As both the TST and IGRAs are indirect tests that indicate a cellular immune response to recent or remote sensitization with mycobacterial antigens, neither test can distinguish between individuals with latent, active, and previous TB infection, nor do they have a high accuracy for predicting active TB [66].

Although the intensity of QFT response does not help to differentiate active from latent TB infection [67], a QFT >2 IU/mL indicates a higher likelihood of the response of the ocular TB to antituberculosis treatment [68]. IGRAs provide greater specificity for M. tuberculosis [23, 69, 70] in pulmonary TB [45, 61, 70, 71] and, unlike the TST (see Table 5.2), do not give false-positive results in individuals vaccinated with the BCG or infected with most NTM. IGRAs may thus be preferred in patients with previous BCG vaccination. In addition, IGRAs have a lower rate of false negatives than the TST in rheumatic patients taking steroids or disease-modifying antirheumatic drugs (DMARDs) [72].

However, IGRAs are not superior to the TST in sensitivity in diagnosing latent TB infections [70, 73, 74] and should not be used as a screening test or first-line investigation in tuberculous uveitis [23, 56, 75], especially in populations where TB is not endemic. Nevertheless, they are a useful adjunct in the diagnosis of tuberculous uveitis when used together with clinical signs suggestive of tuberculosis and TST in BCG-vaccinated populations [57]. A positive IGRA result strongly supports the diagnosis, while a negative IGRA result should be interpreted with caution as it does not exclude the diagnosis [23, 56].

Like the TST, the use of IGRAs in pediatric [76–79] and elderly patients [80] has its limitations. Given the variable T-cell IFN-γ response in children, a substantial proportion of children do not have a determinate IGRA result [81], which is likely due to the immature immune system and primary or acquired immune deficiencies [79, 81]. Similarly, as immunosenescence begins at 50 to 55 years of age [82], an age-related waning of T-cell function [80] is associated with decline in the functional activity of the peripheral blood mononuclear cells, reduced diversity of T cells, and a decreasing trend in IFN-γ production from T cells in response to mitogen and M. Tuberculosis antigens in whole blood [61, 80, 82, 83].

Although IGRAs are objective and require only a single visit [84], they have substantial intra-patient variability particularly in settings with low incidence of TB [64, 85–90] that is largely due to sample processing factors such as delays in incubation and variations in blood volume [86, 87, 90–110]. Reversions and conversions around the existing cut-point should thus be interpreted with caution [111]. Moreover, IGRAs require higher material costs, specialized equipment, and trained personnel to analyze the results since the samples are time and temperature sensitive [88].

Although similar, the T-SPOT.TB may be preferred to the QFT and vice versa, depending on patient demographics. In studies comparing both IGRAs, the T-SPOT.TB was found to be more sensitive for M. tuberculosis infection, especially in immunocompromised individuals [83, 112] and children aged 5 years or younger [38, 113]. This is because the T-SPOT.TB has a better resolution of samples compared to the QFT, which may give “indeterminate” results in individuals with inadequate T-cell numbers. However, the QFT was found to be significantly more accurate than the T-SPOT.TB in identifying true-positive tuberculous uveitis cases among discordant cases [114].

Unlike the QFT which has a distinct cutoff value, the T-SPOT.TB has a range of cutoff values (negative <4 spots and positive >8 spots), resulting in an “equivocal” T-SPOT.TB result if the number of spots is more than the negative cutoff value but less than the positive cutoff value (i.e., between and inclusive of 5–7) [73, 83]. If the T-SPOT.TB result is equivocal, the clinician may repeat the T-SPOT.TB or do the QFT [76]. Individuals less than 13 years old or more than 55 years old are especially likely to have an “equivocal” T-SPOT.TB result [76], and in a setting where TB is moderately endemic, doing the QFT may be preferred as the equivocal T-SPOT.TB may be a false positive. These patients have a high likelihood of a negative QFT result and are unlikely to have latent TB infection [76].

Clinicians commonly adopt one of the four diagnostic strategies: firstly, a two-step approach of TST first, followed by IGRA; secondly, both the TST and IGRA simultaneously at presentation; thirdly, TST only; and lastly, IGRA only [116].The UK-based National Institute for Health and Care Excellence (NICE) guidelines advocate a two-step approach using TST and chest radiography as first-line investigations, with subsequent confirmatory IGRA testing in cases with positive TST results or in those whom the TST is unreliable [122]. In contrast, the US Center for Disease Control and Prevention (CDC) guidelines recommend one-step IGRA-only testing for the diagnosis of TB infection [123].

A study compared the above four strategies for the diagnosis of tuberculous uveitis and found that it was the most cost-effective to do both the TST and IGRA at presentation and least cost-effective to do an IGRA only [124]. It cited two main advantages that likely outweighed the cost of performing both tests simultaneously. Firstly, the dual strategy had higher predictive power which reduced the number of false-positive results and the associated unnecessary costly treatment. Secondly, patients who were initiated on antituberculous treatment based on the positive results from both tests had a decreased likelihood of recurrence of uveitis and lower ocular morbidity [35]. However, this study was conducted in a country with a high cost of IGRA. In countries with a lower cost of IGRA, the two-step approach strategy which the NICE guidelines recommend is more cost-effective than the single-test strategy [124]. If a single-test strategy is used, doing IGRA only may be more cost-effective than TST for the screening of latent TB infection in immigrants from high-risk countries [125].

Although many studies have explored the use of IGRAs in tuberculous uveitis [3, 56, 75, 126–130], their role in the diagnosis and testing strategy of latent TB or tuberculous uveitis is still uncertain [35, 114, 130]. However, based on two systematic reviews, IGRAs should not be used to diagnose active TB in adults because a positive IGRA result may not indicate active TB and a negative IGRA result may not rule out active disease. IGRAs should only be used as an adjunct test in the diagnosis of active TB and should not replace the standard microbiological and radiographic tests [120, 131]. Although IGRAs should ideally be performed prior to the TST, they may still be done in patients who have done the TST as the TST is unlikely to affect the QFT result [65, 76] and the boosting effect is likely to be insignificant if T-SPOT.TB is performed within 3–7 days after the TST [108]. As prevalence of TB affects the interpretation of IGRAs, Harada et al. (2004) [132] suggested that the cutoff value of QFT be reset at a lower level in the context of high prevalence of TB.

Gupta et al. (2007) [10] proposed a diagnostic criteria for intraocular TB, based on clinical signs, ocular investigations, systemic investigations, exclusion of other uveitis entities, and therapeutic test. The algorithm by Vasconcelos-Santos et al. (2009) [130] applies mainly to regions where TB is non-endemic and proposes the use of TST and chest radiograph as the initial investigations, with subsequent options of IGRA , computed tomography of the chest, and/or a trial of antituberculosis therapy depending on the TST and chest radiograph results.

Ang et al. (2009, 2012) [56, 57] recommend a combination of IGRA with TST as performing both tests increases the diagnostic accuracy and avoids negative or indeterminate solo test results [23]. If both the TST and IGRA cannot be performed due to cost or logistical issues, the investigation of choice depends on the prevalence of tuberculous uveitis in the population. IGRA is preferred in populations with low prevalence as it is more specific [114], while the TST is preferred in populations with high prevalence of tuberculous uveitis or in patients in whom there is already a high index of suspicion for tuberculous uveitis [23].

However, doing both the TST and IGRA may give rise to discordant results even in countries with low BCG vaccination. Individuals with concordant positive TST and IGRA results have the highest incidence of progression to TB, compared to those with discordant results. Those with positive IGRA and negative TST have a slightly higher risk of progression to TB as the results tend to represent new or active infections, whereas positive TST and negative IGRA are associated with previous BCG vaccination especially in the young [56].

Besides the chest radiograph, other diagnostic imaging modalities include computer-assisted tomography (CT) scans and positron emission tomography/computed tomography (PET-CT) scans. CT scans may help to delineate concomitant hilar, parenchymal, or pleural disease in highly suspected cases where plain chest radiographs are normal or inconclusive [29, 159]. More recently, PET-CT scans have been shown to be useful in identifying pulmonary and extrapulmonary lesions in patients with presumed ocular TB, which can be biopsied to establish the diagnosis [160, 161]. However, the high cost and lack of availability limit its use for routine investigation.