The near absence of biochemical osteomalacia in people with ‘low’ vitamin D levels suggests a need for a rethink on the criteria for vitamin D deficiency – and the value of testing – according to endocrinologists in New Zealand.
Only one in 5000 vitamin D tests was associated with biochemical osteomalacia and the prevalence was very low (0.23%) even in individuals with the most conservative definition of deficiency, 25-hydroxyvitamin D levels [<25 nmol/L], a study from Auckland University found.
The researchers said their findings might explain other evidence showing that vitamin D supplementation does not improve musculoskeletal health or prevent falls and fractures.
Published in Clinical Endocrinology, their study looked for biochemical osteomalacia in more than 110,000 25OHD measurements from adults in the Auckland region over an 11.5 year period.
Biochemical osteomalacia was confirmed when all three of an elevated ALP (>150 IU/L), low aCa (<2.0 mmol/L) and elevated PTH (>7.3 pmol/L) occurred within six months of the index 25OHD measurement.
Possible biochemical osteomalacia was defined as the presence of two of these three test abnormalities during the index period.
The study first identified five people with osteomalacia and 11 possible cases. On a review of clinical details in hospital records they concluded four of the 11 possibles were also confirmed cases and seven were alternate diagnoses.
It concluded that 0.02% of all eligible 25OHD measurements, 0.06% of 25OHD <50 nmol/L and 0.23% of 25OHD <25 nmol/L were associated with a final diagnosis of biochemical osteomalacia.
“The definition of vitamin D deficiency as 25OHD <25 nmol/L therefore seems inappropriate for adults because it does not indicate a high risk of associated disease,” said the researchers led by Associate Professor Mark Bolland of the Auckland District Health Board Endocrinology Service.
The researchers said defining thresholds for vitamin D deficiency was clinically important.
“If higher thresholds are used (e.g., 50 or 75 nmol/L), then vastly more people are labelled as having vitamin D deficiency and may be advised to take vitamin D supplementation. However, if such groups are not at higher risk of osteomalacia, this represents a misdiagnosis of vitamin D deficiency, raising needless anxiety and wasting resources.”
They said clinicians did not appear to be considering other important tests when assessing low vitamin D status
“…even when low 25OHD measurements occurred, more than 50% did not have measurements of the relevant biochemical markers: 17% of 25OHD <25 nmol/L had no measurement of ALP, aCa or PTH between 6 months before and after the 25OHD measurement; 37% had only one of these measurements, almost always ALP, which may have been measured as part of the broader ‘liver function tests’ panel.”
“This suggests that clinicians either are not sufficiently concerned about the risk of osteomalacia to request these tests when low 25OHD results are obtained, or do not feel that such tests are necessary.”
“Presumably, practitioners were measuring vitamin D for reasons other than clinical suspicion of osteomalacia.”
Commenting on the study, Professor Peter Ebeling from Monash Health told the limbic that a vitamin D level <25 nmol/L was still helpful as a guide.
“What we need to think about is considering more clinical and biochemical features of osteomalacia than just a simple vitamin D level.”
“So what we would be looking for are clinical aspects of muscle weakness, bone and muscle pain and the low calcium, high alkaline phosphatase and secondary hyperparathyroidism as well as the low vitamin D levels. That would be a more complete assessment rather than just looking at the serum 25OHD.”
Professor Ebeling, President-Elect of the American Society for Bone and Mineral Research, said the gold standard test for osteomalacia was a bone biopsy.
He said a German study of 675 people showed histological osteomalacia was highest when the serum 25OHD was <25 nmol/L but was still present, though far less frequently, at levels between 25-50 nmol/L.
“The blood tests are an approximation and we should still be concerned about serum 25OHD levels <30 nmol/L and look for clinical symptoms and signs of osteomalacia, particularly if <18 nmol/L.”