Prevalence of Biochemical Osteomalacia in Adults Undergoing Vitamin D Testing

Objective: Prolonged severe vitamin D deficiency can cause osteomalacia, but the 25-hydroxyvitamin D (25OHD) concentration below which this occurs is unknown. We investigated the prevalence of biochemical osteomalacia in adults with a measurement of 25OHD.

Design, Measurement, And Patients: 25OHD results between 1/1/2009 and 15/6/2020 were obtained from the regional laboratory database, together with measurements of serum calcium, parathyroid hormone (PTH) and alkaline phosphatase (ALP) within 6 months of the index 25OHD. We defined biochemical osteomalacia as all 3 of: albumin-adjusted serum calcium (aCa)<2.0 mmol/L, PTH>7.3 pmol/L and ALP>150 IU/L. Possible osteomalacia was 2/3 criteria with the other test not done. 25OHD measurements associated with significant renal impairment, elevated hepatic transaminases or hypercalcaemia were excluded.

Results: 110,046 25OHD measurements were identified over the 11.5 years period. After removal of ineligible measurements, 42,171 25OHD measurements from 32,386 individuals with at least 2 of aCa, PTH and ALP were included in analyses. Median 25OHD was 63 nmol/L; 8% were <25 nmol/L, and 33% were <50 nmol/L. Five index 25OHD measurements met the definition of biochemical osteomalacia, and another 11 were possible osteomalacia. After reviewing available clinical records for these 16 episodes, we classified 9 cases as osteomalacia and 7 as other diagnoses. Thus, the prevalence of biochemical osteomalacia was 0.02% (9/42,171) for 25OHD measurements and 0.23% (8/3432) for 25OHD<25 nmol/L. All cases of osteomalacia with 25OHD measurements prior to supplementation had 25OHD≤18 nmol/L.

Conclusion: The prevalence of biochemical osteomalacia is very low, even in individuals with 25OHD<25 nmol/L.