Intended Use For the quantitative determination of creatine kinase activity in serum. For in vitro diagnostic use only. Principle The kinetic procedure presented is a modification of Szasz of the Rosalki technique, which optimizes the reaction by reactivation of CK activity with N-actyl-L-cysteine (NAC). CK specifically catalyzes the transphosphorylation of ADP to ATP. Through a series of coupled enzymatic reactions, NADPH is produced at a rate directly proportional to the CK activity. The method determines the NADPH absorbance increase per min at 340 nm.