Assessment of serum total and bone alkaline phosphatase measurement in clinical practice

The aim of the study was to measure serum levels of the bone-specific isoenzyme of alkaline phosphatase in normal subjects and patients with metabolic bone disease by using an immunoadsorption assay. We studied 140 healthy adults, 122 patients affected by metabolic bone disease and 15 patients with cholestatic liver disease. Mean values of the bone-specific isoenzyme of alkaline phosphatase in healthy men were significantly higher than those found in premenopausal women (17.8 ± 4.2 U/l vs 15.6 ± 4.6 U/l, p <0.02); postmenopausal women had significantly higher levels of bone-specific isoenzyme of alkaline phosphatase (22.6 ± 6.4 U/l) than premenopausal women (p <0.0001). After the menopause total alkaline phosphatase increased by 46%, while the increase in bone-specific isoenzyme of alkaline phosphatase was 39%. No significant correlations were found between bone-specific isoenzyme of alkaline phosphatase and either age or years since menopause, in postmenopausal subjects. In patients with bone-specific isoenzyme of alkaline phosphatase above the upper limit of normal, the assay had a sensitivity of 100% only in patients with Paget's disease of bone. In patients with cholestatic liver disease we found no correlation between bone-specific isoenzyme of alkaline phosphatase and either total alkaline phosphatase and γ-glutamyl transpeptidase, while a positive correlation was found between total alkaline phosphatase and γ-glutamyl transpeptidase. Our results confirm the role of bone-specific isoenzyme of alkaline phosphatase assay in clinical research; however, its usefulness in clinical practice is unclear once liver involvement has been excluded.