Indeterminate test results of T-SPOT.TB performed under routine field conditions

¹ Dept of ambulatory care and community medicine, University of Lausanne
² BBR-LTC Laboratories, Lausanne
³ Division of Pulmonary Diseases, Geneva University Hospital
⁴ Oxford Immunotec, Oxford, UK

^* To whom correspondence should be addressed. E-mail: zellwegerjp{at}swissonline.ch.

	Abstract

Interferon Gamma Release Assays can give indeterminate results. We assessed the prevalence of indeterminate test results (ITR) among T-SPOT.TB tests.

A retrospective analysis of samples processed in 2005. ITR were assessed by age, gender, immunosuppression, distance to the laboratory, and season. A subgroup of tests performed for specific indications (contact tracing, migrants with positive tuberculin skin test, TB suspects, and immunosuppression) were analyzed separately.

Among 1429 tests, 49 (3.4%) were indeterminate. ITR were significantly associated with old age (>75 year old vs. 5–75 years old, OR=7.97 95%CI 3.968–15.438, p=0.006) and the season during which samples were transported (autumn & winter vs. spring & summer, OR=3.47 95%CI 1.753–7.514, p=0.0007). The rate of ITR was 2.0% among TB contacts (n=302), 1.6% among immigrants (n=75), 3.0% in TB suspects (n=156) and 3.0% among immunosuppressed patients (n=32). Gender, young age and the distance were not associated with the rate of ITR. Among 13 ITR tests that were repeated, 10 gave a clear positive or negative answer.

ITR with T-SPOT.TB under routine conditions are infrequent. ITR were more common among elderly persons >75 years than among children and adults. The rate of ITR is low and similar among healthy TB contacts, immigrants with a positive TST, TB suspects and immunosuppressed. The conditions of transportation may influence the rate of ITR

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