Plasma alkaline phosphatase activity: a screening test for rickets in preterm neonates.

Both rickets and raised plasma alkaline phosphatase activity are common in the preterm infant. Measurement of plasma alkaline phosphatase activity is valuable in screening for active disease and in diagnosis, but normal reference data are not available for preterm babies. In 30 consecutive preterm infants (birthweight 1580 +/- 410 g, gestational age 31 +/- 2.5 weeks) serial measurements of plasma alkaline phosphatase activity, plasma levels of calcium and inorganic phosphorus, and the pattern of alkaline phosphatase isoenzymes were made. Four patterns of changes in plasma alkaline phosphatase activity with time were seen. 4 of the 30 infants were shown to have rickets; these children and 14 of the 26 non-rachitic infants showed an increasing/peak/decreasing pattern with increasing age, but levels were much higher in the rachitic infants. The activity in all 30 was raised above the adult and childhood reference ranges at some point in time. The data suggest that an activity of five times the upper limit of the normal adult reference range is acceptable in the preterm infant but an activity higher than this may suggest rickets.