Comparison of same day versus delayed enumeration of TB-specific T cell responses.

Background/objective: 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.

Methods: 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 [approximately 24-h (day 1) or approximately 32-h (day 2)] with and without adding T-Cell Xtend.

Results: 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.

Conclusion: 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.