SMA20

Alternative names 
CHEM-20; Sequential multi-channel analysis with computer-20; SMAC20; Metabolic panel 20

Definition

Chem-20 is a group of 20 chemical tests performed on serum (the portion of blood without cells). Electrolytes are ionized salts in blood or tissue fluids (ions are atoms or molecules that carry an electrical charge). Electrolytes in the body include sodium, potassium, chlorine, and many others.

The rest of the tests focus primarily on chemicals related to the body’s metabolism and the breakdown of various substances. These are tests that evaluate the function of the liver and kidneys.

How the test is performed

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.

Infant or young child:
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.

How to prepare for the test
The patient should try not to eat overnight before the test.

Infants and children:
The preparation you can provide for this test depends on your child’s age, previous experience, and level of trust. For general information regarding how you can prepare your child for a test, see the following:

     
  • Infant test or procedure preparation (birth to 1 year)  
  • Toddler test or procedure preparation (1 to 3 years)  
  • Preschooler test or procedure preparation (3 to 6 years)  
  • Schoolage test or procedure preparation (6 to 12 years)  
  • Adolescent test or procedure preparation (12 to 18 years)

How the test will feel
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.

Why the test is performed
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.

Normal Values

     
  • Albumin: 3.9 to 5.0 g/dl  
  • Alkaline phosphatase: 44 to 147 IU/L  
  • ALT (alanine transaminase): 6 to 59 IU/L  
  • AST (aspartate aminotransferase): 10 to 34 IU/L  
  • BUN (blood urea nitrogen): 7 to 20 mg/dl  
  • Calcium - serum: 8.5 to 10.9 mg/dl  
  • Serum chloride: 101 to 111 mmol/L  
  • CO2 (carbon dioxide): 20 to 29 mmol/L  
  • Creatinine: 0.8 to 1.4 mg/dl  
  • Direct bilirubin: 0.0 to 0.3 mg/dl  
  • Gamma-GT (gamma-glutamyl transpeptidase): 0 to 51 IU/L  
  • Glucose test: 64 to 128 mg/dl  
  • LDH (lactate dehydrogenase): 105 to 333 IU/L  
  • Phosphorus - serum: 2.4 to 4.1 mg/dl  
  • Potassium test: 3.7 to 5.2 mEq/L  
  • Serum sodium: 136 to 144 mEq/L  
  • Total bilirubin: 0.2 to 1.9 mg/dl  
  • Total cholesterol: 100 to 240 mg/dl  
  • Total protein: 6.3 to 7.9 g/dl  
  • Uric acid: 4.1 to 8.8 mg/dl

Key to abbreviations:

     
  • IU = international unit  
  • L = liter  
  • dl = deciliter = 0.1 liter  
  • g/dl = gram per deciliter  
  • mg = milligram  
  • mmol = millimole  
  • mEq = milliequivalents

Electrolytes:

     
  • Positively charged ions (cations) include sodium, potassium, calcium, and magnesium (usually drawn separately; see magnesium - serum).  
  • Negatively charged ions (anions) include chloride, bicarbonate (essentially the same as CO2), protein (see total protein), phosphorous, SO4, and organic acids.

When an electrolyte test is performed, the measured ions usually include sodium, potassium, chloride, and bicarbonate. Calcium and magnesium levels are also obtained in many institutions as part of this test.

What abnormal results mean
See the individual tests.

What the risks are

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

Special considerations
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.

Johns Hopkins patient information

Last revised: December 3, 2012
by Levon Ter-Markosyan, D.M.D.

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