Biochemical osteomalacia in adults undergoing vitamin D testing in the North-East of Scotland

BackgroundInternational guidelines give greatly varying definitions of 25-hydroxyvitamin D (25OHD) insufficiency and deficiency. Vitamin D testing is increasing despite 2016 UK guidance for adults advising routine vitamin D supplementation October-March and year-round for high risk groups. A service evaluation of vitamin D testing and biochemical osteomalacia in the North-East of Scotland (57-58°N) could inform definitions and testing guidance.MethodsWe identified adult 25OHD requests 8/7/2008-29/2/2020 and albumin-adjusted serum calcium (aCa), parathyroid hormone (PTH) and alkaline phosphatase (ALP) within 6 months of 25OHD testing. After excluding renal impairment and liver disease, we defined biochemical osteomalacia as ALP >130 IU/L and aCa <2.0 mmol/L and elevated PTH >9.2 or >6.8 pmol/L, depending on the assay. Possible biochemical osteomalacia was defined as 2 of these abnormalities in the absence of the third measurement. From these cases anonymised clinical data were then examined to confirm the diagnosis of osteomalacia.Results25,379 eligible patients had 25OHD measured: 25% were <25 nmol/L (6,258/25,379) and 18% <20 nmol/L (4,536/25,379). 0.5% (126/25,379) of eligible patients had biochemical or possible biochemical osteomalacia. After reviewing clinical records, only 0.1% (29/25,379) had clinically confirmed osteomalacia, equivalent to 2-3 cases/y for a population of 0.5 million, none of the untreated cases of clinically confirmed osteomalacia had 25OHD >25 nmol/L. For the entire tested population, when 25OHD was <25 nmol/L untreated osteomalacia confirmed by clinical records was rare (0.4%).ConclusionsOsteomalacia is rare in North-East Scotland. Our data call into question designating 25OHD 25-50 nmol/L 'insufficiency'. The risk of osteomalacia even when 25OHD is <25 nmol/L is very low.