Why Magnesium Is Important
Magnesium is involved in hundreds of processes in your body, including muscle and nerve function, heart rhythm, blood sugar control, sleep, and bone health. Yet many people don't get enough through their diet.
The Problem With a Standard Magnesium Test
The standard test measures serum magnesium—the magnesium in the liquid part of your blood. But less than 1% of your body's magnesium is in your serum. Most is stored in your bones, muscles, and inside your cells.
Your body works hard to keep serum magnesium stable, even pulling from your bones if needed. That means serum magnesium can look normal even when your total body stores are low.
What Is RBC Magnesium?
The red blood cell (RBC) magnesium test measures magnesium inside your red blood cells. Because it reflects magnesium inside cells, some clinicians believe it may better reflect longer-term magnesium status than serum levels.
Limitations to Know
- RBC magnesium isn't perfect, and there's no single "gold standard" test for magnesium status
- Reference ranges vary between labs
- Research on how well RBC magnesium predicts health outcomes is limited
- It's best interpreted along with symptoms, diet, and other labs
Signs You May Be Low in Magnesium
- Muscle cramps or twitching
- Fatigue
- Trouble sleeping
- Headaches or migraines
- Irregular heartbeat
- Constipation
Who's at Higher Risk?
- People with type 2 diabetes
- Heavy alcohol use
- Digestive conditions like Crohn's or celiac disease
- Long-term use of acid-reducing medications or certain diuretics
- Older adults
- Diets low in whole foods
How to Get More Magnesium
- Foods: Pumpkin seeds, almonds, cashews, spinach, black beans, whole grains, avocado, and dark chocolate
- Supplements: Forms like magnesium glycinate or citrate are commonly used. Too much can cause diarrhea, and people with kidney disease should avoid supplements unless advised by a provider
Learn more about lab testing here: Heva Lab Testing
The Bottom Line
Serum magnesium can miss low body stores, while RBC magnesium looks inside cells and may give additional insight. Neither test is perfect, so results are best paired with symptoms and diet.