Long-term stability of glucose in lyophilized quality control serum. A study utilizing data from the Quality Assurance Service (QAS) Program of the College of American Pathologists.

The long-term stability of glucose in 45 commercial pools of lyophilized quality control serum is evaluated. The pools have been used in conjunction with the Quality Assurance Service (QAS) of the College of American Pathologists by laboratories participating in Regional Quality Control Programs. Directional instability of glucose was detected by at least one analytic method in 69% of the pools studied. Two distinct and opposite directional trends in glucose concentration were found. Increasing concentration of analyte, averaging 4.7 mg/dl per year, were observed with manual and automated glucose oxidase methods and the automated neocuproine procedure in approximately one fourth of pool--method combinations. Decreasing concentrations of glucose, averaging 3.0 mg/dl per year, were found with automated ferricyanide and hexokinase methods and the manual orthotoluidine procedure in approximately one third of pool--method combinations. The results are best explained by postulating that in affected pools there is a gradual diminution of free and bound glucose and/or a shift of glucose from the protein-bound to the free state.