Use of the refractive index to measure the adequacy of glycerol removal from previously frozen erythrocytes.

A common method for measuring deglycerolization of previously frozen erythrocytes is to determine the osmolality of the supernatant fluid after completion of the wash cycle. We attempted to compare this type of examination with the simpler, faster, and cheaper determination of refractive index (RI) using the Goldberg refractometer. One hundred twenty units of deglycerolized red blood cells, some with glycerol added so as to exceed an acceptable 1% glycerol content, had measurements made of the post-wash supernatant fluid by refractive index and osmometry. The responsiveness of refractive index to residual glycerol was essentially the same as osmolality. Although there is more interference from supernatant hemoglobin, this did not preclude the use of RI to measure glycerol removal. Readings up to 28 had glycerol levels of less than 1%. Readings between 29 and 31 should be checked by osmometry, and readings over 31 should cause the unit of blood to be rejected.