Factors Associated with Vitamin D Testing: A Population-Based Cohort Study in Queensland, Australia

Background/Objectives: Vitamin D testing has increased significantly in developed countries in recent decades. We aimed to describe trends in vitamin D testing rates and factors associated with testing and vitamin D deficiency in Queensland, Australia (2011-2019). Methods: We used data from the QSkin Sun and Health Study (n = 40,417), a prospective population-based cohort study with linkage to the Medicare Benefits Schedule, Pharmaceutical Benefits Scheme, and pathology laboratories. Main outcomes included age-standardized incidence rate of vitamin D testing; having ≥1 vitamin D test during follow-up; vitamin D deficiency (25-hydroxyvitamin D concentration <50 nmol/L) in the first vitamin D test; and repeat vitamin D tests. Results: The age-standardized incidence rate of testing increased by 2% per quarter during follow-up. Of the 35,250 participants analyzed for associations with testing (median age of 57 years, 52% female), 45% had ≥1 vitamin D test. Among those tested, 56% had no apparent clinical indication for their initial vitamin D test, 21% were vitamin D deficient in their initial test, and 58% had a repeat test. Repeat testing occurred in 56% who were not deficient in their prior test, while only two-thirds of those deficient received a follow-up assessment. Participants who visited a general practitioner ≥2 times in the year prior to follow-up were 60% more likely to have ≥1 vitamin D test compared with those with no visit, but general practitioner (GP) visits were not associated with risk of vitamin D deficiency. Conclusions: These results suggest that initiatives are needed to help clinicians target vitamin D testing in alignment with clinical guidelines.