Alkaline phosphatase activity measurement in the UK by AMP-buffered methods: an appraisal of current practice

Most UK clinical laboratories use alkaline phosphatase (ALP) methods similar to that proposed by the International Federation of Clinical Chemistry (IFCC), based on the use of 2-amino-2-methyl-1-propanol (AMP) buffer. We present evidence of significant differences in results produced by apparently similar commercial ALP methods using an AMP buffer. We compared Bayer DAX, Dade Dimension and Boehringer Mannheim Hitachi 717 methods. Boehringer and Dade results were higher than Bayer results (Bland and Altman analysis, log transformed data): Boehringer (+23.0%, limits of agreement 1.16-1.31 times Bayer); Dade (+21.9%, limits of agreement 1.13-1.32 times Bayer). Biases were predominantly due to differences in reagents rather than analyser characteristics. Compared to a reagent system prepared exactly as described by the IFCC, Bayer was sub-optimal and Dade and Boehringer methods produced results higher than the IFCC method. Reference ranges and results on patients' samples by the various methods showed large differences but no clinically significant difference was observed in external quality assessment schemes either between Bayer and Boehringer or against method means. Apparently similar methods produce different results in patients' sera: external quality assessment schemes are not useful in highlighting these differences.