CHEM-7 is a group of 7 chemical tests performed on serum (the portion of blood without cells).

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children:

The area is cleaned with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to the puncture site if there is any continued bleeding.

Fast for 8 hours before the test.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Blood chemistry measures the levels of a number of chemical substances that are released from various tissues in the body. The amounts of these chemicals in the blood may reflect abnormalities in the tissues secreting them.

The measured chemicals with their normal reference intervals are:

• BUN (blood urea nitrogen): 7 to 20 mg/dl
• Serum chloride: 101 to 111 mmol/L
• CO2 (carbon dioxide): 20 to 29 mmol/L
• Creatinine: .8 to 1.4 mg/dl
• Glucose test: 64 to 128 mg/dl
• Serum potassium: 3.7 to 5.2 mEq/L
• Serum sodium: 136 to 144 mEq/L

Key to abbreviations:

• L = liter
• dl = deciliter = 0.1 liter
• mg = milligram
• mmol = millimole
• mEq = milliequivalents

See the individual tests.

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.