Increase in Serum Potassium Levels After Refrigerated Storage: A Component of Blood Clot Contaminates the Serum Layer Over the Separator Gel.

Objectives: The aim of this study was to determine the cause of elevated serum potassium levels when blood collection tubes containing separating gel are stored under refrigeration. Methods: Fifty-seven hospitalized patients and 11 healthy volunteers were recruited. Venous blood samples were obtained using Insepac II, Neotube, and Venoject^® II, without anticoagulant. After centrifugation under different processing conditions, the capped tubes were stored at 4°C without aliquoting, and serum potassium levels were measured for up to 14 days. Correlation between the increase in potassium levels and blood cell counts was assessed. Furthermore, serum was replaced with a saline solution and potassium levels were determined after refrigeration. Results: Refrigerated samples stored in Insepac II tubes had significantly higher serum potassium levels on day 14 than on the day of blood collection. The increase in serum potassium levels was positively correlated with the number of red blood cells, but not white blood cells and platelets in venous blood. Furthermore, potassium levels were elevated when serum was replaced with a saline solution. Using Venoject II, which has a larger tube diameter and thicker separating gel than those of Insepac II and Neotube, did not increase serum potassium levels after storage. Increase in the serum potassium level was markedly suppressed by centrifugation at 2330 g for 15 minutes relative to other processing conditions. Conclusions: Potassium levels increase when serum is refrigerated in collection tubes containing separating gel. This can be attributed to contamination of the serum layer by blood cell components beyond the separating gel.