The only surefire way to know for certain the extent of your vitamin D deficiency is to ask for a 25-hydroxyvitamin D test, also called a 25(OH) D test.
Again, this is the circulating form of vitamin D that the liver generates and that then becomes activated by the kidneys. While it’s intuitive to think you’d want to test for the body’s “active form” rather than just a precursor, testing for the activated vitamin D (1,25-vitamin D) actually does not give an accurate portrayal of your vitamin D status. And here’s the rub: many doctors order the wrong test, and when the results come back showing a normal level of activated vitamin D, they think everything is D-okay. You could, however, be suffering from a serious deficiency even though your activated levels appear normal—or even elevated. This may sound incomprehensible, but not once you understand a few facts about vitamin D’s biology.
Any doctor can order the 25(OH)D test for you, but if they use the wrong coding when submitting the claim to the insurance company, they won’t get reimbursed and you will wind up having to pay for the test. (The actual cost of vitamin D testing can vary significantly depending on the lab used and whether a large number of assays are being performed by an individual doctor’s office or hospital. The assay cost can range from approximately $40 to $225 per test, so ask your health care provider. Medicare currently reimburses you for
$40.) The code for vitamin D deficiency is 268.9, or if the doctor has a code for osteoporosis or for other related diseases for vitamin D deficiency, he or she may be able to use that. But 268.9 is for vitamin D deficiency, and using that can help minimize costs and maximize reimbursements. (Other codes of interest: osteomalacia = 268.2;
osteoporosis = 733.00; osteopenia = 733.90.)
The active form of vitamin D circulates at one thousand times lower concentration than 25-vitamin D, which, when created by your body from a supplement, has a half-life in your circulation of two to three weeks. The 25-vitamin D made by your body in response to sunlight, however, lasts twice as long in the body. Half-life simply refers to the amount of time it takes the body to eliminate one half of the total amount of 25-vitamin D and 1,25-vitamin D in your blood. On the other hand, the active form of vitamin D has a half-life of only two to four hours, meaning that the concentration of active D in your bloodstream is reduced by half every two to four hours. As you become vitamin D deficient, the body immediately responds by increasing the production of parathyroid hormone, which tells your kidneys to activate vitamin D, which is why your activated vitamin D (1,25-vitamin D) levels are either normal or elevated when you are vitamin D deficient or insufficient.
How is that possible? And how can you be vitamin D deficient if the kidneys then crank out more activated D? My guess is that your target tissues, namely your intestines and bones, still can’t get enough, even though your blood levels are normal. Serum calcium is usually normal in the vitamin D-deficient state.
Most physicians also trip up on calcium levels. That is to say, if they measure calcium levels in the blood and see nothing off the mark, they automatically assume this translates to a corresponding normal vitamin D status. But in fact
this isn’t necessarily true. Neither calcium levels nor activated vitamin D levels in the blood can tell you whether or not you are vitamin D deficient. You must measure 25-vitamin D; do not accept any other marker no matter what your doctor tells you.
So, where should your 25-vitamin D levels be? The unit of measurement used for vitamin D is nanograms per milliliter, and most experts—myself included—
agree that vitamin D deficiency is defined as 25-vitamin D of less than 20 nanograms per milliliter. Vitamin D insufficiency is between 21 and 29 nanograms per milliliter. And vitamin D sufficiency begins at 30 nanograms per milliliter. To obtain the full benefits of vitamin D for health, many experts recommend that blood levels be closer to 40 nanograms per milliliter. The reason for this is that higher levels have been associated with a decreased risk for cancer, heart disease, autoimmune disease, and so on. Vitamin D intoxication is typically not seen until blood levels are above 150 to 200 nanograms per milliliter; lifeguards, for example, are famous for touting levels of 25-vitamin D in the 100-nanograms-per-milliliter range, and you don’t hear about them falling victim to vitamin D toxicity—principally because they’re getting most of their D from the sun and it’s impossible to get too much from this source. Similarly, tanners who use tanning beds once a week typically maintain levels in the 40-to 60-nanograms-per-milliliter range throughout the winter. If you rely on supplements for your vitamin D, it takes about a month to fall below 20 nanograms per milliliter if your regular supplementation goes on hiatus.
The assay for 25-vitamin D is now the most ordered assay in the United States.
Here’s another way to look at it: for every 100 IU of vitamin D you ingest through, say, a supplement, your blood level of 25-vitamin D increases by 1 nanogram per milliliter. This is why both children and adults need to be on at least 1,000 IU of vitamin D a day when they have inadequate sun exposure to satisfy their body’s vitamin D requirement. I personally take about 2,700 IU of vitamin D daily from supplements and milk (this is in addition to any vitamin D I get from sensible sun exposure), and my blood level is 50 nanograms per milliliter throughout the year.
Reading Different Assays
There are two main assays that test a person’s vitamin D status, one of which is better than the other.
Liquid chromatography tandem mass spectrometry (I know, it’s a mouthful usually abbreviated as just LCMSMS) is, in my opinion, the state-of-the-art, gold-standard assay that determines your blood levels of both 25-vitamin D2 and 25-vitamin D3. What you care about is the total 25-vitamin D, so you’d simply add up the results of this assay to arrive at your grand total. For example, if your results show 20 nanograms per milliliter of vitamin D2 and 15 nanograms per milliliter of vitamin D3, then your total vitamin D is 35 nanograms per milliliter.
What makes this assay best is that it can differentiate between the two types of vitamin D. Remember, vitamin D2 is available only as a pharmaceutical. So if you’re being treated for a deficiency using pharmaceutical-grade D2 but your blood level of D2 does not go up, that indicates that your body is not absorbing vitamin D2.
The other assay out there is a radioimmunoassay. It measures only your total 25-vitamin D and cannot distinguish between D2 and D3. If you don’t have a choice in which assay is used and are stuck with the radioimmunoassay, it’s still a reliable source that can suffice. If possible, however, request the LCMSMS assay for a more in-depth analysis.