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Does this test have other names?

Carbon dioxide test, CO2 test

What is this test?

This test measures the amount of bicarbonate, a form of carbon dioxide, in your blood.

Bicarbonate, also known as HCO3, is a byproduct of your body's metabolism. Your blood brings bicarbonate to your lungs, and then it is exhaled as carbon dioxide. Your kidneys also help regulate bicarbonate. Bicarbonate is excreted and reabsorbed by your kidneys. This regulates your body's pH, or acid balance. Bicarbonate also works with sodium, potassium, and chloride, also called electrolytes. These are usually measured at the same time as bicarbonate.

This test is usually part of a comprehensive series of blood tests to check for certain health conditions.

Why do I need this test?

You may have the bicarbonate test to monitor issues that affect pH levels in your blood. You may also have this test if you have kidney disease, liver failure, or other conditions related to metabolism.

What other tests might I have along with this test?

Your doctor may also order several other tests. These may include:

  • Arterial blood gas analysis

  • Electrolyte (sodium, potassium, and chloride) testing 

  • Urine pH testing

  • Anion gap blood testing

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

Results are given in milliequivalents per liter (mEq/L) or millimoles per L (mmol/L). Normal bicarbonate levels are:

  • 23 to 30 mEq/L in adults

  • 20 to 28 mEq/L in infants or children

  • 13 to 22 mEq/L in newborns  

A high level of bicarbonate in your blood can be from metabolic alkalosis, a condition that causes a pH increase in tissue. Metabolic alkalosis can happen from a loss of acid from your body, such as through vomiting and dehydration. It may also be related to conditions including anorexia and chronic obstructive pulmonary disease, or COPD.

A low level of bicarbonate in your blood may cause a condition called metabolic acidosis, or too much acid in the body. A wide range of conditions, including diarrhea, kidney disease, and liver failure, can cause metabolic acidosis.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Eating acidic fruits could affect your results. Taking diuretics, or medications that remove water from your body, and glaucoma drugs can also affect results.

How do I get ready for this test?

You don't need to prepare for this test. But be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.