A serum magnesium test is a measurement of how much magnesium there is in the blood.

This test is done when your doctor suspects you may have abnormal levels of magnesium in your blood.

About half of the body's magnesium is found in bone. The other half is found inside cells of body tissues and organs.

Magnesium is necessary for nearly all biochemical 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.

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

There is no special preparation for this test.

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.