Characteristics of patients with borderline T-SPOT.TB test and retest results

<bold>Background:</bold> T-SPOT.<italic>TB</italic> test is an interferon gamma release assay (IGRA) used to identify tuberculosis infection (TBI). Unlike other IGRAs, the T-SPOT.<italic>TB</italic> test includes a borderline result in addition to the positive and negative results. We aimed to characterise patients with borderline test and their retest results. <bold>Methods:</bold> From 2010-2017, T-SPOT.<italic>TB</italic> test results, indications for testing, patient demographics, comorbidity status and incident/prevalent TB were analysed for patients at Department of Respiratory Diseases and Allergy, Aarhus University Hospital, Denmark, using data from medical records and national TB databases. <bold>Results:</bold> Among 11,053 valid test results, we found 9132 (83%) negative and 1402 (13%) positive test results. Notably, 518 (4.8%) borderline test results originating from 489 unique patients. Their median age was 43 years (IQR 30-56), 53% (n=259) were male, 3.7% (n=18) had prevalent TB, and 1.2% (n=6) developed incident TB (2.0 per 1000 person years). A third (n=160/489) of borderline patients were retested; 64% (n=102) turned negative, 21% (n=33) positive, and 16% (n=25) remained borderline. A second retest was performed in 39 patients. Of these, all who tested borderline (n=9/39) during their first retest obtained a definitive result from the second retest. A notable number of patients who obtained definitive results during the first retest showed conversion to positive or reversion to negative (n=5/39) or returned to borderline (n=3/39) in the second retest. <bold>Conclusion:</bold> A borderline T-SPOT.<italic>TB</italic> test result is common, also among patients with active TB. Most borderline tests turn negative upon retesting, but conversion and reversion may occur upon second retesting.