TESTING & WARNINGS

IS THERE A BLOOD TEST TO CHECK MY VITAMIN D LEVEL?

Yes, but first, testing is expensive and it is reasonable not to test everyone’s vitamin D level. Healthy persons not in the sun could reasonably take supplemental doses of vitamin D without testing. (Not exceeding 20,000 IU/week)

Individuals with many conditions (bone diseases, inflammatory disorders, kidney disease and intestinal mal-absorption, to name a few) should have their levels tested. Vitamin D levels rise fairly slowly and fall slowly, so it is not unusual to wait 3-6 months before retesting, if that is necessary.

WHAT TO MEASURE

The substance that is measured to assess body stores of vitamin D is 25 OH vitamin D (also known as 25 OH cholecalciferol, 25 OH D3, calcidiol and by more technical names).

Levels below 80 nmol/L (or 32 ng/ml) have been associated with reduced calcium adsorption, osteoporosis and increased fracture risk. Levels from 45 to 60+ ng/ml more closely approximate those found in sun exposed populations. 

TESTS AVAILABLE

Several blood tests are now available for clinical use. The "gold standard" method is called liquid chromatography, tandem mass spectrometry (LC/MS/MS) and is able to distinguish between animal (human) and plant forms of 25 OH vitamin D. This methodology is being used by Quest Diagnostics, Mayo Clinic Labs and the NIH.
There are other standardized methods available which, when preformed in capable laboratories, are suitable for clinical use.  All of the methods available require an experienced and fairly large lab.  The laboratory is probably the most important issue in clinical testing.  In addition vitamin D is very sensitive to light and heat.  Specimen handling has been another major source of error.  Home testing lacks appropriate specimen preparation and dedicated transportation methods and is not recommended.

Other factors influencing choice of testing may include local economic and insurance considerations.

For those wishing more in depth information and with access to a medical library or wishing to purchase a review, this article would provide a good entry point for more technical information.

The Journal of Clinical Endocrinology & Metabolism Vol. 93, No. 5 1804-1808

Correlation among 25-Hydroxy-Vitamin D Assays
N. Binkley, D. Krueger, D. Gemar and M. K. Drezner
http://jcem.endojournals.org/cgi/content/abstract/93/5/1804

 

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