Previously, sensitivity and specificity have been studied in groups of patients with already known microscopy- or culture-confirmed TB, and the aim of this prospective study was to evaluate the sensitivity and specificity of the QFT-RD1 test for the diagnosis of active TB. The patients in the present study were enrolled prospectively, and data were analyzed without knowing the final diagnosis, thereby allowing us to compare sensitivity of the QFT-RD1 test directly with microscopy and culture. To the best of our knowledge, this is the first study to compare the QFT-RD1 test with conventional microbiological diagnostic methods. Our study showed a high sensitivity of the QFT-RD1 test (85%) compared to microscopy (42%) and culture (59%), with a positive predictive value of 80% (41/51). Interestingly, we found that as many as 87% of the patients with negative microscopy and negative culture were QFT-RD1 positive, and by combining microscopy and culture with the QFT-RD1 test, the overall sensitivity increased to 96%, together suggesting that the QFT-RD1 may be of use for the diagnosis of active TB.
Diagnosis of extrapulmonary TB and culture-negative TB remains a challenge; our study demonstrates that the QFT-RD1 test has a particular advantage in this group of patients and may greatly improve the diagnosis of extrapulmonary TB and microscopy- and culture-negative TB. In line with previous studies (21
), we found no difference in the sensitivity of the QFT-RD1 test between patients with pulmonary TB and extrapulmonary TB, whereas the sensitivity of the QFT-RD1 test (85%) was significantly higher than microscopy (42%) and culture (59%). In addition, we found that 87% of the patients who were negative by culture or microscopy were detected by the QFT-RD1 test, suggesting the QFT-RD1 test could improve diagnosis of active TB. Taken together, our findings suggest that the RD1 test may be a useful supplementary tool in the diagnosis of extrapulmonary TB and microscopy- and culture-negative TB. Studies based on larger groups of well-characterized TB patients are warranted to confirm these findings.
In healthy BCG-vaccinated controls without known exposure to M. tuberculosis
, an IFN-γ response to the RD1 antigens was detected in only 1 out of 39 (3%), indicating a high M. tuberculosis
specificity. In contrast to the RD-1 antigens, IFN-γ responses to PPD were present in both patients and healthy controls, confirming the lack of specificity of PPD in healthy BCG-vaccinated individuals (6
). High specificity of the QFT-RD1 is a consistent finding in populations selected because they have a very low risk of exposure to M. tuberculosis
). Given the previously reported very high specificity of the RD1-based tests in low-risk populations (3
), it was interesting to find that 10 of the 25 non-TB patients (40%) were QFT-RD1 positive, resulting in a lower specificity of 60% for the diagnosis of active TB. Thus, although the QFT-RD1 test detected most patients with active TB, it was also positive in a number of patients in whom active TB could not be demonstrated. ESAT-6 and CFP-10 have not been shown to be able to discriminate between active and LTBI, and the non-TB patients had one or several risk factors for TB, suggesting that the positive QFT-RD1 responses were due to the presence of LTBI.
Since the QFT-RD1 test detects both active TB and LTBI, the applicability of the test for active TB will depend on the risk—in the study population—of being exposed to M. tuberculosis
. The present study comprised patients with risk factors, such as immigrants from high-endemicity regions, homeless people, and patients with intravenous drug abuse, possibly latently infected and thus QFT-RD1 positive but suffering from non-TB diseases. In low-risk groups, i.e., patients with chronic obstructive lung diseases or patients under investigation for tumor suspect infiltrates on X-ray, the accuracy of this test should be higher, and preliminary data have shown that only 17% of such patients with no known M. tuberculosis
exposure were positive by QFT-RD1 (unpublished data). Thus, the positive and negative predictive value of the test for active TB will vary with the sample size and the prevalence of LTBI in the study population. The latter may be a particular problem in high-TB-endemicity regions, where there is a high prevalence of both active and LTBI and where 30 to 50% of healthy individuals have been shown to respond to the RD-1 antigens (8
). However, a negative test result for healthy individuals may be useful as a tool to exclude TB infection, and a positive result may help in identifying persons who are candidates for preventive chemotherapy or intensified clinical follow-up.
At present we cannot predict who or how many of the ESAT-6- or CFP-10-responsive non-TB patients will develop TB. We have previously shown an association between ESAT-6 responsiveness and later progression to TB in a group of healthy exposed contacts (9
). In the present study, 10 QFT-RD1 responsive non-TB patients did not develop active TB within the observation period of almost 2 years, but 1 patient with pneumonia and pericarditis, who was initially categorized as a non-TB patient, progressed clinically and developed miliary TB after having received intensive immunosuppressive treatment. The patient who developed TB was QFT-RD1 positive when presenting with symptoms 3 months before progressing to miliary TB, but at that time active TB was not found despite intensive investigations (24
). This case suggests that RD-1-responsive patients may be at risk of progressing to active TB during immune suppression. The use of immune suppression with biological agents like anti-tumor necrosis factor alpha antibodies is increasing, and reactivation of LTBI is one of the most feared side effects (14
). Screening for LTBI with the QFT-RD1 test may turn out to be extremely useful in this situation.
Immunosuppression due to HIV or severe TB is a question of potential concern with any assay based on cellular immune responses. In the present study, seven TB patients were QFT-RD1 negative. Four of these were positive by microscopy and culture. Interestingly, three of these four individuals suffered from severe TB, and the fourth was HIV positive with a low CD4 cell count. These data indicate that more-advanced disease may be associated with weak T-cell responses, which has been suggested in previous studies (22
). The performance of this test in immunocompromised patients is under investigation. Together, these findings underline the necessity of combining the QFT-RD1 test with conventional microscopy and culture for maximal diagnostic sensitivity.
In conclusion, our data suggest that the QFT-RD1 test could be a very useful supplementary tool for the diagnosis of active TB, especially in patients with microscopy- and culture-negative and extrapulmonary TB. Together with negative microscopy and culture, a negative QFT-RD1 test may be used to exclude TB in immunocompetent individuals.