AN ENZYMATIC ASSAY FOR ERYTHROCYTE CREATINE AS AN INDEX OF THE ERYTHROCYTE LIFE TIME

Objectives: To establish and estimate an enzymatic measurement of crea tine in erythrocytes as an index of the erythrocyte lire time. Design and Method: The measurement of creatine in erythrocytes was performed using an enzymatic assay kit that was developed for serum and urine cr eatine. An erythrocyte sample was subjected to creatine measurement af ter hemolysis and deproteinization. Performance of the method for crea tine measurement in erythrocytes was estimated. Effects of age and gen der on the creatine content of erythrocytes were also estimated in 305 normal subjects. Results: The method showed within-run CVs varying fr om 0.7 to 1.0% (n = 20), and between-day CVs from 1.3 to 1.7% (15 days ). Good linearity was observed at least up to 1000 mu mol/L as creatin e value in hemolyzed sample. The analytical recovery was calculated to be 98.1 +/- 1.3% on average. No considerable interference by various substances, including guanidino compounds and amino acids, with the as say was observed. Excellent correlation was observed between the prese nt method and high performance liquid chromatography. With the unit of mu mol/g Hb: slope, 1.034 +/- 0.003 (mean +/- SD); intercept. -0.059 +/- 0.012 (mean +/- SD); correlation coefficient, 0.9996; and Sy.x, 0. 069. With the unit of mu mol/L RBC: slope, 1.033 +/- 0.003 (mean +/- S D); intercept, -18.23 +/- 3.55 (mean +/- SD); correlation coefficient, 0.9996; and Sy.x, 20.40. A significant increase in erythrocyte creati ne was observed in females aged 11- to 50 years old as compared with m ales in the corresponding age bracket, however, a gender difference wa s not observed in other age bracket. This finding suggests the possibi lity of a slight decrease in the erythrocyte life time due to menstrua tion in females. Conclusion: This study showed that the present method is favorable for quantifying erythrocyte creatine, and has analytical characteristics suitable for routine work in clinical laboratories. C opyright (C) 1998 The Canadian Society of Clinical Chemists.