Magnesium is essential for hundreds of enzymatic reactions, muscle and nerve function, and cardiac rhythm. The magnesium test measures serum magnesium, but interpretation has limitations — serum magnesium reflects only a tiny fraction of total body magnesium and may be normal even with significant deficiency. This guide covers magnesium testing and clinical interpretation.
Reference ranges
- Serum magnesium: 1.7-2.2 mg/dL
- RBC magnesium: 4.2-6.8 mg/dL (more reflective of total stores)
The serum magnesium limitation
Only about 1% of total body magnesium is in blood. Most is in bones (~60%) and cells (~40%). Serum levels are tightly regulated and may remain normal despite significant total body deficiency.
This means low normal serum magnesium can still represent deficiency in symptomatic patients. Clinicians often supplement based on symptoms rather than waiting for low serum values.
Causes of low magnesium
- Inadequate dietary intake
- Diarrhea or malabsorption
- Diuretic use (loop and thiazide)
- Alcohol use
- Diabetes (often)
- Proton pump inhibitor long-term use
- Refeeding syndrome
Symptoms of magnesium deficiency
- Muscle cramps and fatigue
- Tremors
- Hyperreflexia
- Cardiac arrhythmias
- Insomnia
- Anxiety and depression
- Migraines (in some patients)
Causes of high magnesium
- Excessive supplementation
- Magnesium-containing antacids/laxatives
- Kidney failure
- Adrenal insufficiency
Significant hypermagnesemia is uncommon outside kidney failure or massive supplementation.
Magnesium repletion
Oral supplementation:
- Magnesium glycinate or citrate (well-absorbed)
- 200-400 mg daily for repletion
- Magnesium oxide is less absorbed but cheaper
IV magnesium for severe deficiency or specific situations (preeclampsia, severe asthma, torsade de pointes).
Frequently Asked Questions
Should I get magnesium tested for cramps or fatigue?
Reasonable. Many providers also empirically supplement given testing limitations.
Is serum magnesium reliable?
Only partially. Normal serum levels don’t exclude deficiency; low levels confirm deficiency.
What’s the best magnesium supplement?
Magnesium glycinate or citrate are well-absorbed. Magnesium oxide is cheaper but less absorbed and more likely to cause GI side effects.
Can I overdose on magnesium supplements?
Difficult with oral supplements unless very high doses or kidney impairment. GI side effects (diarrhea) typically occur first.
Do I need fasting?
Generally no.
The bottom line on the magnesium test
Magnesium testing has limitations — serum levels may not reflect total body stores. Clinical evaluation and empirical supplementation are often used alongside testing. Discuss with your healthcare provider for individualized recommendations.