Overview

A chemistry screen is a test that measures the levels of several substances (such as electrolytes) in the blood. A chemistry screen provides information about your general state of health and helps your doctor detect certain abnormalities or determine whether treatment for a specific problem is effective.

There are several variations of a chemistry screen test. The most complete form of a chemistry screen (called a chem-20, SMA-20, or SMAC-20) measures 20 substances in the blood. Other types of chemistry screens (such as an SMA-6, SMA-7, or SMA-12) measure fewer substances. The type of chemistry screen you have done depends on which substances your doctor wants to evaluate.

A chemistry screen is done on a blood sample taken from a vein.

Why It Is Done?

A chemistry screen may be done:

As part of a routine physical examination. A chemistry screen can provide valuable information about the general state of your health. Your doctor can use information obtained from a chemistry screen to recommend dietary or lifestyle changes.
To help your doctor detect abnormalities.

To help evaluate a specific health condition and monitor the effectiveness of its treatment.

How to Prepare?

Generally, no special preparation is required before having a chemistry screen. If a fasting blood sugar test is being done as part of the chemistry screen, do not eat or drink anything other than water for 12 to 14 hours before the blood sample is taken. No special preparation is required before having a random blood sugar test.

How It Is Done?

The person drawing blood will wrap an elastic band around your upper arm to temporarily stop the flow of blood through the veins in your arm. This makes it easier to put a needle into a vein properly because the veins below the band get larger and do not collapse easily.

The needle site is then cleaned with alcohol and the needle is inserted. More than one needle stick may be needed if the needle does not get placed correctly or if the vein cannot supply enough blood.

When the needle is properly placed in the vein, a collection tube will be attached to the needle. Blood will flow into the collection tube. Sometimes more than one tube of blood is collected.

When enough blood has been collected, the band around your arm will be removed. A gauze pad or cotton ball is placed over the puncture site as the needle is withdrawn. Pressure is applied to the puncture site for several minutes and then a small bandage is often placed over it.

How It Feels?

You may feel nothing at all from the needle puncture, or you may feel a brief sting or pinch as the needle goes through the skin. Some people feel a stinging pain while the needle is in the vein. However, many people do not feel any pain (or have only minor discomfort) once the needle is positioned in the vein. The amount of pain you feel depends on the skill of the person drawing the blood, the condition of your veins, and your sensitivity to pain.