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Dr Cannell on vitamin D blood tests

Posted on: September 26, 2011   by  John Cannell, MD


Many people obtain vitamin D blood tests without knowing the differences and limitations of current testing technology. I feel testing is important, not so much to see if a person is deficient, but to convince both the patients and the doctor that the deficiency is present. Except during the summer, most vitamin D levels obtained will show deficiency.


In the October 2011 issue of the American Journal of Clinical Pathology, Dr. Matthew Krasowski, of the University of Iowa Hospitals and Clinics, published an excellent paper discussing vitamin D testing, absorption, and several other issues.

Krasowski MD. Pathology consultation on vitamin d testing. Am J Clin Pathol. 2011 Oct;136(4):507-14.

Let’s start by looking at the normal ranges laboratories use to report vitamin D levels:

  • ARUP Laboratories: range 30-80 ng/ml
  • Mayo Medical Laboratories: range 25-80 ng/ml
  • LabCorp: range 32-100 ng/ml
  • Liaison: range 30-100 ng/ml
  • ImmunoDiagnostics ISYS: 30-100 ng/ml
  • Quest Diagnostics: 30-100 ng/ml

I don’t know about you, but when I get a blood test result, not only do I like it to be normal, but I like it to be in the middle of normal, so it is extra normal, not just in the normal range. For example, if my platelet count is 150 and the normal range is 150-450, I’m not that pleased, I’d rather see it about 300, in the middle of the normal range. Now, as a physician, I know this doesn’t make medical sense but it does make common sense. I’d like my blood tests to be normal normal.

Look back at the ranges for vitamin D levels. What is normal normal? It is about 50 ng/ml, maybe a little more. In a previous blog, I’ve given four very different reasons to keep you levels at 50 ng/ml. This is just one more common sense reason.

The second reason I like Dr. Krasowski’s paper is his warning that too many physicians are still ordering the wrong vitamin D test. Everyone who has had a vitamin D blood test should either look at a copy of the report or specifically ask the doctor, “Is this test a 25-hydroxy vitamin D or is it a 1,25 di-hydroxy vitamin D?” If it is the latter, the test is worse than useless, as it will assure you that you are not vitamin D deficient, when in fact, you may well be. Only a 25-hydroxy vitamin D test [25(OH)D] can answer the question.

Dr. Krasowski reminds us that sometimes vitamin D is not absorbed in people taking cholestyramine or colesevelam, and in people with cystic fibrosis, celiac disease, Whipple disease, Crohn’s disease, and gastric bypass surgery. He advises either increased supplementation for such people or the use of tanning beds (at sub-tanning intensities) but remember to ask for the regular beds, not the high-pressure beds, or get one of the Mercola UVB beds for your home.

Also, a group of drugs can lower vitamin D levels (carbamazepine, phenobarbital, phenytoin, rifampin and St. John’s wort). Finally, he reminds us that in a rare group of diseases (Primary hyperparathyroidism, sarcoidosis, granulomatous tuberculosis, and other granulomatous diseases like histoplasmosis), calcium in the blood may go up with vitamin D supplementation, and those patients should seek medical care before taking vitamin D supplements.

Don’t forget that the Vitamin D Council offers a very reasonably priced in-home accurate vitamin D test. It saves you one or two visits to the doctor as well as the laboratory charge. ZRT’s support for the Council is important, so we very much appreciate it when you use their vitamin D blood test.


1 Response to Dr Cannell on vitamin D blood tests

  1. [email protected]

    I was told that Medicare will only pay for one 25(OH)D test per year.
    I believe this is a recent change. If so, it is not helpful when you are trying to raise your Vitamin D levels with supplementation and want to check your levels at least twice a year.

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