My serum magnesium blood level was elevated at 2.8 but my RBC magnesium was normal at 4.9. How is this possible?? Which is more accurate?

Means nothing. Serum magnesium means little if you are even close to reference range. And the reference ranges are themselves set so that a few percent of healthies fall out on either side. Stop worrying about this stuff and be glad for good health.

Related Questions

My serum magnesium blood level was 2.5. Is it possible I'm still deficient since I didn't get my RBC magnesium checked?

Yes it may be. Rbc magnesium is the test is the most precise way to assess intracellular magnesium status this test does not require fasting and just don't take any suuplements the day of your blood test.You can take other prescription medicines. Read more...
Magnesium. An ionized magnesium level might be slightly more accurate however magnesium is mostly an intracellular trace mineral so lab measurements are just reasonable estimations still. Read more...