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Search Health Information    Chemistry Screen

Chemistry Screen

Test Overview

A chemistry screen is a blood test that measures the levels of several substances in the blood (such as electrolytes ). A chemistry screen tells your doctor about your general health, helps look for certain problems, and finds out whether treatment for a specific problem is working.

Some chemistry screens look at more substances in the blood than others do. The most complete form of a chemistry screen (called a chem-20, SMA-20, or SMAC-20) looks at 20 different things in the blood. Other types of chemistry screens (such as an SMA-6, SMA-7, or SMA-12) look at fewer. The type of chemistry screen you have done depends on what information your doctor is looking for.

For more information about specific parts of a chemistry screen, see:

Why It Is Done

A chemistry screen may be done:

  • As part of a routine physical examination.
  • To help you and your doctor plan changes in your meal plan or lifestyle.
  • To look for problems, such as a low or high blood glucose level that may be causing a specific symptom.
  • To follow a specific health condition and check how well a treatment is working.
  • Before you have surgery.

How To Prepare

How you prepare for a chemistry screen depends on what your doctor is looking for in the test.

  • You may be instructed not to eat or drink anything except water for 9 to 12 hours before having your blood drawn. This is called a "fasting blood test." Fasting is not always necessary, but it may be recommended.
  • Usually, you are allowed to take your medicines with water the morning of the test.
  • Do not eat high-fat foods the night before the test.
  • Do not drink alcohol before you have this test.

Many medicines may change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?) .

How It Is Done

The health professional drawing blood will:

  • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein. More than one needle stick may be needed.
  • Attach a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Apply a gauze pad or cotton ball over the needle site as the needle is removed.
  • Apply pressure to the site and then a bandage.

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.

References

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.

Credits

By Healthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer Thomas M. Bailey, MD - Family Medicine
Last Revised March 26, 2012

This information does not replace the advice of a doctor. Healthwise, Incorporated disclaims any warranty or liability for your use of this information. Your use of this information means that you agree to the Terms of Use. How this information was developed to help you make better health decisions.

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