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Blood test
Blood test

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

Alternative Names:
Low density lipoprotein

How the test is performed:
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill 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 tourniquet 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.

Infant or young child:
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. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.
How to prepare for the test:
Fasting for 12 hours may be advised. The health care provider may advise you to withhold drugs that may affect the test (see special considerations).

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
How the test will feel:
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed:

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

Cholesterol is an important normal constituent of the body. It is part of the structure of cell membranes, bile acids, and steroid hormones. Since cholesterol is water insoluble, most cholesterol is carried in the blood by lipoproteins (large protein-like molecules, including chylomicrons, VLDL, LDL, and HDL). 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 high density lipoprotein (HDL) and very low density lipoprotein (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.
Normal Values:

A healthy LDL level is one that falls in the optimal or near-optimal range.

  • Optimal: Less than 100 mg/dL
  • Near Optimal: 100-129 mg/dL
  • Borderline High: 130-159 mg/dL
  • High: 160-189 mg/dL
  • Very High: 190 mg/dL and higher
What abnormal results mean:
High levels of LDL may be associated with: Lower-than-normal levels of LDL may be caused by: Additional conditions under which the test may be performed:
What the risks are:
  • Excessive bleeding
  • Fainting or feeling light-headed
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)
  • Multiple punctures to locate veins
Special considerations:
Drugs that can increase lipoprotein levels include aspirin, oral contraceptives, phenothiazines, corticosteroids, and sulfonamides.

Veins and arteries vary in size from one patient to another, and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Review Date: 10/27/2001
Reviewed By: A.D.A.M. editorial (8/23/02). Previous review: Jeffrey Heit, M.D., Department of Medicine, University of Pennsylvania Health System, Philadelphia, PA. Review provided by VeriMed Healthcare Network (10/27/01).
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