Quick Facts
- Sample: Blood
- Fasting: No fasting or other special preparation required
- Turnaround: 5-8 business days. (This test is only run on Mondays and Thursdays) May take longer based on weather, holiday, or lab delays.
What This Test Measures
This test measures the level of magnesium found specifically within your red blood cells, rather than in the liquid portion of your blood. This is sometimes called the RBC magnesium test or Mg, RBC. It is different from a standard serum magnesium test, which measures magnesium circulating in the blood serum rather than inside the red blood cells.
When your body becomes low in magnesium, it can draw magnesium out of red blood cells and into the serum, which means a standard serum test may still appear normal even when red blood cell magnesium has dropped. Measuring magnesium within the red blood cells can offer a more sensitive look at this particular marker than a standard serum magnesium measurement alone.
Benefits
- Measures magnesium specifically within red blood cells
- May detect changes a standard serum magnesium test can miss
- Offers insight relevant to muscle and bone health
- Provides a data point for cardiac and neuromuscular symptom evaluation
- Supports ongoing monitoring alongside healthcare provider guidance
- Uses a simple blood draw with no special preparation
Who Is This Test For?
- Individuals whose healthcare provider wants to evaluate red blood cell magnesium
- People with symptoms such as muscle cramping or weakness
- Individuals experiencing cardiac arrhythmias under medical evaluation
- People with a history of seizures being evaluated by a provider
- Individuals taking magnesium supplements who want data for their provider
- People whose provider suspects a standard magnesium test missed relevant findings
How It Works
- Order Your Test Online
- Visit a Local Lab
- Get Results Online
FAQ
1. What is the purpose of this test?
This test measures magnesium levels within your red blood cells rather than in the liquid portion of blood (serum). It's typically ordered when there is reason to believe someone may have low magnesium, since red blood cell magnesium can offer a more sensitive look at this marker than a standard serum magnesium test alone — when the body becomes low in magnesium, it can draw magnesium out of red blood cells and into the serum, so a standard serum test may still appear normal even when red blood cell magnesium has dropped. The result reflects magnesium stored in red blood cells, not a diagnosis on its own, and should be reviewed with a healthcare provider.
2. How does this test differ from other nutritional tests?
Because it looks specifically inside the red blood cells rather than in serum, this test can reflect changes that a standard serum magnesium test may not capture, since red blood cells can retain magnesium even after serum levels shift. It is not interchangeable with other magnesium or mineral tests that use different specimen types, as each reflects a different pool of magnesium in the body.
3. Who should consider this test, and when should it be ordered?
This test may be appropriate when a healthcare provider wants to evaluate red blood cell magnesium as part of assessing symptoms such as muscle cramping, weakness, cardiac arrhythmias, or seizures. It may also be useful for people taking magnesium supplements who want that data available for their provider, or in cases where a standard serum magnesium result doesn't align with ongoing symptoms. This test does not replace a full medical or nutritional evaluation.
4. Do I need to fast or prepare beforehand?
No fasting or other special preparation is required for this test. You can take your medications and dietary supplements as you normally would. Let your doctor or the lab know before testing if you are taking a magnesium supplement, as this can affect your results.
5. How quickly will I get results?
Results are typically available 5-8 business days after testing begins. This test is only run on Mondays and Thursdays, so timing may vary, and results may take longer based on weather, holidays, or other lab delays.
6. How do I interpret the results?
Interpreting your red blood cell magnesium result depends on your individual health history, symptoms, diet, supplement use, medications, and other laboratory findings. A low or high result does not automatically confirm a deficiency or excess on its own, and a normal result does not automatically rule out a nutritional concern. Your provider may consider this result alongside other relevant lab work.
Disclaimer: Reference ranges may vary by laboratory. Listed ranges are general guidelines and may differ from those used by the performing lab. Always consult your healthcare provider for interpretation.
7. What should I do if my results are abnormal?
Review results that fall outside the expected range with a qualified healthcare provider, who can help interpret them in the context of your symptoms, medical history, and other lab findings, and may recommend additional evaluation if appropriate. Do not stop prescribed medication unless instructed by your healthcare provider.
8. How often should I get tested?
Testing frequency should be based on your individual health needs and determined as directed by your healthcare provider. There is no universal schedule for repeat testing of red blood cell magnesium, and your provider can advise whether follow-up testing is appropriate for your situation.
Pre-test preparation
No fasting or other special preparation is required prior to giving blood for this test. You are able to take your medications and dietary supplements as you normally would the morning of the test. Let your doctor or the lab know before testing if you are taking a magnesium supplement, as this can affect test results. Do not stop prescribed medication unless instructed by your healthcare provider.
