Bone Abstracts

Objectives: Nutritional rickets results from the interaction of poor vitamin D status and limited calcium intake. Elevated serum alkaline phosphatase is a marker of impaired mineralization in many forms of rickets. We assessed the reliability of serum alkaline phosphatase in identifying nutritional rickets in calcium-deprived Nigerian children.

Methods: We reanalyzed data from a case-control study of Nigerian children with active rickets (cases) and age-, sex-, and weight-matched control subjects without rickets in an area where dietary calcium insufficiency is common (J Pediatr 2000;137:367–73). Children were classified as having radiographically active rickets prior to obtaining blood for biochemical analysis. We performed a multivariate logistic regression to assess the odds of rickets associated with varying alkaline phosphatase values, adjusting for age and sex.

Results: A total of 122 children with rickets and 121 control children had sufficient data (n=243) for analysis. Rachitic children had a mean (±S.D.) age of 54±29 months, and 55 (45.1%) were male. Cases and controls had similarly low mean dietary calcium intakes (216±87 and 214±96 mg/day, respectively). Mean alkaline phosphatase values in cases and controls were 812±415 and 245±78 U/L, respectively (P<0.001). Serum alkaline phosphatase was negatively associated with serum 25-hydroxyvitamin D values (r=−0.34; P<0.001). In the model, the odds ratio (95% confidence interval) for rickets was 6.7 (4.1–12.2) for each 100 U/L increase in alkaline phosphatase. The area under the receiver operating characteristic (ROC) curve was 0.98, indicating a strong relationship between alkaline phosphatase and having rickets. Age and sex were not significant confounders (P=0.71). Test characteristics of alkaline phosphatase >350 U/L for identifying nutritional rickets in Nigerian children were: sensitivity 0.93, specificity 0.92, positive likelihood ratio 11.3, and negative likelihood ratio 0.07.

Conclusion: In Nigerian children with a low dietary calcium intake, an alkaline phosphatase >350 U/L effectively discriminated between children with and without nutritional rickets. Alkaline phosphatase is a low cost biochemical test that could be used to screen for nutritional rickets, but cut-point values require validation in other populations. Laboratory values for alkaline phosphatase need to be standardized for widespread population studies of nutritional rickets.

Disclosure: The authors declared no competing interests.