Dr. Carolyn Dean states "The magnesium RBC (red blood cell) test is more accurate than the serum (blood) magnesium test, but it's not as accurate as the ionized magnesium test, which is only available in research labs." She adds, "The current magnesium range for most labs is 4.2 - 6.8 mg/dL, but recently I've seen some labs go even lower, to 3.5 - 6.0 mg/dL, as the population gets more and more deficient! About 80 percent of the population is deficient in magnesium, so I tell people that for optimal health, we want to be at the high end of the range, at or above the 80th percentile, or about 6.0 - 6.5 mg/dL."
One reason the magnesium RBC test is not a perfect test is because there are no mitochondria in red blood cells. There are mitochondria in all other cells of the body but not RBCs. For example, liver cells have between 1000 and 2000 mitochondria. There is a lot more magnesium in the cells which have mitochondria (the energy powerhouse of the cell) so you can see why the RBC magnesium test cannot be accurate.
Serum magnesium tests are highly inaccurate because only 1 percent of the body's total magnesium is in the blood. The other 99 percent is working within cells in an ionized form and is never measured by this test. So, the serum magnesium tests are also inherently not accurate.
Over the years, the focus for lab testing of minerals has changed from looking at whole blood to isolating minerals inside cells. Current state-of-the-art testing involves assessing mineral ion levels within the cell. This gives a much more accurate picture of what is happening inside the body. Unfortunately, the test is not yet available outside the 50 university research laboratories in the U.S.
These magnesium test limitations are why Dr. Dean recommends listening to your body as you supplement magnesium. Foods rich in magnesium are rice and wheat bran, cocoa powder, seeds, nuts, and peanut butter. For real, whole-food recommendations for optimal health, see my book.