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

Acta Paediatr. 2000 Jul;89(7):867-73.


We wanted to improve detection of low bone mineral density in preterm infants by combining serum measurements of total alkaline phosphatase, its bone-type isoenzyme and serum inorganic phosphate in a prospective design. The subjects were 43 preterm infants. Total and bone isoenzyme activity of alkaline phosphatase was determined at 3 wk chronological age and at 3 and 6 mo corrected age. The main outcome measure, apparent bone mineral density (BMAD) at the distal forearm and forearm shaft, was assessed by dual energy X-ray absorptiometry at 3 and 6 mo corrected age. An apparent density below 95 mg/cm3 at 3 mo corrected age was considered to indicate bone disease, based on the distribution of BMAD values of children with non-complicated courses of prematurity. At 3 mo corrected age, total alkaline phosphatase activities exceeding 900 IU/l revealed low bone mineral density with 88% sensitivity 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 sensitivity was only 50%.

Conclusion: A combination of the criteria "serum total alkaline phosphatase activity above 900 IU/l" and "serum inorganic phosphate concentrations below 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 preterms.

MeSH terms

  • Absorptiometry, Photon
  • Age Factors
  • Alkaline Phosphatase / blood*
  • Bone Density*
  • Bone Diseases, Metabolic / diagnosis*
  • Calcium Phosphates / blood*
  • Humans
  • Infant, Newborn
  • Infant, Premature
  • Isoenzymes / blood
  • Prospective Studies
  • Regression Analysis
  • Sensitivity and Specificity


  • Calcium Phosphates
  • Isoenzymes
  • Alkaline Phosphatase