Magnesium
What it measures, and what changes the number.
This test measures magnesium in serum, the liquid part of blood. Magnesium is a mineral used by many enzymes, and it plays a role in muscle, nerve and heart function. Most of the body's magnesium sits inside cells and in bone, so the blood level reflects only a small portion of the total.
It is often ordered with other electrolytes and minerals, especially calcium, potassium and phosphate, because their balance is linked, and with PTH and kidney tests such as creatinine, since the kidneys regulate magnesium excretion.
Red blood cells contain more magnesium than serum, so hemolysis (damaged red cells in the tube) or a delay in separating the serum can raise the result. Fasting is usually not required. Some labs also offer red blood cell magnesium, a different test not interchangeable with serum magnesium. Labs may print it as Magnesio.
Often read alongside: Calcium (total), Potassium, Phosphate, PTH (intact), Creatinine