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.

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

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
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
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.
  • Put pressure on the site and then put on a

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


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
  • 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.


A chemistry screen is a blood test that
measures the levels of several substances in the blood (such as

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.

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
  • 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.


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.


ByHealthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD – Internal Medicine
Adam Husney, MD – Family Medicine
Martin J. Gabica, MD – Family Medicine
Specialist Medical Reviewer Martin J. Gabica, MD – Family Medicine

Current as ofOctober 9, 2017