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, P. W., Longhurst, S., Joel, S. P., Slevin, M. L., & Balkwill, F. R., https://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.