Currently, the concentration of unbound IL-6 is typically measured in serum, i.e. blood samples are clotted and refrigerated-centrifuged within 25-40 minutes after collection. The serum is then suited for use in several immunoassays, e.g., ELISA, CLIA, ECLIA, and RIA. Alternatively, the serum can be frozen and stored at -20 ℃ or lower for later use (.<ref name="Arti25">Plasma and Serum Preparation, ThermoFisher Scientific, 2007, https://www.thermofisher.com/nl/en/home/references/protocols/cell-and-tissue-analysis/elisa-protocol/elisa-sample-preparation-protocols/plasma-and-serum-preparation.html </ref><ref name="Arti26">Measuring cytokine levels in blood. Importance of anticoagulants, processing, and storage conditions. Journal of Immunological Methods, 153(1–2)(, 115–124, 1992, Thavasu et al, P. W., Longhurst, S., Joel, S. P., Slevin, M. L., 1992) (Todd et al& Balkwill, F. R., 1979https://doi.org/10.1016/0022-1759(92). 90313-i </ref> In addition, commercial assays also allow for measurement in plasma. Whole blood is collected into anticoagulant-treated tubes e.g., EDTA-treated or citrate-treated. Cells are removed from plasma by centrifugation for 10 minutes at 1000-2000 x g using a refrigerated centrifuge. Centrifugation for 15 minutes at 2000 x g depletes platelets in the plasma sample.