Objective To determine if alkaline phosphatase (AP) levels are a useful screening test for vitamin D-deficient rickets in children who have been breastfed.

Methods The study was conducted in Seattle, Washington, USA. Blood for AP determination was obtained from eligible patients who were healthy children 6–15 months old who were exclusively breastfed for >6 months and did not receive supplemental vitamin D. Radiographs were obtained on children with values classified as elevated by the reference laboratory (females 281 U/l, males 324 U/l). Radiographs were classified as rickets or no rickets by the study radiologist. Children with normal AP levels and those with normal radiographs were categorised as not having rickets. Separate analyses were conducted using a cut-off value for AP of 500 U/l.

Results AP levels were obtained on 221 children; levels were elevated in 30 (13.6%); levels were >500 in nine patients (5.0%). Radiographs were obtained in 17 patients with elevated AP levels. No rickets was seen in all eight children with elevated levels <500 for whom radiographs were obtained. Rickets was present on radiograph in 4/9 of those with AP levels >500. If 500 is used as the cut-off value, the specificity of AP as a screen for rickets was 97.5% (97.5% CI 94.4% to 99.2%); the prevalence of rickets in this population was 1.9% (95% CI 0.5% to 4.9%).

Conclusion AP determination, an inexpensive test requiring a small amount of blood, could be a useful screen for rickets in healthy young children who have been breastfed.