Evaluation of a modified interferon-gamma release assay for the diagnosis of latent tuberculosis infection in adult and paediatric populations that enables delayed processing

Abstract

The objective of the study was to evaluate the specificity of a modified interferon-gamma release assay (IGRA) procedure that allows storage of blood samples for up to 32 h before processing. A total of 116 subjects were enrolled in the study. Two blood samples were collected from each volunteer; 1 specimen was processed within 8 h and analyzed using the T-SPOT^®.TB test and the second specimen was stored overnight and processed 23–32 h later after addition of the T-Cell Xtend™ reagent and then analyzed using the T-SPOT.TB test. A total of 108 paired T-SPOT.TB and T-SPOT.TB plus T-Cell Xtend tests were analyzed on specimens from 97 adults and 11 children. The median age of the subjects was 28 y with 68.5% female and 78.7% white. The overall agreement between the 2 tests was 98.2% (106/108). The specificity of the T-SPOT.TB test was 99.1% (107/108) and for T-SPOT.TB plus T-Cell Xtend was 97.2%. The 2 tests were comparable in results. Increasing storage time of the collected blood specimen prior to processing provides flexibility for clinicians and laboratories. Additional studies in larger and diverse patient populations including immunocompromised and paediatric patients, and patients with active TB disease or latent tuberculosis infection are needed.

Acknowledgements

The authors would like to thank Amy Rhinehart, MA and Kassi Spealman, RN for assistance with recruitment and enrolment, Bridget Vesosky, PhD for reviewing and verifying T-SPOT.TB assay spots, and Dr Roger Miller, Dr Mary Elam, and Judy Hild for facilitating the use of the OSU Student Health Center for recruitment and enrolment. Statistical support for Dr Haikady N. Nagaraja: National Institute of Health, Clinical and Translational Science Award (NIH CTSA) grant UL1RR025755 awarded to The Ohio State University. The authors also acknowledge Oxford Immunotec Ltd, UK who contributed towards the costs of this study and to the provision of assay materials only.

Declaration of interest: There is no conflict of interest.