A recent article in the New York Times highlighted an ongoing problem with the accuracy of vitamin D testing at the largest commercial clinical laboratory, Quest Diagnostics. It has become clear from shared experience among vitamin D experts, including myself, that Quest Diagnostics has a problem with seemingly random over-estimation of vitamin D levels.
In a corporate statement they claim to have corrected the problem before November of 2008. However, I have experienced three cases of over estimation since then, one by more than 50 ng/mL difference. Clearly the problem has not been addressed adequately.
The problem is the method of measuring vitamin D. Mayo Clinic perfected a liquid chromatography tandem mass spectrometry assay to measure both D2 and D3. Mayo Clinic strictly controls the assay conditions and regularly validates their results. This method is very accurate but is user dependent.
The high volume of vitamin D testing now being done through Quest Diagnostics is likely compromising the assay conditions and limiting validation. As a result you get random errors, in this case mostly over-estimating vitamin D levels.
The method of measurement used by the Centers for Disease Control (NHANES), Harvard Nurses Study, Harvard Health Professions Study and the Framingham Study is an immunoassay developed by Drs. Bruce Hollis and Ronald Horst and now produced and marketed by Diasorin Inc. This method allows for large numbers of samples to be measured in an automated fashion with less error. Lab Corp, the nation’s second largest clinical laboratory uses this method to measure vitamin D.
Things to Remember:
1. Know your ‘D’ number. Don’t accept a ‘normal’ result. Get the actual number in ng/mL. It should be 35-100 year round and ideally 45-70.
2. Take The Vitamin D Cure Risk Quiz. You may want to be re-measured at a laboratory using the Diasorin immunoassay if your result is not consistent with your risk.
3. Get the book, The Vitamin D Cure. I will tell you how to replace your vitamin D safely and accurately.