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.
A chemistry screen may include tests for:
- Alkaline phosphatase.
- Alanine aminotransferase (ALT).
- Aspartate aminotransferase (AST).
- Bilirubin (total and direct).
- Blood glucose.
- Blood urea nitrogen.
- Calcium (Ca) in blood.
- Carbon dioxide (bicarbonate).
- Chloride (Cl).
- Cholesterol and triglycerides.
- Creatinine and creatinine cClearance.
- 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 isn't always needed. But it may be recommended.
- In most cases, 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.
How It Is Done
A health professional uses a needle to take a blood sample, usually from the arm.
How It Feels
When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.
There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.
Results are usually available in 1 to 2 days.
Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.
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.