Introduction The T-SPOT^®.TB assay involves the collection of peripheral blood samples for analysis of the T cell response to TB-specific antigens. Traditionally the blood sample has been required to be processed on the same day as it is collected. This study was designed to investigate a new procedure, for use in combination with the T-SPOT.TB assay, which offers the potential to initiate the antigen stimulation the day following blood collection, thereby allowing overnight transportation of samples from the point of collection to a processing laboratory.

Methodology Subjects were enrolled at two sites; one in the UK and one in South Africa. Patients all provided informed consent and a brief medical history; blood samples were taken into two lithium heparin tubes.

One tube was selected for immediate testing by the T-SPOT.TB assay. The second tube was stored overnight at room temperature. Immediately prior to testing the stored blood sample, the T-Cell Xtend reagent (Oxford Immunotec Ltd, UK) was added to the sample and incubated for 20 min.

Results A total of 208 participants were enrolled into the study and 3/208 (1.4%) subjects were excluded due to blood collection errors at enrolment. Of the 205 samples available for analysis, 10/205 (4.9%) failed to yield a valid T-SPOT.TB test result with either the fresh blood (3 samples) or the stored blood (7 samples) leaving 195 paired results with both fresh and stored blood samples for analysis (see Abstract P164 Table 1).

The overall agreement for the study was 97.9% (191/195); 95% CI 94.8 to 99.4%, with a kappa value of 0.943 which indicates excellent agreement.

Conclusions The T-SPOT.TB assay on overnight stored blood samples, when combined with a pre-incubation step utilising the T-Cell Xtend reagent, yields results that are equivalent to those obtained with fresh blood samples. This allows, when combined with overnight shipment to a central testing facility, greater flexibility in when and where blood samples can be collected.