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Chemistry Screen

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks

There is very little chance of a problem from having blood sample taken from a vein.

  • You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
  • In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
  • Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

Results

A chemistry screen is a blood test that measures the levels of several substances in the blood (such as electrolytes).

Normal values vary from lab to lab and depend on which tests were included in your chemistry screen. Results are usually available in 1 to 2 days.

Many conditions can change chemistry screen test levels. Your doctor will talk with you about any abnormal results that may be related to your symptoms and medical history.

For more information about normal and abnormal values, see:

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Taking medicine. Some medicines can cause changes in the normal values of a chemistry screen.
  • Eating high-fat foods or drinking alcohol.
  • Recent intravenous (IV) fluids, such as fluids given during surgery.
  • Vomiting.
  • Diarrhea.
  • Dehydration.

What To Think About

There are several different chemistry screens. For example, an SMA-7 looks at 7 substances in the blood, including uric acid, potassium, and sodium. A complete chemistry screen (or SMA-20) looks at the same things as an SMA-7 plus 13 others (such as phosphorus, carbon dioxide, and bilirubin). Which chemistry screen your doctor orders depends on why you are having the test, your symptoms, and whether you have any specific conditions or diseases.

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.

  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

WebMD Medical Reference from Healthwise

Last Updated: March 26, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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