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Are Your Vitamin D Test Results Valid?

Vitamin D deficiency is rampant not just in the U.S., but worldwide, and research into vitamin D status and its effects on your health has grown tremendously in the past couple of years, showing that previously recommended levels are insufficient to reap any major health benefits.

This has led to a surge in vitamin D testing, with labs like Quest doing some 500,000 tests per month!

Unfortunately, recent developments in 2008 have made it clear that there are irregularities in the values obtained from the different testing methods. Although results from any of the three commonly used assays may be analytically accurate, they might not be clinically accurate, which is, ultimately, what matters.

First -- Order the Correct Test

First of all, it is very important that your physician orders the correct test as there are two vitamin D tests -- 1,25(OH)D, and 25(OH)D.

25(OH)D is the better marker of overall D status. It is this marker that is most strongly associated with overall health.

The correct test is 25(OH)D, also called 25-hydroxyvitamin D.

Second – Measure Your Results Against Optimal Values

Please note the difference between normal and optimal. You don't want to be average here; you want to be optimally healthy.

These optimal values have been confirmed by more recent studies into vitamin D status and its effects on health and reduced disease risks.

Optimal 25-hydroxyvitamin D values are:

45-50 ng/ml or
115-128 nmol/l

Normal 25-hydroxyvitamin D lab values are:

20-56 ng/ml
50-140 nmol/l

Your vitamin D level should NEVER be below 32 ng/ml. Any levels below 20 ng/ml are considered serious deficiency states and will increase your risk of breast- and prostate cancer, and autoimmune diseases like MS and rheumatoid arthritis.

If you have the above test performed, please recognize that many commercial labs are using the older, dated reference ranges. The above values are the most recent ones based on large-scale clinical research findings.

Third -- Make Sure Your Lab Uses the STANDARD Assay

There are a number of different companies that have approval to perform vitamin D testing, but the gold standard is DiaSorin. Their radioimmunoassay (RIA) method for measuring total vitamin D levels has become the gold standard, not because it’s more accurate than the others, but because it’s the one used in almost every major vitamin D study, on which the recommended blood levels for clinical efficacy are based.

Therefore, in order for any other testing method to offer clinically relevant results, the test values must agree with DiaSorin RIA results, since those were used to establish the recommended levels.

Vitamin D status is measured by looking at blood levels of 25-hydroxyvitamin D3. There are three common methods used for measuring vitamin D3:

1. LC-MS/MS -- This test measures 25-hydroxyvitamin D2 and D3 separately
2. RIA (DiaSorin) -- Developed in 1985, it accurately measures total 25-hydroxyvitamin D (It does not separate D2 and D3)
3. Liaison (DiaSorin) -- a more recently developed automated immunoassay by DiaSorin that has largely replaced the RIA

The LC-MS/MS (liquid chromotatography-mass spectrometry) method is the preferred method for many labs, including the Mayo Clinic, Quest Labs, Esoterix, ZRT, and others, while Liaison is favored by other testing labs like LabCorp.

Since the DiaSorin assay (RIA) was used in the major clinical studies that led to the recommended vitamin D levels, any lab using the LC-MS/MS method need to make sure their test correlate with the RIA test values in order to accurately determine your vitamin D status.

WARNING: Your Lab Can Give You the WRONG Results!

The test Quest uses, while analytically accurate, gives vitamin D values that are consistently about 25-40 percent higher than the DiaSorin assay.

What does this mean?

If you are using Quest labs you are getting FALSE reassurances that your levels are accurate and may even stop or discontinue treatment thinking that you are in the toxic range when you still need more vitamin D.

Some labs, such as ZRT and the Mayo Clinic, have recognized the severity of this problem and have already calibrated their LC-MS/MS results against the DiaSorin RIA values -- in order to be clinically applicable -- whereas Quest has failed to do so

This is a serious problem since Quest probably runs the most vitamin D assays of any lab in the U.S.

Which Lab Should You Use?

The single biggest concern in this area is Quest Labs as their lab is reporting results as much as 40 percent higher than those found with the DiaSorin assay. That means, if you are seeking to obtain levels similar to those that have been confirmed optimal in the scientific literature, you will be falsely reassured by their test results.

At this point in time Quest has NOT recalibrated their test values to be aligned with DiaSorin, and it appears unlikely that they will do so anytime in the near future. Making matters worse, Quest is currently doing over 500,000 tests a month -- much of this increase has occurred over the past year -- and the consistent reports obtained from the field is that many split samples being sent to them are coming back widely different, indicating a lack of consistency and quality.

The LC-MS/MS is clearly a highly accurate test, but only in the hands of experienced technicians who have the time to do the test properly, and only if it’s calibrated against the RIA.

Liaison, which is a more recently developed DiaSorin test that renders clinically accurate results, is also a much more accurate testing method for high volume throughput of tests, and does not depend as much on lab technician’s expertise.

LabCorp uses this method, which makes them better able to handle large volumes of tests without sacrificing clinical accuracy or worrying about the qualifications of the staff.

Additionally the charge for the Lab Corp test should be less than half of that of the Quest test.

For all these reasons it is now strongly recommend to use LabCorp for these reasons until Quest can guarantee accurate, usable results.



Reference Source 116
Septembert 3, 2008
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