Therapeutic drug level are laboratory tests to look for the presence and the amount of specific drugs in the blood.

With most medications, you need a certain level of drug in your bloodstream to obtain the desired effect. Some medications are harmful if the level rises too high and do not work if the levels are too low.

Monitoring the amount of the drug found in your blood allows your health care provider to make sure the drug levels are within an effective range.

Drug level testing is especially important in people taking drugs such as:

• Procainamide or digoxin used to treat abnormal beating of the heart
• Dilantin or valproic acid used to treat seizures
• Gentamicin or amikacin, antibiotics used to treat infections

Testing may also be done to determine how well your body breaks down the drug ( metabolism), or how it interacts with other necessary drugs.

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 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.

See also: Venipuncture

The sample is then taken to the laboratory, where it is checked for the particular drug specified by your health care provider.

Some drug level tests require preparation. Your health care provider will tell you how to prepare.

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.