Flexibility in sample processing may improve test utility of the quantitative antigen-specific T cell assay (T-SPOT^®.TB). We investigated whether delayed sample processing with and without the use of T-Cell Xtend, a proprietary reagent, impacted upon test accuracy.

Blood samples obtained from 363 sequentially recruited tuberculosis suspects or treated patients were processed immediately (day 0) and at different times after receipt of the sample [24-h (day 1) or 32-h (day 2)] with and without adding T-Cell Xtend.

T-Cell-Xtend-independent median ELISPOT counts (spot forming cells per million peripheral blood mononuclear cells) were significantly higher at day 1 versus day 0 (114 vs. 100; n=66; p=0.03); inter-time-point agreement between the results was 95.45% and the conversion/reversion rate was 4.55%. By contrast, counts on day 0 without T-Cell Xtend versus day 1 with T-Cell Xtend were similar (56 vs. 56; n=215), inter-time-point agreement between the results was 97.17%, and the conversion/reversion rate was 2.83%. Counts performed at day 2 with T-Cell Xtend were not significantly different from day 0. These findings were independent of HIV status.

There was high agreement between results when samples were processed immediately and after a 24-h delay. However, although the use of T-Cell Xtend appeared to reduce the number of conversions/reversions this reduction was not statistically significant. Larger studies are required to clarify these findings.