Stratus automated creatine kinase-MB assay evaluated: identification and elimination of falsely increased results associated with a high-molecular-mass form of alkaline phosphatase.

We compared the performance of an automated assay of creatine kinase MB isoenzyme (CK-MB) mass (Stratus) with that of a CK-MB enzymatic assay routinely used at our institutions. Both of these assays use the same CK-MB-specific monoclonal antibody to immunocapture CK-MB, thus providing a direct means of comparing a mass assay with an activity assay. Routine CK-MB measurements for 206 samples within the analytical range of both assays revealed the following relationship: Stratus (micrograms/L) = 0.67 (activity U/L) + 0.18 (r = 0.95, Sx.y = 4.45). The linearity, sensitivity, and precision of the Stratus assay were acceptable for routine clinical use. Icteric, lipemic, and hemolyzed samples do not interfere with the assay. During our evaluation we identified a single, clinically significant false-positive sample. Because this patient had alkaline phosphatase values greater than 1100 U/L, we investigated additional samples with increased activities of alkaline phosphatase and found that samples from 12 of 23 patients selected for alkaline phosphatase values greater than 460 U/L produced falsely increased CK-MB values. We determined that a membrane-associated, high-molecular-mass form of alkaline phosphatase was a cause of these falsely increased values and instituted an approach to identify falsely increased Stratus CK-MB values. Samples from 23 of 1933 patients were falsely increased, the increase being clinically significant in samples from 14 of these patients. Consultation with the manufacturer resulted in the successful reformulation of the substrate/wash solution to minimize interferences from high-molecular-mass forms of alkaline phosphatase.