It is probably best to work with a cooperative local physician to have your (25) OH D3 level tested if you are supplementing. If your Doctor won't test you, or this is not an option for you, there are home testing kits available through several sources.
Reader Steve tells of an article in the New York Times regarding a large national laboratory that had some quality control problems relating to Vitamin D testing. Quest pharmaceutical had a number of erroneously high D measurements and had to do a "recall" of sorts, offering retests to to potentially affected patients.
From the Times article:
Some doctors who advocate vitamin D use said they had begun noticing some unusually high test results in 2007 and had begun complaining publicly in the summer of 2008.
M any laboratory tests, including Quest’s vitamin D test, do not require approval from the FDA.
"If you get your vitamin D level measured in the Mayo Clinic, the Cleveland Clinic and the Timbuktu Clinic, it would be nice if it came out the same value,” said Dr. Neil C. Binkley, associate professor of medicine at University of Wisconsin.
Dr. Binkley said that a few years ago he sent a sample of his blood to six laboratories and got results that ranged from 14 nanograms a milliliter, which would be a deficient level, to 41 nanograms — a level three times as high and considered adequate. While the tests’ consistency has improved since then, there can still be substantial variability, he said.
....the F.D.A. is considering increasing its role in regulating diagnostic tests. Now, test kits sold to labs, hospitals and doctor’s offices must be approved by the agency. But tests developed and offered by a single laboratory, like the Quest vitamin D test, do not.
That last paragraph is interesting. At the least, it seems there is little reason to assume that behemoth labs will necessarily be more reliable than those offering test kits. That might include the at-home test kits.
The Vitamin D council is under direction of Dr. Cannell, who I have cited previously and who seems to be a pretty reliable character. The council has partnered with ZRT laboratories to offer a home test kit that is based on assessment of a spot of dried blood from a simple fingerstick. The test is a liquid chromatography/tandem mass spectrometry assay from the later reconstituted dried blood drop.
Interassay coefficients of variation were 13%, 13%,
and 11% at concentrations of 14, 26, and 81 ng/ml, respectively, for 25-hydroxy vitamin D3
The 25(OH)D3assay was linear from 3.5 to 75 ng/ml (R> 0.99).
Blood spot and serum values showed excellent correlation for 25(OH)D3(R= 0.91,n= 83).
Maybe I have a reader who knows the inter-test variablility (precision) of serum LC measurements of (25) OH D3. This blood spot technique seems accurate enough to base your D replacement regime on, though, and in any case is more accurate than taking pills and just guessing.
The council offers the test kit, which you mail in, for a mere $65, or better yet, 4 tests for $220. They donate $10 of each test to the council for its non-profit promotion of Vitamin D health. It's a good deal at $65. ZRT charges $75 to consumers on their own web site.
If you live in New York State, you are out of luck. Patient empowerment is apparently illegal there. (Get ready for more of that with the coming "health care reform")
You can also go to grassrootshealth which is another good D advocacy site. There, if you agree to participate in a survey with them on D levels, the test is only $40.