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.
How to prepare for the test
The health care provider may tell you to stop taking certain medicines before the test.
Drugs that can increase LDH measurements include anesthetics, aspirin, clofibrate, fluorides, mithramycin, narcotics, and procainamide.
How the test will feel
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.
Why the test is performed
This test is usually done when your doctor thinks you might have high LDH levels. Measurement of LDH isoenzymes helps determine the location of tissue damage.
LDH is found in many body tissues like the heart, liver, kidney, skeletal muscle, brain, blood cells, and lungs.
LDH exists in 5 forms, which differ slightly in structure.
LDH-1 is found primarily in heart muscle and red blood cells.
LDH-2 is concentrated in white blood cells.
LDH-3 is highest in the lung.
LDH-4 is highest in the kidney, placenta, and pancreas.
LDH-5 is highest in the liver and skeletal muscle.
All of these can be measured in the blood.
What abnormal results mean
Because LDH can be found in many tissues in the body, total LDH is not specific for heart disease.
Normally, the level of LDH-2 is greater than LDH-1. However, after a heart attack, LDH-1 is generally higher than LDH-2. This is called a "flipped" LDH pattern.
The LDH level rises within 24 - 72 hours after a heart attack, peaks in 3 - 4 days, and returns to normal in about 14 days.
There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
Fainting or feeling light-headed
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
Abraham NZ Jr, Carty RP, DuFour DR, Pincus MR. Clinical enzymology. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: Saunders Elsevier; 2006:chap 20.
Linda J. Vorvick, MD, Medical Director, MEDEX Northwest Division of Physician Assistant Studies, University of Washington, School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.