Impact of Delayed Centrifugation on Interleukin 6 Determination in Human Blood

<b>Background/Objectives</b>: Clinical experience indicates that the determination of interleukin 6 (IL-6) in human blood can vary depending on time span between sample collection and centrifugation. Here, we evaluated confounding effects in various blood specimens. <b>Methods</b>: The blood of healthy individuals and critically ill patients was collected in EDTA-, heparin-, and serum collection tubes. Tubes were facultatively incubated (20 °C, 24-48 h) before centrifugation, and IL-6 was measured in the supernatant. <b>Results</b>: The preincubation of the blood collection tubes increased the IL-6 values in heparin plasma (in 17/20 samples up to 50-fold) and serum (in 17/20 samples up to 12-fold). These changes were relevant since the normal values were thereby lifted above the upper confidence limit in 12/20 heparin plasma samples and 4/20 serum samples. These IL-6 increases were probably due to in vitro synthesis as opposed to the release of preformed IL-6 from blood cells because subjecting uncentrifuged collection tubes to mechanical cell lyses had negligible effects on IL-6, while incubation with microbial stimulators dramatically increased these values. In the case of EDTA blood, collection tube preincubation induced IL-6 decreases in 17/20 samples from healthy individuals and 20/23 samples from critically ill patients. <b>Conclusions</b>: IL-6 determination in heparin plasma and serum is compromised by delayed centrifugation. This effect is relevant for normal values. It increased the number of false high results by >50%. The delayed centrifugation of EDTA blood decreased the IL-6 values, which caused a single false-negative result in 1/43 healthy and critically ill people. The false-negative rate is possibly higher in EDTA blood from non-critically ill out-patients, exhibiting moderately increased IL-6 levels.