Impact of Delayed Centrifugation on Interleukin 6 Determination in Human Blood

Background/Objectives: Clinical experience indicates that determination of interleukin 6 (IL-6) in human blood can vary depending on time span between sample collection and centrifugation. Here, we have evaluated that confounding effect in various blood specimen. Methods: Measurement of IL-6 before and after pre-centrifugation incubation (20 °C, 24 - 72 h) in heparin-plasma, serum and EDTA-plasma from healthy individuals and critical ill patients. Results: Pre-incubation induced IL-6 increases in heparin-plasma (in 17/20 samples up to 50-fold) and serum (in 17/20 samples up to 12-fold). Normal values were thereby lifted above the confidence limit in 12/20 heparin plasma samples and 4/20 sera. The observed increases are probably due to in vitro synthesis/incretion as opposed to release of cell-bound IL-6, because microbial stimulators evoked > 1000-fold increases while cell lyses had negligible effects on IL-6. In EDTA-plasma, pre-incubation induced IL-6 decreases by up to 30 % in 17/20 samples, which entailed one false-negative result. Conclusions: IL-6 determination in heparin-plasma and serum is significantly compromised by delayed centrifugation, which increases the rate of false-high results by > 50 %. IL-6 determination in EDTA-plasma is less affected by delayed centrifugation, which decreases the results moderately but induces false-negative results in < 10% of cases.