A standard enzyme assay may help screen for rickets in breast-fed infants not given vitamin D supplements
Research Activities, September 2011, No. 373
Testing for increased alkaline phosphatase (AP) activity in the blood of breast-fed infants, who are not getting vitamin D supplements, can be effective in screening this population for rickets, a new study finds. Rickets in children is associated with interruption in bone growth and mineralization, and possible limb deformities. In 2003, the American Academy of Pediatrics recommended that all breast-fed infants receive 200 international units (IU) of supplemental vitamin D daily, later raised to 400 IU daily. Because rickets is known to be associated with increased levels of blood AP, James A. Taylor, M.D., of the University of Washington, and colleagues decided to test blood samples from breast-fed infants for the enzyme's level. They used radiography to confirm the presence of the disorder.
Based on the analysis of AP in blood from 246 children, enzyme levels were found to be elevated in 33 children (13.4 percent). Parents of 18 children (54.5 percent) with elevated AP levels agreed to allow radiographs to be taken. The researchers found that the AP level with the highest sensitivity and specificity for rickets screening was 552 U/L, and 11 children (4.5 percent) in the study had AP values above this level. At this cutoff, most children with normal AP levels would not need further testing for rickets (test specificity = 97.4 percent) and two-fifths of those with AP levels above the cutoff were found to have rickets (positive predictive value [PPV] = 40.0 percent). This is comparable to or higher than the PPVs for other pediatric screening tests, the researchers note.
Participating practitioners came from six private-practice pediatric offices and one inner-city pediatric clinic. AP analysis was done on infants or toddlers who had blood samples taken for other reasons, as part of health supervision office visits. Children eligible for the study were 6 to 15 months old, predominantly breast-fed for the first 6 months of life, and did not routinely receive supplemental vitamin D. The study was funded in part by the Agency for Healthcare Research and Quality (HS16029).
More details are in "The utility of alkaline phosphatase measurement as a screen test for rickets in breast-fed infants and toddlers: A study from the Puget Sound Pediatric Research Network," by Dr. Taylor, Monica Richter, M.D., Ph.D., Stephen Done, M.D., and others in Clinical Pediatrics 49(12), pp. 1103-1110, 2010.