Vitamin D Newsletter


Vitamin D testing

American Association of Clinical Chemists: Vitamin D Testing—What's the Right Answer?
College of American Pathologists: Vitamin D intrigues, but not a done deal

In the above two reports, what really caught my eye above was at the Cleveland Clinic, Vitamin D blood tests jumped from 1,500 tests a month in 2006 to 12,000 a month in 2009. Cleveland Clinic switched to DiaSorin Liaison method to keep up with the demand. That tells me no matter what the Food and Nutrition Board does, patients and doctors are catching on: Vitamin D deficiency is best treated.

If you want to know about the problems with Vitamin D blood testing, read the above two articles. However, my recommendation is not to read them. It will just upset and confuse you. Even if you are a doctor, maybe especially if you are a doctor, don't read them. You expect lab tests to be accurate, give the same result with the same blood sample. Well, okay, believe that if you want.

Robert Michel, publisher of the Dark Report, just reported on his latest experience with Vitamin D testing. The results are not good, especially for Quest Diagnostics. Michel sent 24 aliquots, or identical samples, of his blood, all drawn the same day, to two different reference labs, which in turn sent them, over a three week period, for 24 Vitamin D blood tests. Again, 24 blood samples, drawn from the same person at the same time, so, in a perfect world, all 24 samples would test the same.

However, the results varied from 36 ng/mL to 66 ng/mL! Quest's results: 36, 42, 51, 54, 55, and 66. The Mayo Clinic, which uses the same technique that Quest uses, did better, 48, 48, 51, and 61. The good news was the immunoassay methods used by LabCorp, Clinical Pathology Labs, and ARUP clustered around 44 ng/mL and all 11 samples were within 4 points of 44 ng/mL with the highest 48 and the lowest 39.6.

Long story short, if you use Quest Diagnostics, divide by 1.3 and hope they continue to work at improving their process. Mayo's is better but Dr. Singh must be getting tired of all those Vitamin D tests, which are hard to do on mass spec. If your lab sends out to LabCorp, ARUP, or Clinical Pathology Labs, you are fine.

If you use ZRT's in-home vitamin D test kit, know that it is a mass spec technique; it has to be mass spec to be done on a blood spot. ZRT is also harmonized to the gold standard, that is, corrected to the gold standard. By gold standard I mean the method that the scientific studies use when they study cancer, heart disease, autoimmune disease, etc. When you see an article that says a new study showed higher Vitamin D levels are associated with longer life, etc., that study almost always used DiaSorin RIA, the gold standard, or DiaSorin Liaison, which gives almost identical results to the DiaSorin RIA.

I see that Dr. Graham Carter, a great proponent of accurate Vitamin D testing, slammed me and ZRT in a recent paper in Clinical Chemistry.

Graham is angry, perhaps, because it was not his watchdog organization, DEQAS, that first detected the problem with inaccurate Vitamin D testing at Quest? Instead, he admits, it was the Vitamin D Council who first blew the whistle on Quest Diagnostics.

Dr. Carter said, correctly, that ZRT home testing "cannot easily be monitored by external proficiency testing schemes." Graham is right, schemes, such as Graham's DEQAS, cannot easily monitor home testing by ZRT, because ZRT uses blood on a blotter paper and not serum. ZRT may be able to be modified to participate in DEQAS, if ZRT can afford it, ZRT is a small lab. I'll ask ZRT if they can find a way to participate.

For those who do not know, this is what DEQAS does. Participating commercial labs pay DEQAS a fee (that is not disclosed on their website but reportedly substantial) so DEQAS will check that lab's precision. DEQAS then sends participating labs batches of standardized Vitamin D samples. In other words, it seems that the major reference labs keep DEQAS in business.

The problem with DEQAS is they refuse to send the test samples blind, like Robert Michel did for the Dark Report. In reality, the commercial labs all recognize the DEQAS batches when they come in the mail and all the commercial labs run their DEQAS samples very very carefully. The best DEQAS can hope for is to find out if commercial labs can do it right, not if they do it right.

In the best of all possible worlds, all commercial Vitamin D testing would be accurate, patients would not have to seek in-home Vitamin D levels because their physicians would already have done so in the office, and everybody could afford commercial lab fees, which can range up to $200.00 per test. In the best of all possible worlds, if doctors did order a Vitamin D test, they would order the correct test and finally, in the best of all possible worlds, doctors would know how to correctly interpret the tests that they ordered.

Until then, if you have health insurance or can afford it, I recommend using LabCorp, ARUP, Clinical Pathology Labs, or Cleveland Clinic. If you use the home Vitamin D test kit from ZRT, they have already corrected for the DiaSorin RIA/mass spec uniform variance but realize it is a mass spec technique. ZRT also submitted, at my request, samples for comparison with RIA and they were quite accurate. Plus, I review ZRT's results; I know they are not artificially high; in fact, way too many of ZRT results are incredibly low. Falsely elevated results is where the danger lies, thinking you are fine when you are deficient.

Page last edited: 08 August 2011