A serum magnesium test is done to see how much magnesium there is in the blood.

Blood is 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. This applies pressure to the area and causes the vein to swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects in an air-tight vial or syringe attached to the needle. During the procedure, the band is removed to restore circulation. 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, the site is cleaned with germ-killing medicine (antiseptic) and punctured with a sharp needle or a lancet. The blood may be collected in 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.