Blood Gas Tests

What are blood gas tests?

Blood gas tests measure the levels of oxygen and carbon dioxide in your child’s blood. The tests also measure the balance of acids and bases in your child’s blood.

There are 2 kinds of blood gas tests: one is done using blood from an artery (ABG) and one is done using blood from a vein (VBG). The ABG is used to measure oxygen. Both the ABG and VBG tests can measure carbon dioxide and acids in the blood. The ABG test is better in some cases of heart problems or severe breathing problems. Arteries are more sensitive than veins when they are poked by a needle. For this reason, the ABG test may hurt more than the VBG test.

Why are these tests done?

Blood gas tests can measure how well the body uses oxygen and gets rid of carbon dioxide and acid waste. The body breathes out carbon dioxide to get rid of it. Other wastes are eliminated by the kidneys. Blood gas tests may be done to:

• Check how well your child’s lungs are working.
• Check how well oxygen therapy or other breathing treatments are working.
• Check how well your child’s heart is delivering blood to rest of the body.
• Check how well the kidneys are working to get rid of acid waste.
• Provide information about whether your child’s blood has the right balance of acids and bases.

Blood gas tests may be done when your child has an injury that affects breathing, or if your child is having trouble breathing. Blood gas tests may also be done if your child has problems with acid build up in the body caused by lactic acidosis or diabetic ketoacidosis.

How do I prepare my child for these tests?

There is usually no special preparation for these tests. If your child is using oxygen therapy, be sure to tell the technician how much oxygen your child is on, for example, 2 liters per minute, and how many minutes or hours your child has been on oxygen before the test.

How are the tests done?

For the VBG, a small amount of blood is taken from a vein in your child’s arm with a needle.

For the ABG, a small amount of blood is taken from an artery in your child’s arm with a needle. The artery most commonly used for this test is the artery in the wrist where your child’s pulse is usually checked. For newborns, blood may be taken from the umbilical cord. If your child is in intensive care and needs a lot of blood tests, a small tube may be placed in an artery or vein so that your child doesn’t need to have as many painful needlesticks.

Because blood pressure is stronger in arteries than in veins, the puncture hole from the needle may take longer to close. Firm pressure is applied to the site for at least 5 minutes after the test to stop bleeding.

The blood is collected in tubes and tested in a lab right away to get the most accurate results.

Having this test will take just a few minutes. There is no risk of getting AIDS, hepatitis, or any other blood-borne disease from this test.

What does the test result mean?

The usual results include measurements of the oxygen level, carbon dioxide level, bicarbonate level, and pH (the measure of the acid-base balance). Normal values for children are usually:

• PaO2: 70 to 100 mm Hg (millimeters of mercury) for the oxygen level in the ABG
• PaCO2: 35 to 45 mm Hg for the carbon dioxide level
• HCO3: 19 to 25 mEq/L (milliequivalents per liter) for the bicarbonate level (which shows how well the kidneys are working)
• pH: 7.35 to 7.45 for the acid-base balance (VBG results are slightly lower)

Many health problems can cause abnormal blood gas results. Examples of possible causes of an abnormal result are:

• Your child’s body is not getting enough oxygen.
• Your child’s body is not getting rid of enough carbon dioxide.
• Your child’s kidneys are not working properly.
• Your child has had severe vomiting or diarrhea.
• Your child’s heart is not delivering enough blood to the rest of the body.
• Your child has diabetic ketoacidosis, which can happen when your child’s sugar is very high.

What if my child’s test result is not normal?

Test results are only one part of a larger picture that takes into account your child’s medical history and current health. Sometimes a test needs to be repeated to check the first result. Talk to your child’s healthcare provider about what your child’s results mean.

If your child’s test results are not normal, ask your child’s healthcare provider:

• if your child needs more tests
• what kind of treatment your child might need
• when your child needs to be tested again