Measurement of peripheral blood cytokines and other immunomodulatory proteins is a useful and popular tool for assessing human immune responses to a wide range of assaults. A common challenge in this work is obtaining fresh, high-quality samples and limiting the time between blood collection and the separation of plasma or serum from cells. In this study we sought to determine the effect of sample age at the time of processing on the measured levels of 41 soluble immune mediators. Two cohorts were examined: healthy lab donors and trauma patients, who have significant immune perturbation. Whole-blood samples were aliquoted, and plasma was isolated, at days 0, 1, 2, and 3 after collection. Multiplexing techniques were used to measure protein concentrations, and general estimating equations were used to determine if there was a significant change over time. Over the 3-day period examined, only 15 of the 41 proteins showed no significant change in either cohort. Among the remaining proteins both increases and decreases were observed, with changes ranging from 2.4% per day to 325% per day. Proteins with significant changes in one cohort did not always show significant changes in the other group. These results support the need to separate plasma or serum from whole blood as quickly as possible and/or to standardize the length of time to processing within a given study of peripheral blood protein concentrations. When this is not possible, care should be taken to account for differences due to sample age.
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