Bone isoenzyme of serum alkaline phosphatase and serum inorganic phosphatein metabolic bone disease of prematurity

We wanted to improve detection of low bone mineral density in preterm infan ts by combining serum measurements of total alkaline phosphatase, its bone- type isoenzyme and serum inorganic phosphate in a prospective design. The s ubjects were 43 preterm infants. Total and bone isoenzyme activity of alkal ine phosphatase was determined at 3 wk chronological age and at 3 and 6 mo corrected age. The main outcome measure, apparent bone mineral density (BMA D) at the distal forearm and forearm shaft, was assessed by dual energy X-r ay absorptiometry at 3 and 6 mo corrected age. An apparent density below 95 mg/cm(3) at 3 mo corrected age was considered to indicate bone disease, ba sed on the distribution of BMAD values of children with non-complicated cou rses of prematurity. At 3 mo corrected age, total alkaline phosphatase acti vities exceeding 900 IU/1 revealed low bone mineral density with 88% sensit ivity and 71% specificity. Measurements of bone isoenzyme activity did not improve diagnostic performance. Serum inorganic phosphate levels below 1.8 mmol/l reflected low bone density with high specificity (96%), but the sens itivity was only 50%. Conclusion: A combination of the criteria "serum total alkaline phosphatase activity above 900 IU/1" and "serum inorganic phosphate concentrations bel ow 1.8 mmol/l" yielded a sensitivity of 100% at a specificity of 70%. This was the best available screening method for low bone mineral density in pre terms.