Biomarker
RBC Magnesium
Last updated Sat May 30 2026 00:00:00 GMT+0000 (Coordinated Universal Time)
Observational— Better than serum but no established prognostic data
Why serum magnesium fails
Less than 1% of body magnesium is extracellular. Serum magnesium is tightly regulated by renal handling and remains normal even with significant intracellular depletion. Population studies suggest ~50% of US adults have inadequate magnesium intake; few would show low serum.
What RBC magnesium adds
Red cells live ~120 days and reflect average magnesium status over weeks. RBC magnesium correlates better with muscle magnesium content than serum.
When to test
- Persistent symptoms of deficiency (muscle cramps, fatigue, palpitations, PPI use, refractory hypertension or arrhythmia).
- PPIs are a major cause of magnesium deficiency that may not show on serum tests.
- Diuretic users, type-2 diabetics, alcohol-use disorder.
- Trial-and-response: a 200–400 mg/day magnesium glycinate or citrate trial is reasonable in symptomatic patients with low-normal results.
Limitations
- Not standardised across labs — reference ranges vary.
- No outcomes trials demonstrating that targeting RBC magnesium improves hard endpoints.
- Ionised magnesium would be ideal but is not widely available.
Related entries
References
- Costello, R. B. et al. Perspective: the case for an evidence-based reference interval for serum magnesium. Adv. Nutr. 7, 977–993 (2016).