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The first study of its kind on vitamin D retesting calls for a greater focus on monitoring in certain groups

Trinity College Dublin (TCD) researchers have found that while there is significant expenditure on vitamin D retesting, those who are deficient or most at risk of deficiency (young adults aged 18-to-39 years) are the least likely to have their vitamin D levels monitored or rechecked by GPs in Dublin.
The study, published in the Journal of Clinical Chemistry and Laboratory Medicine, has for the first time gathered and analysed data on vitamin D retesting. It establishes the need for a better understanding of the role of vitamin D testing in the population and for greater focus to be placed on the monitoring of vitamin D status in particular groups.

In recent years, there is greater public awareness of vitamin D, its importance in bone health and potential effects on immunity, the Covid-19 virus and other medical conditions. This has led to a surge in the number of vitamin D tests in Ireland and globally, which is placing increased pressure on healthcare systems.
While there are Irish guidelines for vitamin D testing, there are no clear recommendations as to who should be retested (that is, have repeat tests). This study aimed to identify what proportion of patients are retested, whether this was appropriate, and if vitamin D levels improved after retesting. The cost of inappropriate or unnecessary retests was also identified.

The key findings include:. 

  • One-in-four patients had their vitamin D retested.
  • One-in-10 tests were requested earlier than recommended, a third were done too frequently, and more than half were in patients with adequate vitamin D levels, resulting in significant costs.
  • While young adults were most likely to have low vitamin D, they were the least likely to have it rechecked
  • Women were also more likely to have repeat tests, however they were less likely to be deficient.
  • Levels of vitamin D deficiency fell considerably between initial and repeat tests, however nearly one-in-four (23 per cent) still remained deplete after two or more retests.
  • Findings suggest that vitamin D testing is common, but the right people are not being tested at the right time. 

These findings will help inform guidelines for GPs on vitamin D retesting, such as for patients with metabolic bone disorders where monitoring of vitamin D status can be important, said the authors. The study also suggests that a user-friendly GP ordering system for vitamin D requests that restricts it to pre-defined criteria, such as limiting retests within three months or more than two per year, should be considered. By implementing these measures, the number of inappropriate tests and the costs could be hugely reduced and the efficiency of the system improved, they added.

Diet and supplements

Ms Helena Scully, Mercers Glanbia Bone Research Fellow and Lead Author, MISA Institute, School of Medicine, TCD, commented: “We have found that one-in-four patients have their vitamin D frequently checked by their GP, and yet some remain deficient after several tests. Young adults and males were most likely to have low vitamin D, but were retested the least. This shows those who are at most risk of vitamin D deficiency are not being assessed, leading to misdirection of resources from those who need it most. Clear guidelines on who should have their vitamin D retested and when are needed to better identify deficiency in the population.
“Instead of going to their GP and requesting multiple vitamin D tests, the public should focus on getting enough vitamin D via getting good sources of vitamin D in their diets (oily fish, egg yolk, fortified milk/dairy products) and by taking a vitamin D supplement (10ug/day for those age 5-65 years).”
Dr Kevin McCarroll, co-author and Consultant Physician at James’s Hospital and Clinical Senior Lecturer, School of Medicine, TCD, added: “The study shows that vitamin D retesting is prevalent, yet a large proportion of tests are not being done on the right people or at the right time. It highlights the needs for a better understanding of the role of vitamin D retesting in the population and of the importance of getting enough vitamin D in the diet and/or via supplements, rather than having to check levels frequently.”