Infection with Mycobacterium tuberculosis (Mtb), one of the most common infections worldwide, remains an important public health problem in the United States. Although the incidence of tuberculosis (TB) cases has declined over the past 2 decades, data from 2013 to 2015 show that rates have plateaued at 3.0 per 100,000 persons (1). In 2015, 9,563 TB cases were reported in the United States (1).
Latent TB infection (LTBI) is defined as a nontransmittable and asymptomatic form of TB, but it can progress to active TB disease. Successful control of TB warrants prompt identification of infected hosts. The detection of LTBI permits early treatment and reduces the chance of developing active disease. Until recently, the tuberculin skin test (TST) was the primary mode of screening for latent TB. The TST was prone to false-positive results in individuals who had been vaccinated with Bacillus Calmette–Guérin (BCG) (2,3) and those who were exposed to nontuberculous mycobacteria. In addition, the TST required manual administration and assessment of the response 48–72 hours later and is prone to errors arising from incorrect administration and/or interpretation (4).
QuantiFERON-TB Gold In-Tube (QFT-GIT) Assay serves as an improved LTBI serologic screening method through indirect measure of Mtb infection. QFT-GIT is an interferon-gamma (IFN-γ) release assay which detects the in vitro adaptive immune response to specific TB antigens by their production of IFN-γ. Host blood is collected into three samples: the Nil tube, Mitogen tube, and TB Antigen tube. The amount of IFN-γ produced in each tube is measured by enzyme-linked immunosorbent assay. IFN-γ values from the Nil tube are subtracted from the TB Antigen tube to correct for the amount of IFN-γ which is not induced by the specific TB antigens but considered as background. TB Antigen minus Nil IFN-γ levels over 0.35 IU/mL indicate a positive test. In addition, Nil tube values must not be higher than 8.0 IU/mL, which could otherwise indicate a false-positive result. Tests with corresponding Mitogen tube responses less than 0.5 IU/mL higher than the value of the Nil tube are labeled “indeterminate,” as this tube is used as an indication of the patient's immune status and maintenance of proper testing techniques (5). Unlike the TST, the QuantiFERON test is reliable in individuals vaccinated with BCG. A positive QuantiFERON test result indicates that TB infection is likely; however, a positive QuantiFERON test cannot discern between LTBI and active TB disease.
In our practice, because TB disease can mimic many neuro-ophthalmic disorders, QuantiFERON has replaced the TST as the screening test for TB. We hypothesized that the QFT-GIT is not beneficial as a screen for TB disease in patients at low-individual risk for TB. We retrospectively reviewed 99 cases of QFT-GIT testing in 2 neuro-ophthalmologic clinics in Houston, TX to determine the diagnostic yield of such testing.
METHODS
Patient records from 2 academic ophthalmology centers, located in a private (Hospital A) and a public (Hospital B) healthcare setting, were reviewed from January 2012 to February 2016. QFT-GIT results were divided into 3 groups: 1) negative, 2) indeterminate, and 3) positive. All patients with positive test results had at least 6 months of follow-up. Patients were excluded if they had a previous diagnosis of active TB disease or LTBI. Patients were defined as having LTBI if they had a positive QFT-GIT but negative bacteriologic cultures (if performed), and no clinical symptoms or signs, bacteriologic confirmation, or chest radiographic evidence of active TB disease (6). Patients were defined as having active TB if they had clinical, bacteriologic, and/or radiographic evidence of active TB (6). Patients were defined as not having active TB which affects ocular function if they had a negative reaction to a QFT-GIT test, regardless of previous exposure to TB (6).
Quality control mechanisms at the 2 testing laboratories were investigated. Protocols for the retesting of positive, indeterminate, and borderline specimens were intact for the verification of test results (7). In addition, strict criteria for the handling and processing of samples were in place at both testing locations.
RESULTS
A total of 99 patients with QFT-GIT testing were included. Of these 99 patients, 72 (73%) had negative QFT-GIT testing, 18 had positive, and 9 had indeterminate results. Of the 18 patients with positive tests, 12 had documentation of chest radiographs or computed tomography which showed no evidence for either active TB or LTBI, indicating potential false-positive QFT-GIT results. Four patients had chest imaging indicative of potential LTBI, while two could not be contacted regarding results of chest radiography. None of these 18 patients had symptoms of active TB, and none developed active TB within the follow-up period. These results are summarized in Table 1. Demographic data, examination findings, laboratory and imaging results, and clinical course are shown in Supplemental Digital Content, (see Table E1, https://links.lww.com/WNO/A216). QFT-GIT testing results are found in Table 2.
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TABLE 1.: Results of QuantiFERON-TB gold in-tube assay and chest imaging
TABLE 2.: Summary of QuantiFERON-TB Gold In-Tube (QFT-GIT) results in patients with positive QFT-GIT testing and negative chest imaging for pulmonary tuberculosis
Hospital A had a rate of LTBI detection of 8.4% between 2012 and 2015. This rate was significantly higher at Hospital B, where 22% of patients were shown to have LTBI by QuantiFERON testing performed between 2012 and 2016 (8). Despite the disparity, a similar prevalence of positive QFT-GIT testing with negative neuro-ophthalmic findings was found at both hospitals: 16% for Hospital A (not including 2 patients who did not have chest radiographs) and 15.6% for Hospital B (Table 1).
DISCUSSION
Despite its widespread use for the diagnosis of LTBI, QFT-GIT testing has been shown to be neither sensitive nor specific for detection of active TB disease (9,10). In our study, we had a rate of 67% (12/18 patients) false-positive QFT-GIT testing. LTBI was identified through chest radiography in 4 of our 18 patients who had positive QFT-GIT tests. Six of the 12 patients with positive QFT-GIT testing and negative chest imaging for LTBI showed no sign of active TB during the follow-up period. The high number of patients with normal chest imaging in our study is consistent with findings of Fong et al (11) who found that only 4.1% of 484 health care workers with positive QFT-GIT test results had abnormalities on chest radiographs, and none developed active pulmonary TB. It is notable that most of our cases of false-positive results were found in patients presenting with optic neuropathies and/or optic atrophy (see Supplemental Digital Content, Table E1, https://links.lww.com/WNO/A216). Further study in a larger patient cohort might determine which clinical presentations yield a higher risk for development of TB-related ocular disease.
We believe that routine testing with QFT-GIT in a low-risk cohort is unnecessary and we do not recommend routine testing for such low-risk individuals. Factors which would indicate a patient is at low risk for TB affecting ocular function include low prevalence of TB in a particular geographic region, lack of potential patient exposure (e.g., close contacts with individuals with TB, residence in an endemic country, recent incarceration, or homelessness), examination findings indicative of an alternative disease process, and lack of medical history increasing the patient's risk for active TB, such as immunosuppressive states including HIV, severe kidney disease, AIDS fewer than 5 years, and immunosuppressive and immunomodulatory therapy (12–16).
Low-risk status lowers the pretest probability of TB detected by QFT-GIT leading to a lower positive predictive value of QFT-GIT and a higher rate of false-positive results. For progression to active TB, the positive predictive value for interferon-gamma release assays (IGRAs) such as QFT-GIT, which measure the in vitro immune response to antigens from IFN-γ–releasing cells, increased significantly with high-risk individuals. This supports our recommendation of targeted testing based on pretest probability (17,18). Notably, a low cut point when retesting a low-risk population could increase the false-positive rate of QFT-GIT testing (9,19–21). We advocate a directed approach to determine the need for QFT-GIT based on pretest probability of TB disease in patients with neuro-ophthalmic disorders.
Several well-described limitations exist regarding use of the QFT-GIT, including a significant amount of variability (11,22). A systematic review of 26 studies, including those with both identical and nonidentical protocols, found variability in IFN-γ response (±0.47 IU/mL) to QuantiFERON even under identical conditions. In nonidentical protocols, the range of variability was even larger (±1.4 IU/mL) (23). Variations in laboratory techniques, such as varied incubation time and inoculum volume, can lead to indeterminate or false-positive results, rather than increased test sensitivity (24,25). Although current recommendations to confirm a positive QFT-GIT test in a low-risk setting include repeat testing, the rate at which positive tests will revert to a negative value on repeat testing has been shown to range from 7% reversion at immediate retesting with the same plasma sample to 87.5% reversion with repeat testing at 1 year (7,11,20,21,26–29). As the QFT-GIT test measures released quantities of IFN-γ from T cells in the peripheral blood, daily variations in this level could be explained by the number of T cells at any one time, especially if these fluctuations are close to the cutoff of 0.35 IU/mL (11,30). Based on inherent ranges of for test variability, Metcalfe et al (21) found that positive QFT-GIT test results less than 0.59 IU/mL could be expected to revert back to negative on repeated testing. Three of our 12 positive tests were less than 0.59 IU/mL. In addition, Thanassi et al (31) found that if the QFT-GIT result was positive between 0.35 and 1.11 IU/mL, then a 75% risk of reversion occurred with sequential testing, and an 80% risk of reversion occurred if the QFT-GIT result was between 0.35 and 0.72 IU/mL. Eight of our test results fell into the former category, whereas 5 patients fell into the latter data range, indicating high likelihood for reversion.
There is an additional IGRA for detection of LTBI: the T-SPOT.TB test. In this assay, peripheral blood mononuclear cells are incubated with 2 of the 3 active synthetic peptides found in Mtb that are used in QFT-GIT, and then T cells producing IFN-γ are measured with an immunospot assay (18). As multiple studies have indicated, T-SPOT.TB and QFT-GIT tests have shown discordant results. It is possible that the T-SPOT.TB assay might potentially yield more accurate results for detection of TB in neuro-ophthalmic patients (18,28). However, in comparing IGRAs (including both T-SPOT.TB and QFT-GIT) and TST, it was found that no available tests for LTBI have a high prognostic value, and that only a weak-to-moderate association exists between positive IGRA assays and the development of active TB (32).
We recognize the limitations of our study, including the retrospective analysis, the relatively small sample size, and the short duration of follow-up. Although we only have followed our patients for 5 months to 4 years, the lifetime risk of LTBI converting to an active TB infection is 5%–10%. The loss of follow-up of 2 of the 18 positive QFT-GIT testing patients, with failure to undergo chest imaging, is another limitation. Our study did not analyze the 9 of the 99 patients with indeterminate results; retesting could have changed the proportion of patients with positive or negative results. Although no patient with indeterminate results developed TB disease during the follow-up period, the implications for indeterminate testing remain unclear. We did not determine the rate of false-negative QFT-GIT testing. To our knowledge, none of our 99 patients developed active TB disease over the follow-up period, and none of the patients with negative testing progressed to LTBI using alternative testing methods.
In summary, the 18 positive QFT-GIT assays studied in a neuro-ophthalmologic setting did not have prognostic significance in the detection or development of ocular TB in patients from 5 months to 4 years of follow-up. The results of this retrospective cohort study, alongside current literature demonstrating the intrinsic variability of the QFT-GIT and high rates of false positivity in low-risk populations, lead us to recommend against universal screening for ocular TB using QFT-GITs in favor of directed testing toward individuals at high risk for disease with the most sensitive and specific test available.
STATEMENT OF AUTHORSHIP
Category 1: a. Conception and design: L. M. Little, M. Rigi, A. Suleiman, S. V. Smith, E. A. Graviss, R. Foroozan, and A. G. Lee; b. Acquisition of data: L. M. Little, R. Foroozan, and A. G. Lee; c. Analysis and interpretation of data: L. M. Little, M. Rigi, A. Suleiman, S. V. Smith, E. A. Graviss, R. Foroozan, and A. G. Lee. Category 2: a. Drafting the manuscript: L. M. Little, M. Rigi, A. Suleiman, S. V. Smith, E. A. Graviss, R. Foroozan, and A. G. Lee; b. Revising it for intellectual content: L. M. Little, M. Rigi, A. Suleiman, S. V. Smith, E. A. Graviss, R. Foroozan, and A. G. Lee. Category 3: a. Final approval of the completed manuscript: L. M. Little, M. Rigi, A. Suleiman, S. V. Smith, E. A. Graviss, R. Foroozan, and A. G. Lee.
ACKNOWLEDGMENTS
The authors thank Dr. Charles Stager, Dr. Robert Atmar, and Anthony Cordova for supplementation of data for QuantiFERON-Gold In-Tube Assay results within the Harris Health System.
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