Long-term stability of enzymes, total protein, and inorganic analytes in lyophilized quality control serum

The stability of total protein and various enzyme and electrolyte analytes in numerous pools of lyophilized quality control sera is evaluated. Using data obtained between 1973 and 1976 from Regional Quality Control Programs utilizing the Quality Assurance Service of the College of American Pathologists, monthly group mean values are studied as a function of time, by means of linear regression analysis. In the 28 pools studied, unstable analyte-pool combinations were detected in approximately 35% of the cases. Total protein was the most universally stable. In a large percentage of pools, minimally increasing concentrations of sodium, chloride, and potassium were found, while inorganic phosphorus decreased in the majority of pools. In pools with changing enzymatic activiteis, alkaline phosphatase tended to increase and creatine phosphokinase generally decreased. Changes in levels of calcium, serum glutamic oxaloacetic transaminase, and lactate dehydrogenase were inconsistent. Changes in analyte concentration were judged in relation to the precision (SD) with which the analytes were measured. The ratios of rate of change in analyte value to SD were lower for electrolytes than for other analytes.