LDL is a test that measures the amount of low-density lipoprotein (LDL) cholesterol in blood serum.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. 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 area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to the puncture site if there is any bleeding.

You may be told not to eat anything for 12 hours before the test. 

The health care provider may tell you to stop taking certain drugs before the procedure.

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.

This test is usually performed as part of an evaluation of coronary risk factors.

Cholesterol is an important, normal part of the structure of our cell membranes, bile acids, and steroid hormones. Since cholesterol does not dissolve in water, most cholesterol is carried in the blood by lipoproteins -- large protein-like molecules, including chylomicrons, very low density lipoprotein (VLDL), high density lipoprotein (HDL), and LDL. Chylomicrons are lipoproteins that are present shortly after a meal, but disappear within about 2 hours in "normal" people.

The main function of LDL seems to be to carry cholesterol to various tissues throughout the body. The laboratory actually measures the cholesterol portion of the LDL molecule, rather than the actual concentration of LDL in the blood. This is also true for HDL and VLDL. The total cholesterol level is the sum of LDL, HDL, and VLDL cholesterol.

Excess cholesterol in the blood has been correlated with cardiovascular disease. LDL is sometimes referred to as "bad" cholesterol, because elevated levels of LDL correlate most directly with coronary heart disease.

According to the National Heart, Lung, and Blood Institute, your LDL cholesterol level is a better indicator of your risk for a heart attack and stroke than total cholesterol. The lower your LDL, the lower your risk for heart disease or stroke.