We have shown that several commercial control sera containing reversibly inactivated creatine kinase are reactivated by pre-incubation in N-acetyl cysteine assay diluent. The increase in activity can proceed for up to 4 h. This reactivation is not seen in patients' sera even after storage for 14 days at −20°C, 7 days at 4°C, or 2 days at 20°C, during which time no loss of activity was evident. This study brings into question the validity of using consensus values to standardise enzyme assays.
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