Mc. Backstrom et al., Bone isoenzyme of serum alkaline phosphatase and serum inorganic phosphatein metabolic bone disease of prematurity, ACT PAEDIAT, 89(7), 2000, pp. 867-873
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.