Magnesium, RBC is a specialized test that measures magnesium levels within red blood cells, providing a more accurate assessment of long-term magnesium status than serum measurements. This test is essential for evaluating magnesium deficiency, which can cause muscle weakness, tremors, cardiac arrhythmias, and neurological symptoms. Magnesium is a critical cofactor for over 300 enzymatic reactions involved in energy production, protein synthesis, and muscle and nerve function. Red blood cell magnesium levels are more stable and better reflect tissue magnesium stores than serum levels, which can fluctuate with dietary intake and other factors. This test is particularly valuable for patients with chronic diseases, those on medications that affect magnesium absorption, and individuals with unexplained muscle or neurological symptoms.