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.
- Put pressure on the site and then put on 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.
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
ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Adam Husney, MD - Family Medicine
Martin J. Gabica, MD - Family Medicine
Specialist Medical ReviewerMartin J. Gabica, MD - Family Medicine
Current as ofOctober 14, 2016