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Low Sensitivity of a Whole-Blood Interferon-γ Release Assay for Detection of Active Tuberculosis

  1. Puneet K. Dewan1,
  2. Jennifer Grinsdale2, and
  3. L. Masae Kawamura2
  1. 1International Research and Programs Branch, Division of Tuberculosis Elimination, National Center for HIV, STD, and TB Prevention, Centers for Disease Control and Prevention, Atlanta, Georgia
  2. 2San Francisco Department of Public Health, San Francisco, California
  1. Reprints or correspondence: Dr. Puneet K. Dewan, International Research and Programs Branch, Div. of Tuberculosis Elimination, National Center for HIV, STD, and TB Prevention, Centers for Disease Control and Prevention, 1600 Clifton Rd., MS E10, Atlanta, GA 30333(pdewan{at}cdc.gov).
 
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Abstract

The sensitivity of an interferon-γ assay (Quantiferon-TB Gold; Cellestis) was evaluated for the detection of tuberculosis among 242 persons with suspected tuberculosis in San Francisco, California. Thirty-seven subjects had culture-confirmed tuberculosis. Excluding 1 indeterminate result, 23 (64%; 95% confidence interval, 48%–78%) of 36 subjects had positive results using the QuantiFERON-TB Gold assay. The 64% sensitivity suggests that the QuantiFERON-TB Gold assay should not be used alone to exclude active tuberculosis.

In December 2004, the US Food and Drug Administration approved the Quantiferon-TB Gold (QFT-G; Cellestis), a new assay for the detection of Mycobacterium tuberculosis infection. The QFT-G assay measures the amount of IFN-γ released after blood is incubated with synthetic antigens (early secretory antigen 6 and culture filtrate protein 10) that simulate proteins present in M. tuberculosis. Although the QFT-G assay is reported to detect M. tuberculosis infection with high specificity, it does not distinguish between active tuberculosis and latent tuberculosis infection [1–7]. Furthermore, the sensitivity of QFT-G for detection of active tuberculosis has not been well defined among individuals with suspected tuberculosis in the United States.

The San Francisco Department of Public Health (San Francisco, CA) has used the QFT-G assay as a component of routine evaluation for patients with suspected tuberculosis since February 2005. We sought to evaluate the sensitivity and predictive value of QFT-G for the detection of active tuberculosis among persons with suspected tuberculosis at a public health clinic.

Methods. San Francisco Department of Public Health Tuberculosis Clinic guidelines require QFT-G testing of patients who are suspected of having active tuberculosis (defined as American Thoracic Society tuberculosis class 5), in conjunction with routine clinical, microbiologic, and radiographic examinations [8]. An individual with suspected tuberculosis is defined as a patient with clinical or radiographic evidence consistent with active tuberculosis for whom laboratory confirmation of the final disease classification has not been received. Because specimens require same- and subsequent-day processing, the QFT-G assay is not used if patients are evaluated at the San Francisco Department of Public Health Tuberculosis Clinic after 3 P M on Fridays or weekends, or if phlebotomy is unsuccessful or declined. The assay is performed in accordance with the manufacturer's instructions: specimens are processed <8 h after phlebotomy, and incubation time was standardized at 16–18 h [9]. A tuberculin skin test (TST) is not required for patients with suspected tuberculosis, but results are often available.

In this retrospective evaluation, we reviewed clinical records for all consecutive patients from San Francisco who were reported to have suspected tuberculosis during the period from 1 March through 31 December 2005. We excluded patients who transferred out of the jurisdiction or were treated for >14 days with antituberculosis drugs before QFT-G testing. We collected information about TST results obtained within 2 weeks of the QFT-G test date that were reported to San Francisco Department of Public Health by the referring provider or that were obtained at the tuberculosis clinic using standard methods [8]. TST indurations ⩾ 5 mm in diameter were classified as positive results. We defined “high clinical suspicion” as suspected tuberculosis in a patient who was receiving empirical antituberculosis medication before the availability of diagnostic microbiologic test results. Cases were classified by final diagnosis as either a verified case of tuberculosis or as negative for tuberculosis [10].

Ninety-five percent CIs for sensitivity were calculated using the Wilson score method [11]. Proportions were compared using the χ2 test or, if there were <5 observations, Fisher's test.

Results. In the evaluation period, the San Francisco Department of Public Health noted 522 consecutive patients with suspected tuberculosis; 41 (8%) were excluded as transfers out, and 39 (7%) had not yet received a final diagnosis as of 15 March 2006. Of the remaining 442 subjects with suspected tuberculosis, 242 (55%) were tested using the QFT-G assay ( table 1). Persons who underwent QFT-G testing did not significantly differ from those who did not with regard to distribution of age, HIV infection status, high clinical suspicion of tuberculosis, or final diagnosis of tuberculosis. Patients who underwent QFT-G testing were more likely to be foreign born or Asian and were less likely to declare non-Hispanic black or Hispanic white ethnicity/race.

Table 1
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Table 1

Characteristics of patients evaluated for tuberculosis, San Francisco, California, March–December 2005.

Among the 242 persons with suspected tuberculosis, 45 (19%) received a diagnosis of tuberculosis, including 37 (82%) who had culture-confirmed disease (table 2). Among these 45 patients with tuberculosis, QFT-G results were positive for 25 (55%), negative for 17 (38%), and indeterminate for 3 (7%). Excluding indeterminate results, we calculated an overall QFT-G sensitivity for this population of 60% (95% CI, 44%–73%) and a negative predictive value of 86% (95% CI, 79%–91%). False-negative QFT-G results were distributed equally throughout the 46-week evaluation period (data not shown). We observed similar results among the 37 patients with culture-confirmed tuberculosis (sensitivity, 64% [95% CI, 48%–78%]; negative predictive value, 89% [95% CI, 83%–94%] and among persons with no history of prior tuberculosis infection (including those with latent tuberculosis infection and active disease) or tuberculosis treatment ( table 2). Only 3 HIV-infected patients received a diagnosis of tuberculosis, precluding meaningful evaluation of this subgroup.

Table 2
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Table 2

QuantiFERON-TB Gold (QFT-G; Cellestis) results among 242 persons with suspected active tuberculosis, by patient population subgroup.

We compared the clinical characteristics of patients with verified tuberculosis and true-positive QFT-G results (25 patients) with those who had false-negative QFT-G results (17 patients) (table 3). We observed no significant differences with regard to age distribution, proportion of patients without chronic medical conditions, HIV infection status, or duration of treatment of active tuberculosis at the time of testing. Patients with false-negative QFT-G results were more likely to have extrapulmonary tuberculosis (35% vs. 4%; P < .05). We did not observe cases of multiple-site or miliary tuberculosis in this cohort of patients who underwent QFT-G testing.

Table 3
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Table 3

Characteristics of patients who had verified cases of tuberculosis diagnosed.

TST results were available for 24 (53%) of the 45 patients with verified tuberculosis who were tested using the QFT-G. Of these 24 patients, the QFT-G and TST results were both positive for 12 (50%) and both negative for 1 (4%). Nine patients (38%) had negative QFT-G and positive TST results, and 2 (8%) had indeterminate QFT-G and negative TST results. No patient with verified tuberculosis had negative TST and positive QFT-G results. Eighteen (75%) of the 24 patients' TST results were verified by the San Francisco Department of Public Health, and 6 (25%) results were reported in induration (in mm) by community providers; between these groups, the proportion of discordant TST and QFT-G results did not significantly differ (data not shown).

Discussion. The QFT-G IFN-γ assay was demonstrated to have 64% sensitivity and 89% negative predictive value for culture-confirmed cases of active tuberculosis among patients with suspected tuberculosis in San Francisco. Excluding indeterminate results, we observed negative QFT-G results in 17 (40%) of 42 patients with tuberculosis, including 13 (36%) of 36 patients with culture-confirmed tuberculosis and 10 (40%) of 25 patients with no prior history of tuberculosis infection, disease, or treatment. Our results reinforce the recent recommendation that negative results should not be used alone to exclude active tuberculosis and that results should be interpreted in conjunction with other clinical and diagnostic findings [12]. The combination of low sensitivity for active tuberculosis and the intrinsic inability of the test to distinguish between latent infection and active disease suggests that the QFT-G assay has a limited role in the evaluation of patients with suspected tuberculosis.

The sensitivity of the QFT-G assay for the detection of active tuberculosis cases observed in San Francisco was slightly lower but similar to that observed in referral hospitals [13]. Lee et al. [5] reported a sensitivity of 70% among 87 patients who received a diagnosis of tuberculosis; the subjects in that study differed from our population in that 55 patients (63%) in that study had culture-confirmed tuberculosis, and 29 (33%) were classified as immunocompromised. Among patients with culture-confirmed tuberculosis, Kang et al. [3] reported 81% sensitivity among 54 patients, and Mori et al. [4] reported 89% sensitivity among 118 patients. Lee et al. [5] suggested that using lower IFN-γ response values for classification of a positive QFT-G result may increase the assay's sensitivity with minimal trade-off in specificity and that this should be considered when testing patients with a high pretest probability of active tuberculosis, such as close contacts or patients with suspected tuberculosis. This approach, however, has not been validated.

Among patients with extrapulmonary tuberculosis, we observed that 6 (86%) of 7 had negative QFT-G results, including 3 (75%) of 4 with culture-confirmed cases. Poor IFN-γ response to culture filtrate protein 10 has been previously reported among patients with extrapulmonary tuberculosis [14].

Our retrospective evaluation had several limitations. Our results may have been affected by the fact that 8 (47%) of 17 patients with tuberculosis diagnoses who had false-negative QFT-G results had received 1–14 days of antituberculosis treatment. Our evaluation lacked the power to assess the possible effects of treatment [15, 16]. However, similar QFT-G sensitivity was observed among the subset of patients with no prior history of antituberculosis treatment or M. tuberculosis infection; thus, this limitation has likely not affected our conclusions. Because the TST was only performed for a subset of patients and by multiple health care providers, our evaluation should not be interpreted as a comparative study with the TST. The limitations of the TST in the diagnosis of active tuberculosis are well established, and clinical recommendations suggest that the TST should not be used alone to exclude active tuberculosis [17].

Our results are specific for the QFT-G assay and should not be assumed to apply to other IFN-γ assays. Newer commercial assays, such as the T-SPOT.TB (Oxford-Immunotec) and the QuantiFERON-TB Gold In Tube (Cellestis) assays, have many differences and may perform with higher sensitivity [5, 18]. Nevertheless, the QFT-G assay is currently the only IFN-γ assay commercially available in the United States, and providers should be aware of the limitations we have observed. Similarly, these findings should not be extrapolated to assume that the QFT-G or other IFN-γ assays lack adequate sensitivity for the detection of latent tuberculosis infection. Prospective studies are critically needed to characterize the future risk of tuberculosis among persons with different responses to IFN-γ in these commercial assays.

In conclusion, we observed a low sensitivity for the QFT-G assay for the detection of active tuberculosis among persons with suspected tuberculosis at a public health clinic in San Francisco. As with the TST, providers should be aware of the limitations of this assay and should not use negative QFT-G results alone to rule out active disease. Prospective, community-based clinical trials should be conducted for newer IFN-γ assays before any adjustments are made in current diagnostic practices [19].

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Acknowledgements

We acknowledge Sally Liska and Ernest H. Wong (Public Health Laboratory of the San Francisco Department of Public Health), for their invaluable contributions to IFN-γ assay implementation in San Francisco; Robert Fallstead, for data entry assistance; and the frontline staff of the Tuberculosis Control Section, San Francisco Department of Public Health. We also thank Dr. Andrew Vernon (Centers for Disease Control and Prevention [CDC]; Atlanta, GA), for his review of this manuscript, and Ann Lanner (CDC), for editorial assistance.

Potential conflicts of interest. All authors: no conflicts.

  • Received July 19, 2006.
  • Accepted August 23, 2006.
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  1. Clin Infect Dis. (2007) 44 (1): 69-73. doi: 10.1086/509928
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