An Enzymatic Assay for Erythrocyte Creatine as an Index of the Erythrocyte Life Time

Objectives: To establish and estimate an enzymatic measurement of creatine in erythrocytes as an index of the erythrocyte life time. Design and Method: The measurement of creatine in erythrocytes was performed using an enzymatic assay kit that was developed for serum and urine creatine. An erythrocyte sample was subjected to creatine measurement after hemolysis and deproteinization. Performance of the method for creatine measurement in erythrocytes was estimated. Effects of age and gender on the creatine content of erythrocytes were also estimated in 305 normal subjects. Results: The method showed within-run CVs varying from 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 μmol/L as creatine 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 assay was observed. Excellent correlation was observed between the present method and high performance liquid chromatography. With the unit of μmol/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 μmol/L RBC: slope, 1.033 ± 0.003 (mean ± SD); intercept, −18.23 ± 3.55 (mean ± SD); correlation coefficient, 0.9996; and Sy.x, 20.40. A significant increase in erythrocyte creatine was observed in females aged 11- to 50 years old as compared with males in the corresponding age bracket, however, a gender difference was not observed in other age bracket. This finding suggests the possibility of a slight decrease in the erythrocyte life time due to menstruation 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.