Magnesium test is a blood test used to measure levels of magnesium, an electrolyte that is critical in nearly all metabolic processes.Magnesium is mostly found in the bone and inside cells of body tissues and organs. It is necessary for almost all the processes in the body. It helps maintain normal muscle and nerve function, keeps the bones strong, controls the heart beat, and helps regulate blood pressure. Magnesium also controls blood sugar levels and helps support the body's defense (immune) system. Abnormal levels may indicate renal insufficiency, chronic renal disease, uncontrolled diabetes, diabetic acidosis, Addison's disease, hypothyroidism, malnutrition, malabsorption, hypoparathyroidism, and alcoholism.