Myoglobin and creatine kinase isoenzyme MB mass assays: intermethod behaviour of patient sera and commercially available control materials.
The low biological variation of myoglobin and creatine kinase isoenzyme MB mass (CK-MBm) requires accurate measurements. In the standardization process, in order to effectively measure and correct intermethod variability, the intermethod behaviour of control materials must be the same of patient sera, i.e. they must be commutable. In this work we checked the commutability of some commercially available control materials in pairs of methods for myoglobin and CK-MBm measurements; we assessed the impact of commutable and non-commutable control materials when used for equalizing patient sera results by two different methods and discussed the problems related to external quality assessment schemes. Myoglobin and CK-MBm were measured in sets of 49 and 56 patient sera and in 13 commercially available control materials with two automatic analytical systems. The non-commutability rate was 8.3% for myoglobin and 23.1% for CK-MBm. Recalculation of serum samples results with a control material as calibrator lowered or increased the bias originally present according to whether the material itself was commutable or not. We conclude that also for myoglobin and CK-MBm assays it is necessary to check the commutability of materials to be used in external quality assessment schemes, or to normalize patient results by different methods.