Vitamin D testing in children and adolescents in Victoria, Australia: are testing practices in line with global recommendations?

OBJECTIVE: To describe changing primary care ordering of serum 25-hydroxyvitamin D (25OHD) tests in Australian children. DESIGN: Longitudinal, population-based descriptive study of 25OHD testing using a large administrative dataset of pathology orders and results, 2003–2018. SETTING AND PARTICIPANTS: Three primary health networks in Victoria, Australia. Patients aged ≤18 years with a serum 25OHD test ordered by the general practitioner (GP). MAIN OUTCOME MEASURES: Trends over 15 years in the number of 25OHD tests ordered, proportion indicating low levels or vitamin D deficiency and details of repeat testing. RESULTS: Of 970 816 laboratory tests, 61 809 (6.4%) included an order for a 25OHD test. The 61 809 tests were performed in 46 960 children or adolescents. The odds of ordering a 25OHD test in 2018 was 30.4 times higher compared with 2003 (95%CI 22.6 to 40.8, p<0.001). The odds of detecting a low 25OHD (<50 nmol/L) compared with the baseline in 2003 remained steady (adjusted OR<1.5) over time. Repeat tests (14 849) were undertaken in 9626 patients (median intertest interval 357 days, IQR 172–669 days). A total of 4603 test results indicated vitamin D deficiency (<30 nmol/L), but in only 180 (3.9%) of these was a repeat test performed within 3 months as recommended. CONCLUSION: Testing volumes increased 30-fold, but the odds of detecting low 25OHD remained steady. Current Australian policy and the Global Consensus Recommendations for the prevention and management of nutritional rickets do not support routine 25OHD testing. Education and electronic pathology ordering tools may assist GPs to better align practice with current recommendations.