The role of interferon γ release assays to diagnose tuberculosis associated uveitis is uncertain. Commercially available interferon γ release assays include the T-SPOT.TB (Oxford Immunotec, Oxford, UK) and QuantiFERON-TB Gold In-tube (QFT; Cellestis Incorporated, Carnegie, Victoria, Australia), both of which the authors use in their center.
An equivocal test result is particular to the T-SPOT.TB, due to its range of cut-off values, and occurs when the numbers of "spots" produced are more than the negative cut-off value but less than the positive cut-off value. Since the equivocal T-SPOT.TB is inconclusive, there are currently no clear guidelines on how to interpret the result or what clinicians should do next. Most ophthalmologists disregard the result, repeat the T-SPOT.TB test or perform QFT.
The authors conducted this prospective cohort study to determine the risk factors for obtaining an equivocal 'T-SPOT.TB result in patients suspected of a TB etiology, as well as the clinical implications of an equivocal result in Singapore, where TB infection is moderately endemic. Subjects were 155 consecutive patients with ocular signs consistent with TB-associated uveitis who presented to the Singapore National Eye Center Ocular Inflammation and Immunology Service between 2008 and 2009.
Equivocal T-SPOT.TB results were obtained in 15 subjects (9.67 percent), more than half of whom were Chinese with no sex dominance. No patient was immunocompromised. Patients younger than 13 or older than55 years were more than 21 times more likely to have an equivocal result independent of sex, race, medical history or clinical features of uveitis. These patients were also approximately 15 times more likely to have a negative QFT.
The authors note that since only one patient younger than 13 had an equivocal result, they cannot conclude with certainty that T-SPOT.TB may be less useful in younger patients.
The authors conclude that QFT is a more useful test than T-SPOT.TB in the older age group and that patients with equivocal T-SPOT.TB results are unlikely to have latent TB infection.