The validity of serum alkaline phosphatase to identify nutritional rickets in Nigerian children on a calcium-deprived diet.

Nutritional rickets results from the interaction of low vitamin D status and limited calcium intake. Serum alkaline phosphatase (AP) activity is a biomarker of impaired mineralization in rickets.To assess the performance of serum AP activity in identifying nutritional rickets in calcium-deprived Nigerian children.We reanalyzed data from a case-control study of children with active rickets and matched control subjects without rickets, using a multivariate logistic regression to assess the odds of rickets associated with AP activity, adjusting for age, sex, and weight for age z-score.A total of 122 children with rickets and 119 controls were included. Rachitic children had a mean (±SD) age of 54±29 months, and 55 (45.1%) were male. Cases and controls had low dietary calcium intakes (216±87 and 214±96 mg/day, respectively). Serum AP activity levels in cases and controls were 812±415 and 245±78 U/L, respectively (P<0.001). AP was negatively associated with 25-hydroxyvitamin D values (r=-0.34; P<0.001). In the adjusted model, the odds ratio (95% confidence interval) for rickets was 6.7 (4.1-12.2) for each 100 U/L increase in AP. The area under the receiver operating characteristic curve was 0.978. AP >350 U/L identified nutritional rickets in Nigerian children with sensitivity 0.93, specificity 0.92, positive likelihood ratio 11.3, and negative likelihood ratio 0.07.An AP >350 U/L effectively discriminated between Nigerian children with and without nutritional rickets. AP is a low-cost biochemical test that could be used to screen for nutritional rickets, but cut-off values require validation in other populations, and laboratory values need to be standardized for widespread population studies.

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Authors: Tom D Thacher, Christopher T Sempos, Ramon A Durazo-Arvizu, Philip R Fischer, Craig F Munns, John M Pettifor