Creatine measurement in serum and urine with an automated enzymatic method

Abstract

I describe an automated enzymatic procedure to quantitate creatine in both serum and urine. In this assay, which requires no pretreatment of the sample, creatine kinase (CK; EC 2.7.3.2) and pyruvate kinase (EC 2.1.7.40) are used as auxiliary enzymes and lactate dehydrogenase (EC 1.1.1.27) is used in the indicator reaction. CK is also used as the starting reagent. Data obtained with the present method for creatine measurement in serum were compared with those from the Jaffé method and an enzymatic method: y = 1.13x - 7.58, SE = 4.48, and r = 0.925 (Jaffé); and y = 1.17x + 2.73, SE = 5.06, and r = 0.962 (enzymatic); for creatine measurement in urine: y = 0.63x + 39.74, SE = 296.7 and r = 0.719 (Jaffé). The present method provides improved precision: the total CVs for serum, determined by the present and comparative methods, respectively, were 3.5-8.9%, 8.2-43.0%, and 5.3-16%; for urine, the CVs were 3.3-5.1% and 9.6-21.2% for the present and comparative method, respectively. I established the normal reference interval as 13-74 and 13-89 μmol/L for creatine in serum, and as 175-700 and 150-1200 μmol/24 h for creatine in urine for men and women, respectively.