Evaluation of alkaline phosphatase activity assay for the Reflotron system after substitution of the comparison...

The alkaline phosphatase (ALP) assay for the Reflotron system is a dry chemistry method applicable for decentralized testing. The ALP diethanolamine method has been applied as the comparison method, which is now replaced by the more widely distributed aminomethylpropanol method. The new comparison method measures ALP activities by approximately 59% lower compared to the former method. The analytical performance of the newly calibrated ALP Reflotron method was evaluated at three clinical laboratories. The study of the imprecision showed coefficients of variation ranging from 1.6-3.0% (intraassay) and 2.7-5.7% (interassay). Blood samples from liver disease patients (n=192), adolescents (n=40) and pregnant women in the third trimester (n=34) were collected in order to evaluate the impact of different ALP isoenzymes. The comparison between ALP Reflotron and the new comparison method revealed an excellent agreement in Passing-Bablok regression analysis (slope: 1.004; intercept: 0.14 U/l; median difference: 0.5%) for samples with prevailing liver ALP. However, in plasma samples with predominantly elevated bone or placental isoenzyme the Reflotron method measured lower or higher activities, respectively (slope: 0.912, 1.394; intercept: -4.16, -17.26 U/l; median difference: -10.58, 26.33%). The ALP Reflotron assay is an adequate method for decentralized testing. It shows a good precision and agreement with the comparison method, although the systematic differences for placental and bone ALP isoenzymes have to be considered.