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:

• Albumin.
• Alkaline Phosphatase.
• Alanine Aminotransferase (ALT).
• Aspartate Aminotransferase (AST).
• Bilirubin (total and direct).
• Blood Glucose.
• Blood Urea Nitrogen.
• Calcium (Ca) in Blood.
• Carbon Dioxide.
• Chloride (Cl).
• Cholesterol and Triglycerides Tests.
• Creatinine and Creatinine Clearance.
• Gamma-glutamyl transferase (GGT).
• Lactate dehydrogenase.
• Phosphate in Blood.
• Potassium (K) in Blood.
• Sodium (Na) in Blood.
• Total Serum Protein.
• Uric Acid in Blood.

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.