Placental alkaline phosphatase isoenzyme in quality control materials may be a source of variability in alkaline phosphatase activity

Objectives: To determine the cause of discrepancies in the alkaline phosphatase (ALP) activities of quality control (QC) materials in two different analyzers using IFCC method. Design and methods: ALP activities of patients' samples and QC materials (QC1 and QC2 from Bio-Rad) measured using TBA-200FR and Synchron LX-20 analyzers were compared and isoenzyme electrophoresis was done. Fractional mixing of bone or liver ALP with placental ALP was performed. ALP activities of QC materials were measured in TBA-200FR with pH-modified reagents. Results: ALP activities of QC materials were significantly lower in TBA-200FR than in LX-20. Placental ALP comprised 57% of QC1 and 95% of QC2. Higher placental ALP proportion in the mixture with bone or liver ALP resulted in lower ALP activities in TBA-200FR. The discrepancy in ALPs of QC materials decreased when measured with pH-modified reagents. Conclusion: Placental ALP in QC materials and differences in reagents' pH caused the discrepancy in ALP activities. (C) 2010 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.