses of LTBI in BCG-vaccinated individuals. As the prevalence of LTBI declines in low-prevalence countries, an increasing proportion of positive tuberculin skin test results will be due to prior BCG vaccination. Therefore, the specificity of the ELISPOT makes it an important tool for control programs aimed at eliminating tuberculosis.