Overhead rotation: A simple method to assess the mechanical stability of stored erythrocytes

Erythrocytes loose some functional qualities during storage, which may influence the outcome after transfusion. One of them is mechanical stability, which determines their in vivo survival in the circulation. We have analyzed different forms of mechanical stress and have developed a simple, reproducible test for mechanical stability. Specimens of outdated erythrocyte units stored under routine conditions were investigated. Mechanical stress was applied either by rolling blood-containing 5 ml tubes at 15 rpm (Mixer 820, Swelab, Sweden) or overhead rotation at 10 rpm (Intelli-Mixer RM-2S Elmi, Skyline, Axon Lab AG, Baden, Switzerland). Free hemoglobin (Hb) in the supernatant was used as a parameter of membrane integrity. Stored erythrocyte units at the end of their "shelf-life" of 42 days had a median free Hb concentration of 1.8 g/l (25-75 percentiles: 1.8-2.6 g/l) corresponding to a spontaneous hemolysis rate of 0.31% (0.28-0.46%). In samples subjected to 24 h rolling, free Hb rose to 4.8 (4.0-7.0; p = 0.005). Overhead rotation for 24 h increased free Hb to 17.1 (12.2-27.9) g/l when 1.5 ml blood in 5 ml tubes were used, and to 38.0 (19.6-55.2) g/l when 4.5 ml in 5 ml tubes were used (p = 0.005 between the two groups), indicating that hemolysis during rotation depended on the blood volume. The type of tube also influenced the extent of hemolysis. A large variation was seen between different RBC units. The time course of hemolysis was an inverse exponential function; i.e. 2 h of rotation induced already 45% and 7 h 86% of the hemolysis measured after a 24 h rotation. We conclude that the rate of hemolysis after a standardized overhead rotation is a simple, useful laboratory test to determine the mechanical stability of stored erythrocytes. Large variations between different RBC units suggest that this may be valuable tool for the quality control of stored RBCs.