Stability of interleukin 6, soluble interleukin 6 receptor, interleukin 10 and CC16 in human serum

The stability of interleukin 6 (IL-6), its soluble receptor (sIL-6R), IL-10 and CC16 or uteroglobin (an endogenous cytokine inhibitor) in human serum was examined using an accelerated stability testing protocol according to the Arrhenius equation. Further, the effect of time delay between blood sampling and sample processing, clotting temperature and repeated freeze-thaw cycles on serum levels of these proteins were determined. Paired serum samples were stored at 4°C, 20°C, 30°C and 40°C for 1 to 21 days. We found that IL-6 and CC16 concentrations did not change at 4°C, 20°C and 30°C. Interleukin-6 concentrations significantly declined after 11 days at 40°C. The concentrations of sIL-6R and IL-10 did not change at 4°C but significantly decreased at 20°C (after 21 and 14 days respectively), 30°C and 40°C (after 1 day at both temperatures for both cytokines). Arrhenius-plots indicated that sIL-6R and IL-10 are stable for at least several years at - 20°C and - 70°C, respectively. Since their relative stability, no Arrhenius-plot could be calculated for IL-6 and CC16. The concentrations of the proteins examined were not significantly altered by repeated freeze-thaw cycles, nor by extended clotting times at 4°C or 20°C. We conclude that serum samples for the determination of IL-6, sIL-6R and CC16 can be stored at - 20°C for several years, but for IL-10 determinations, storage at - 70°C is recommended.