Impact of short-term storage of plasma samples on quantitation of ultra-low levels of interleukin-6

OBJECTIVE: To quantitate plasma interleukin-6 (IL-6) levels in healthy individuals and to clarify how these levels are affected by blood sample handling procedures during short-term storage. METHODS: Ethylenediaminetetraacetic acid (EDTA)-treated plasma samples were simultaneously collected from 14 healthy individuals and stored on ice prior to analysis of the IL-6 levels. White blood cells (WBCs), red blood cells, and platelets were counted immediately after blood collection. IL-6 levels were analyzed every 30 minutes using a commercial electrochemiluminescence immunoassay. RESULTS: Correlation coefficients between plasma IL-6 levels and WBC counts ranged between 0.605 and 0.554, higher than those for other cell types. The lowest IL-6 value in healthy individuals was estimated at 0.04 pg/mL and the mean values remained under 2 pg/mL over time. CONCLUSION: Analysis of IL-6 levels in EDTA-treated plasma samples centrifuged within 1 hour and stored on ice can be performed within 90 minutes of short-term storage if the analytical method has a sensitivity in the range of 10 fg/mL.