RBC (red blood cell) magnesium measures magnesium inside your cells, providing a more accurate assessment of your true magnesium status than standard serum magnesium tests.
Magnesium is essential for over 300 biochemical reactions in the body, including muscle function, nerve signaling, blood sugar regulation, blood pressure control, and energy production. Yet an estimated 50-60% of Americans don't get enough magnesium.
Why RBC magnesium is better than serum magnesium:
- Only 1% of body magnesium is in blood; 99% is in cells and bones
- Serum magnesium can appear "normal" even when you're deficient
- The body tightly regulates serum levels, pulling from cells and bones to maintain them
- RBC magnesium reflects what's actually available inside your cells
- Deficiency symptoms often correlate better with RBC than serum levels
If you've been told your "magnesium is fine" but still have symptoms of deficiency (muscle cramps, fatigue, anxiety, poor sleep), RBC magnesium may reveal the true picture.
Price
$15
Results
2-3 days
Lab Network
Quest
Fasting
Not Required
What's Measured
What This Test Measures
This test measures magnesium concentration inside red blood cells, reported in mg/dL. Optimal RBC magnesium is typically 5.0-6.5 mg/dL. Levels below 4.5 mg/dL suggest deficiency. Some integrative practitioners aim for the upper end of the range (6.0-6.5 mg/dL).
Who Should Consider This Test
Anyone with symptoms of magnesium deficiency despite "normal" serum levels, people with muscle cramps, headaches, or sleep issues, those with diabetes or metabolic syndrome, individuals on medications that deplete magnesium (PPIs, diuretics), and those with high stress or alcohol consumption.
Health Insights
Conditions Screened
Related Symptoms
When to Get Tested
No fasting required. RBC magnesium can be tested any time. If supplementing, continue as usual to see your levels on your current regimen. Retest 2-3 months after starting supplementation to assess improvement.
Understanding Your Results
Optimal: 5.5-6.5 mg/dL. Adequate: 4.5-5.5 mg/dL. Low: <4.5 mg/dL. If deficient, magnesium glycinate, citrate, or threonate are well-absorbed forms. It takes weeks to months to replete magnesium stores. Retest in 2-3 months.
What to Expect
After placing your order, you'll receive an email from Rupa Health (our lab order partner) within 24 business hours to sign a consent form and schedule your blood draw at a Quest Diagnostics location.
The blood draw typically takes just a few minutes. Results are usually available within 2-3 business days and will appear in your secure Justlabs dashboard.
Our results include reference ranges and easy-to-understand indicators showing whether your values are within normal limits.
Frequently Asked Questions
$15
Typical hospital cash price: $100–$200
+ $28 in standard lab & physician fees at checkout (no additional bills later)
$18 lab collection + $10 physician order
Need Help Choosing?
Not sure if this is the right test for you? Take our quick questionnaire.
Take the QuizRelated Tests
$15
+ $28 in fees