Objective
to assess the usefulness of the T-SPOT.TB™ interferon-gamma release assay (IGRA), as used in a regional hospital infectious diseases unit in Northwest England, for the diagnosis of active tuberculosis.
Design
Retrospective case series.
Results
T-SPOT.TB™ test was applied to a group of 64 patients, 20 of whom had tuberculosis (mostly extrapulmonary tuberculosis). The T-SPOT.TB™ test had a sensitivity of 83.3% and a specificity of 75% for the diagnosis of active tuberculosis, compared with culture. A positive IGRA approximately doubled the pre-test probability of disease from 0.23 to 0.5. This doubling of probability was true regardless of HIV status, though for HIV+ patients the sensitivity was lower (sensitivity 66.7%, post test probability 0.4 for a positive IGRA result). When extrapolated to the local population the test was most useful for exclusion of disease; post test probability 0.006 (or 1 in 167) for a negative IGRA result.
Conclusion
Although it can add weight to a clinical diagnosis, T-SPOT.TB™ assay is not reliable for the diagnosis of active tuberculosis in a real world setting where the test is often used in patients with smear negative or extra-pulmonary disease. The test is useful for ruling out disease in HIV negative patients.
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