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.
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 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
- 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
- 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
- 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
Many medicines may change the results of this test. Be sure
to tell your doctor about all the nonprescription and prescription medicines
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
medical test information form(What is a PDF document?).
How It Is Done
The health professional drawing blood
- 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
- Put the needle into the vein. More than one needle stick
may be needed.
- Attach a tube to the needle to fill it with
- Remove the band from your arm when enough blood is
- Apply a gauze pad or cotton ball over the needle site as
the needle is removed.
- Apply pressure to the site and then a