What is Being Tested?
After the skin makes previtamin D3 (from exposure to sun’s UV radiation) or consuming vitamin D2 and D3 (from diet and/or supplements), the liver converts the D2 and D3 into 25-hydroxy vitamin D. A blood test for 25-hydroxy vitamin D is considered the preferred test for vitamin D status.
Going further down the pathway in the body, this vitamin D marker will eventually be converted in the kidney to 1,25-dihydroxyvitamin D. This is the most biologically active form of vitamin D. Even though this can also be tested, it’s not normally the preferred measurement due to 25-hydroxy vitamin D having a longer half-life (2 weeks versus 15 hours), being found in higher concentrations, having less physiological variation, and correlating well with bone mineral density.
Why is Vitamin D Tested?
Currently the BC Guidelines state that testing may be indicated in patients at higher risk for vitamin D deficiency, including individuals with:
- high dose vitamin D interventions, combined with evidence of vitamin D toxicity
- hypo- or hypercalcemia/hyperphosphatemia
- hypo- or hyperparathyroidism
- osteopenia or osteoporosis
- malabsorption syndromes
- medications affecting vitamin D metabolism (e.g. phenobarbital, carbamazepine, phenytoin and valproate) – medications interfering with vitamin D absorption (e.g. cholestyramine, colestpiol and orlistat)
- significant renal or liver disease
- unexplained bone pain
- unexplained elevation of alkaline phosphatase
- non-traumatic fractures
It’s currently considered there to be a relatively small number of people who are truly vitamin D deficient, but a significant number with insufficient serum concentrations of vitamin D.
Excluding summer months, if you live at latitudes above 37 degrees north (all of Canada!) or below 37 degrees south of the equator, on average the skin does not make enough previtamin D for the body to convert into active forms. Individuals in these regions are at a relatively greater risk for vitamin D deficiency. Lack of sun exposure wouldn’t be as much of an issue if there were sufficient dietary sources to complement this. Despite fortification, on average Canadians do not get enough vitamin D from the diet to meet the current recommended daily allowance or maintain adequate 25-hydroxy vitamin D. Infants, children and older adults are especially at risk to have inadequate dietary intake of vitamin D. Canadian’s are usually assumed to have insufficient serum concentrations unless supplementing.
In BC, routine testing within a healthy population is generally not recommended and is normally not covered by MSP. Regardless of the test result, it is usually indicated to follow the Health Canada guidelines for Vitamin D intake and supplementation. There is controversy surrounding how insufficient serum concentrations relate to general health, as well as the related appropriate interventions and testing. Currently Health Canada’s daily recommended dietary allowances are based on the maintenance of bone health which is a well established action of vitamin D. In the future, this might evolve to incorporate guidelines based on other actions of vitamin D, such as immune regulation. As a naturopathic physician in private health care, alternative routes might be explored based on the individual need of the patient.
Coverage for Testing:
At this time testing vitamin D is only covered by MSP when the patient is under 19 years old or if a specialist orders the test. In other situations, more rarely, an extended health plan may cover testing. Otherwise, if any other doctor orders the test out of those parameters, it will be at a cost to the patient.
1) BCGuidelines. (Oct. 30, 2019) Vitamin D Testing. Retrieved Dec. 2019 from: https://www2.gov.bc.ca/gov/content/health/practitioner-professional-resources/bc-guidelines/vitamin-d-testing
2) Fookes, C. (Oct. 23, 2018) Vitamin D. Retrieved Dec. 2019 from: https://www.drugs.com/vitamin-d.html
3) Harvard Women’s Health Watch. (April 2008) Time for more vitamin D. Retrieved Dec. 2019 from: https://www.health.harvard.edu/staying-healthy/time-for-more-vitamin-d
4) Holick, M. (2010) The D-lemma: To Screen or Not to Screen for 25- Hydroxyvitamin D Concentrations. Clinical Chemistry. Apr: 56(5); 729-731. DOI: 10.1373/clinchem.2009.139253
5) Hossein-nezhad, A., Holick, MF. (2013) Vitamin D for Health: A Global Perspective. Mayo Clin Proc. Jul;88(7):720–55. doi: 10.1016/j.mayocp.2013.05.011.
6) Janz, T., Pearson C. (2015) Vitamin D blood levels of Canadians. Statistics Canada. Report No.: 82-624–X. Retrieved Dec. 2019 from: https://www.statcan.gc.ca/pub/82-624-x/2013001/article/11727-eng.htm