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A new immunodiagnostic test based on the Mycobacterium tuberculosis-specific antigens CFP-10/ESAT-6(QFT-RD1) has been launched as an aid in the diagnosis of latent tuberculosis (TB) infection (LTBI). The aim of this study was to evaluate this test for the diagnosis of active TB. Eighty-two patients with suspicion of TB and 39 healthy BCG-vaccinated persons were enrolled. Forty-eight had active TB, 25 did not, and 9 were excluded. Sensitivity and specificity of the test for active TB were evaluated in a prospective blinded manner in patients suspected of TB. The sensitivity of the QFT-RD1 was 85% (40/48; confidence interval [CI], 75 to 96), and it was higher than the sensitivity of microscopy, 42% (20/48; CI, 27 to 56; P = 0.001), and culture, 59% (27/46; CI, 44 to 73; P = 0.009). Of patients with extrapulmonary TB, 92% (12/13) were QFT-RD1 positive, whereas only 31% (4/13) were positive by microscopy and 42% (5/12) by culture (P < 0.05), and 87% (13/15) of those who were negative by both microscopy and culture were QFT-RD1 positive. By combining microscopy and culture with the QFT-RD1 test, sensitivity increased to 96% (CI, 90 to 102). Ten of 25 (40%) non-TB patients were QFT-RD1 positive, resulting in a specificity of 60%. However, 80% (8/10) of these had risk-factors for TB, indicating latent infection in this group. In healthy controls, only 3% (1/39) were QFT-RD1 positive. In conclusion, the QFT-RD1 test is sensitive for diagnosis of TB, especially in patients with negative microscopy and culture. The accuracy of the QFT-RD1 test will vary with the prevalence of LTBI. We suggest that the QFT-RD1 test could be a very useful supplementary tool for the diagnosis of TB.
Tuberculosis (TB) remains an important and potentially fatal infection in humans, and it is estimated that one-third of the world population is infected with Mycobacterium tuberculosis (30). The most powerful tools in any TB control program are prompt diagnosis and successful treatment of patients with active contagious disease. Diagnosis of TB is often based on clinical suspicion and appropriate response to anti-TB therapy. While culture confirmation is optimal, this is frequently not possible. Detection of acid-fast bacilli or granulomatous lesions are not optimal means of diagnosing TB but are strongly indicative. The diagnosis of patients with microscopy- and culture-negative or extrapulmonary TB is complicated and is often delayed due to the need for invasive diagnostic procedures, and no definitive diagnostic test can exclude infection with M. tuberculosis. The tuberculin skin test (TST) has been used for almost a century to support the diagnosis of active and latent TB infection (LTBI) (1). The main drawback with the clinical use of the TST is the lack of specificity due to cross-reactivity with proteins present in other mycobacteria, such as the Mycobacterium bovis bacillus Calmette-Guérin (BCG) vaccine strain and Mycobacterium avium (1, 18). Identification and characterization of the two M. tuberculosis-specific antigens ESAT-6 and CFP-10 (4, 5, 26) has led to the development of new specific diagnostic tests for infection with M. tuberculosis (2). ESAT-6 and CFP-10 are contained within the RD1 region of the mycobacterial genome, which is absent from M. bovis BCG, M. avium, and most other nontuberculous mycobacteria (2, 12). A number of studies have demonstrated a high sensitivity and specificity when detecting gamma interferon (IFN-γ) responses to these specific antigens in patients with active TB (6, 17, 20, 21, 25, 28), nonexposed healthy people (3, 6, 7, 13, 17, 20, 21, 25, 28), and recently M. tuberculosis-infected healthy TST converters independently of their BCG vaccination status (3, 7, 10, 16, 27). A high percentage of healthy individuals in TB high-endemicity areas respond to the RD-1 antigens, indicating a high prevalence of LTBI (8, 15, 25, 29). Recently, a simple whole-blood test, QuantiFERON-TB (Cellestis Limited, Australia), for in vitro detection of M. tuberculosis infection in low-endemicity regions, has been approved by the Food and Drug Administration (19). The test is simple to perform, results can be obtained within 24 h, it takes only one patient visit, and it does not boost subsequent tests. The QuantiFERON-TB test, however, is based on purified protein derivative (PPD), and its specificity is impaired by prior BCG vaccination, just as is the case with the TST (6, 11, 13). The aim of the present study was to evaluate the M. tuberculosis-specific RD1 antigens (ESAT-6 and CFP-10) using the QuantiFERON test system (QFT-RD1) for the diagnosis of active TB. The prospective nature of our study allowed a direct comparison between QFT-RD1 test and microscopy and culture.
(Part of this work was presented orally at the 14th European Congress of Clinical Microbiology and Infectious Diseases in Prague, 4 May 2004, abstr. 411.)
Peripheral blood was collected in tubes coated with lithium-heparin and kept at room temperature for a maximum of 12 h before assaying. Blood samples were collected before or within the first week of chemotherapy. Whole-blood stimulation was performed essentially according to the instructions by Cellestis, with the difference that the specific antigens ESAT-6 and CFP-10, produced at Statens Serum Institute, were used along with PPD. Five 1-ml aliquots of heparinized blood were incubated in 24-well culture plates (Nunc, Roskilde, Denmark) at 37°C in a humidified atmosphere for 24 h in the presence of 3 drops of saline (nil control), 1 μg/ml PHA (mitogen control), 5 μg/ml PPD, 5 μg/ml ESAT-6, or 5 μg/ml CFP-10. Supernatants were harvested and immediately frozen at −20°C until further analysis. The amount of IFN-γ released was determined using a sensitive ELISA, and results were expressed as IU/ml, as determined from a standard curve run on each plate. IFN-γ produced in the saline control well was subtracted from the respective antigen-stimulated wells. The cutoff value for a positive response was set at 0.35 IU/ml for ESAT-6 and CFP-10 based on receiver-operator-characteristic curve analysis of 39 healthy nonexposed controls and 26 culture-confirmed TB patients. The cutoff value for a positive response to PPD or PHA was arbitrarily set at 1.5 U/ml, and patients with IFN-γ responses below 1.5 IU/ml to both PPD and PHA were considered inconclusive on suspicion of anergy or failures during processing.
Microscopy for acid-fast bacilli and culturing for mycobacteria were performed at the International Reference Laboratory of Mycobacteriology, Statens Serum Institut, Denmark, which is accredited according to European standard EN45001.
The study was approved by the local ethical committee of Frederiksberg and Copenhagen KF 01-033/00. Written informed consent was always obtained.
Median values and 25th to 75th percentiles are shown. Chi-square and McNemar tests were used to compare proportions using Med Calc and Sigma Stat version 1.01 software. P values of <0.05 were considered significant.
This study was designed as a blinded prospective study. Patients were included if there was a high clinical suspicion of active TB and if relevant material (sputum or other specimen) was available for microbiological diagnosis. Patients with a known history of previous TB, healthy contacts referred for contact investigation, and patients with an incomplete data set were not included. Eighty-two patients were enrolled consecutively from the Department of Pulmonary Disease at Gentofte Hospital or at the Department of Infectious Diseases at Rigshospitalet and Hvidovre Hospital, Denmark. Thirty-nine healthy BCG-vaccinated office employees with no known exposure to M. tuberculosis were enrolled as controls. Patient and clinical information were collected from the patient files.
No definitive diagnostic test can exclude infection with M. tuberculosis, and to ensure the most correct classification of the patients in this study, all TB patients were followed to the end of treatment and all the non-TB patients were followed for up to 31 months (median, 21.5 months; 25th to 75th percentile, 20 to 26 months) to ensure that they did not develop TB at a later time. The follow-up evaluation consisted of clinical evaluation while the patient was still being seen at the relevant outpatient department and also by contacting the Laboratory of Mycobacteriology to ensure that no samples had been submitted from other health care centers which could have been positive for M. tuberculosis. This evaluation resulted in reclassification of five patients. One TB suspect patient, who suffered from pneumonia and pericarditis, was initially classified as a non-TB patient because TST and cultures were negative but developed miliary TB 3 months after inclusion in the study. This patient was reclassified as microscopy/culture-negative TB at the time corresponding to the blood sample. Four patients received anti-TB treatment for less than 8 weeks because the TB diagnosis was disproved and treatment was interrupted (M. avium [n = 2], sarcoidosis [n = 1], and rapid clinical response to penicillin indicating community-acquired bacterial pneumonia [n = 1]).
The patients were finally classified as having active TB if either culture was positive for M. tuberculosis (n = 27) or the physician decided that the patients suffered from TB based on clinical findings, patient history, and/or X-ray, and/or histology indicating mycobacterial infection (i.e., granulomatous necrosis or identification of acid-fast bacilli), and the patients responded clinically and radiologically to a full course of anti-TB treatment(n = 21). The final classification of a non-TB patient was based on the fact that (i) M. tuberculosis was not found by conventional screening and (ii) another diagnosis was found, or the patient recovered without a complete course of anti-TB treatment and (iii) TB was not diagnosed during follow-up.
Baseline information for all 82 patients is shown in Table Table1.1. Nine had inconclusive results due to no responses to both PPD and PHA and were excluded; thus, 73 TB suspect patients were evaluated. Active TB was diagnosed in 48 (66%) subjects, median age was 41 years (range, 34 to 51 years), 33% were female, and 56% had emigrated from TB high-endemicity regions into Denmark, 3 (6%) were human immunodeficiency virus (HIV) antibody positive, and 69% had one or more risk factors. Among the 25 patients without TB, the median age was 47 years (34 to 55), 40% were female, 36% were immigrants from TB high-endemicity regions, 4 (16%) were HIV antibody positive, and 64% had one or more risk factors. Of 48 patients with active TB, 33 (69%) were positive by either microscopy or culture. Of 20 microscopy-positive patients, 15 were positive by microscopy of acid-fast bacilli and 5 of granulomatous lesions. Five were positive by microscopy only, and 13 were positive by culture only. Twenty-five (34%) of the 73 TB suspect patients were classified as non-TB patients, and in 15 of these patients another diagnosis was obtained: bacterial infections responding to antibiotics (n = 10), leukemia (n = 1), vasculitis plus diabetes mellitus (n = 1), HIV positive with fever (n = 1), and M. avium infection (n = 2). No specific diagnosis was obtained despite thorough investigation in 10 patients with respiratory (n = 4) or gastrointestinal (n = 4) symptoms, fever and lymphadenitis (n = 1), or possible sarcoidosis (n = 1).
Figure Figure11 shows the individual IFN-γ responses of 73 patients highly suspect for active TB and 39 healthy controls in response to either ESAT-6 or CFP-10 (the highest for each subject) and PPD. PPD induced high levels of IFN-γ in both the patients and the healthy BCG-vaccinated controls, whereas ESAT-6/CFP-10 induced responses mainly in the TB suspect patients. In total, 51 of the 73 TB suspect patients (70%) recognized at least one of the M. tuberculosis-specific RD1 antigens and were considered QFT-RD1 positive. Only 1 (3%) out of the 39 healthy BCG-vaccinated controls showed a response to the RD1 antigens, which was just above the cutoff. In contrast, both TB suspects (61/73; 84%) and healthy controls (32/39; 82%) showed positive responses in vitro to tuberculin PPD.
The final results of the QFT-RD1 test and clinical and microbiological investigations are depicted in Fig. Fig.2.2. The QFT-RD1 test was positive in 51 persons of whom 41 were diagnosed with active TB, and the QFT-RD1 test was negative in 22 persons of whom 7 had active TB. Sensitivity of the QFT-RD1 for the diagnosis of TB was 85% (41/48; CI, 75 to 96%). In the group of TB patients who were negative by both microscopy and culture, sensitivity was 87% (13/15) (data from Fig. Fig.2),2), and in the patients with either culture-negative or microscopy-negative TB, the sensitivity of QFT-RD1 was 85% (17/20) and 89% (25/28), respectively (data not shown). Not all patients had a TST done, but where the TST was performed, we found that 8/20 (40%) non-TB patients and 20/24 (88%) TB patients had a positive TST (>12 mm).
The prospective design of this study allowed us to compare the sensitivity of the QFT-RD1 test with that of microscopy and culture, and we found that the sensitivity of the QFT-RD1 test for the diagnosis of TB of 85% (41/48; CI, 75 to 96%) was significantly higher than that of microscopy, 42% (20/48; CI, 27 to 56; P = 0.001, McNemar test), and culture, 59% (27/46; CI, 44 to 73%; P = 0.009) (Table (Table2).2). Pulmonary TB is often easily recognized by typical infiltrates on X-ray and a positive sputum smear, whereas the diagnosis of extrapulmonary TB and smear-negative TB is more difficult. In the present study, we found an equally high proportion of patients with pulmonary TB and extrapulmonary TB responding to QFT-RD1, 83% (29/35; CI, 70 to 96%) versus 12/13, 92% (CI, 78 to 107%), respectively (P > 0.05, χ2 test) (Table (Table2).2). The sensitivity of the QFT-RD1 test was then compared with microscopy and culture results in patients with pulmonary and extrapulmonary TB. In patients with pulmonary TB, the QTF-RD1 test was more sensitive (83%; CI, 70 to 96) than microscopy (46%; CI, 29 to 63; P = 0.009), but there was no difference between the sensitivity of the QFT-RD1 test and culture at 63% (CI, 47 to 79; P = 0.121). For extrapulmonary TB, the QFT-RD1 test was more sensitive (92%; CI, 78 to 107%) than both microscopy (31%; CI, 5 to 56; P = 0.013) and culture (42%; CI, 13 to 70; P = 0.041) (Table (Table2).2). The combined sensitivity of microscopy and/or culture was 69% (33/48; data are shown in Fig. Fig.2),2), and by combining microscopy and culture with the QFT-RD1 test for the diagnosis of active TB, overall sensitivity increased to 96% (CI, 90 to 102%) (Table (Table22).
The QFT-RD1 test has previously been shown to discriminate patients with active TB from healthy unexposed individuals, and in this study, only 1 of the 39 healthy unexposed Danish controls recognized CFP-10 or ESAT-6 in the QFT-RD1 test, resulting in a specificity of 97% (CI, 94 to 108) (Fig. (Fig.1)1) in this group of healthy unexposed individuals. However, the primary aim of this study was to evaluate the performance of the QFT-RD1 test for the diagnosis of active TB in a clinical setting, and in this situation we found that 40% (10/25; CI, 20 to 60%) of the 25 non-TB patients were QFT-RD1 positive compared to 85% of the patients with active TB (P < 0.01). Thus, specificity for the diagnosis of active TB was calculated to be 60% (CI, 40 to 80). The data on TB risk factors for the QFT-RD1-positive non-TB patients were examined as part of the initial screening, and of the 10 QFT-RD1-positive non-TB patients, 8 (80%) were found to have TB risk factors suggesting possible LTBI. Table Table33 shows the individual risk factors for each of the QFT-RD1-positive non-TB patients. Only two patients had no known TB risk factors, one was diagnosed with acute myeloid leukemia, and one had granulomatous lesions in the liver interpreted as sarcoidosis, but self-limiting M. tuberculosis infection could have been an alternative diagnosis.
Nine patients were excluded due to no response to PPD and PHA. One non-TB patient and two TB patients did respond to ESAT-6 or CFP-10. It could be claimed that exclusion of these patients might have resulted in altered sensitivity of the assay; however, while recalculating the sensitivity and specificity of the QFT-RD1 test and including these nine patients, sensitivity and specificity were unchanged (84% [43/51] and 36% [11/31], respectively).
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, 22), 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, 7, 11, 13, 18, 20, 21, 28). 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 (3, 6, 17, 20, 21, 28). Given the previously reported very high specificity of the RD1-based tests in low-risk populations (3, 6, 17, 20, 21, 25, 28), 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, 15, 25, 29). 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, 25). 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.
We thank Vita Skov (Statens Serum Institute, Copenhagen, Denmark) for excellent technical support, Eskild Pedersen (University Hospital, Skejby, Denmark) and Kåre Mølbak (Statens Serum Institute, Copenhagen, Denmark) for discussions on the design of the study, Søren Hoff (Statens Serum Institute, Copenhagen, Denmark) for assistance in the clinic, and Klaus Larsen (University Hospital, Hvidovre, Denmark) for statistical counseling.
Peter Andersen and Aase Bengaard Andersen are inventors of patents and patents application files by Statens Serum Institut relating to the discovery of the M. tuberculosis antigen ESAT-6. The rights have been assigned to Statens Serum Institute, and Statens Serum Institute has outlicensed the use of the ESAT-6 and CFP-10 antigens for TB diagnosis.
QuantiFERON TB reagents were supplied by Cellestis Limited, Australia.