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HbA1c decreased in inverse proportion to the increase in erythrocyte creatine because of a shortened mean age of erythrocytes. The abnormally decreased HbA1c value could be assessed with erythrocyte creatine.

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We developed a new, highly sensitive enzymatic method for quantifying creatine in erythrocytes, which comprises creatine amidinohydrolase, sarcosine oxidase, and peroxidase. In the present method, an N-methylcarbamoyl derivative of methylene blue, 10-N-methylcarbamoyl-3,7-bis(dimethylamino)phenothiazine (MCDP), was used as a sensitive chromogenic compound. Potassium ferrocyanide was used to prevent nonspecific oxidation of MCDP. The enzymatic method exhibited good analytical performance: precision, within-run CVs <1.0% and between-day CVs <2.0%; average analytical recovery, 99.3% ± 1.8%; detection limit, 1.0 μmol/L in hemolysate; and linearity, at least up to 500 μmol/L as creatine concentration in hemolysate. Excellent agreement was observed between the present method (y) and HPLC (x), y = 1.029x − 0.002 μmol/g hemoglobin, r = 0.9998, Sy‖x = 0.053 μmol/g hemoglobin (n = 110). No significant interference was produced by various compounds, including guanidino compounds, amino acids, and reducing materials. The reference intervals (mean ± 2 SD) for erythrocyte creatine obtained from 60 males and 60 females were (in μmol/g hemoglobin) 1.18 ± 0.52 (0.66–1.70) for males and 1.35 ± 0.49 (0.86–1.84) for females. Using this method, we documented changes in erythrocyte creatine in patients with various hemolytic conditions, including hemolytic anemia, liver cirrhosis, renal insufficiency, and chronic renal failure treated with hemodialysis with or without the administration of erythropoietin. We conclude that the use of MCDP allows sensitive measurement of erythrocyte creatine and that MCDP with potassium ferrocyanide can improve the sensitivity of assays that use peroxidase for detection of H2O2.