Mass versus activity: validation of an immunometric assay for bone alkaline phosphatase in serum

A detailed investigation of the performance of an immunometric assay for the quantitation of bone alkaline phosphatase (ALP) in serum showed that precision of this assay was similar to that of lectin precipitation and electrophoretic methods. Relationships for mass and activity measurements of the bone isoform using samples from children and patients with Paget's disease were similar for lectin and electrophoretic methods. Purified liver enzyme showed 100% cross reactivity in the immunometric assay. However, comparison of the slopes between bone ALP mass and total ALP activity using only samples with predominantly liver or bone isoforms showed that the cross reactivity of the liver isoform in serum was 18.3%. Experiments in which increasing amounts of a sample containing 90% of liver isoform adding to a serum sample from a patient with Paget's disease showed a cross reactivity of 16.5%. The reference range for bone ALP mass was 7-28 micrograms/L for men (n = 77) and 5-20 micrograms/L for women (n = 110) in the age group 20-50 years. In women over 50 years bone ALP was 28% higher. Increased bone ALP mass was also demonstrated in patients with Paget's disease (n = 59), liver disease (n = 95), chronic renal failure (n = 41) and hyperthyroidism (n = 17).