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

¹ Dept of Ambulatory Care and Community Medicine, University of Lausanne, ² Futurelab, BBR-LTC Medical Analytical Laboratory, Lausanne, and ³ Division of Pulmonary Diseases, Geneva University Hospital, Geneva, Switzerland. ⁴ Oxford Immunotec, Oxford, UK.

CORRESPONDENCE: J-P. Zellweger, Dept of Ambulatory Care and Community Medicine, University of Lausanne, Rue du Bugnon 44, 1011 Lausanne, Switzerland, Fax: 41 213144740. E-mail: zellwegerjp{at}swissonline.ch

Keywords: Interferon- release assays, latent tuberculosis infection, T-SPOT^TM.TB, tuberculosis

Received: September 5, 2007
Accepted November 21, 2007

Interferon- release assays for the diagnosis of tuberculosis (TB) can give indeterminate results. The prevalence of indeterminate test results (ITRs) among T-SPOT^TM.TB tests was assessed.

A retrospective analysis of samples processed in 2005 was performed. ITRs were assessed by age, sex, 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 analysed separately.

Of a total of 1,429 tests, 49 (3.4%) were indeterminate. ITRs were significantly associated with old age (>75 versus 5–75 yrs; odds ratio (OR) 7.97, 95% confidence interval (CI) 3.968–15.438) and the season during which samples were transported (autumn and winter versus spring and summer; OR 3.47, 95% CI 1.753–7.514). The incidence of ITR was 302 (2.0%) among TB contacts, 75 (1.6%) among immigrants, 156 (3.0%) in TB suspects and 32 (3.0%) among immunosuppressed patients. Sex, young age and distance to the laboratory were not associated with the rate of ITR. Of the 13 tests with ITR that were repeated, 10 gave a clear positive or negative result.

Indeterminate test results with T-SPOT^TM.TB under routine conditions were infrequent and more common in individuals aged >75 yrs than in children and younger adults. The incidence of indeterminate test results was low and similar among healthy tuberculosis contacts, immigrants with a positive tuberculin skin test, tuberculosis suspects and the immunosuppressed. The conditions of transportation may influence the incidence of indeterminate test results.