Doctors can check people’s blood for normal or abnormal carbon dioxide (CO2) levels to help diagnose certain medical conditions. For example, a carbon dioxide test or CO2 test can help diagnose kidney and respiratory problems.
The CO2 test is a simple blood test that measures the amount of CO2 gas in a person’s blood. It is usually part of a larger test called an electrolyte panel.
The kidneys and lungs maintain the concentration of CO2 in the blood. If a person’s blood CO2 levels are above the normal range, a doctor may request further tests to check kidney and lung function, to assess other gases in the blood, or to check for fluid retention.
In this article, we look at the CO2 test, normal blood CO2 levels, and what abnormal results might mean.
The CO2 blood test measures the amount of carbon dioxide in the blood, which is present in the form of CO2, bicarbonate (HCO3), and carbonic acid (H2CO3). It mainly occurs in the form of bicarbonate.
As part of its normal functions, the human body naturally produces certain acids and bases that balance each other. Any disruption to this balance can be an early sign of an underlying medical condition.
Carbon dioxide is an “acidic” component because it combines with water to form carbonic acid. This makes the blood acidic. The CO2 blood test detects changes in the blood’s acidic content.
A healthcare professional can measure the levels of CO2 in a person’s blood using a simple blood test. They may take blood from either an artery (an arterial blood draw) or a vein (a venous blood draw).
A venous blood draw provides a measure of bicarbonate levels. An arterial blood draw measures the particle pressure of carbon dioxide. Both components are indicators of blood oxygenation.
The test procedure usually takes around 2–5 minutes. A nurse or healthcare professional will make a tiny prick in the person’s arm and collect blood in a test tube to send away for testing.
It is normal to experience a slight feeling of discomfort from where the nurse drew the blood, but this is temporary and will resolve on its own.
A healthcare professional may order a CO2 as part of a routine checkup or to investigate the underlying cause of certain symptoms. They may also recommend it in the case of a medical emergency or just before surgery.
People experiencing the following symptoms may receive a CO2 blood test:
- nausea or vomiting
- shortness of breath
- breathing difficulties
- confusion or feelings of faintness
A healthcare professional will likely perform a CO2 blood test as part of an electrolyte panel to check a person’s bicarbonate, a form of CO2, levels.
This test measures levels of electrolytes, including sodium, chloride, and potassium in the blood, along with bicarbonate levels. Electrolytes keep the body’s fluid balance in check and help maintain heart rhythm, muscle contractions, and brain function.
The electrolyte panel tells doctors how well a person’s kidneys are functioning. High or low bicarbonate levels can indicate that a person has metabolic alkalosis or acidosis, respectively.
Both arterial and venous blood draws can provide an overall picture of a person’s electrolyte status and the balance of acids and bases in their blood.
Results of the CO2 test may vary depending on the person’s age, sex, and medical history, as well as the test administration method and whether the person takes any type of medication.
The American Association of Clinical Chemistry report the normal ranges of total blood CO2 to be as follows. Measurements are in milliequivalents per liter (mEg/l) for conventional units and millimoles per liter (mmol/l) for International System of Units (SI units):
| Age range | Conventional units | SI units |
| 18–59 | 23–29 mEq/l | 23–29 mmol/l |
| 60–89 | 23–31 mEq/l | 23–31 mmol/l |
| 90+ | 20–29 mEq/l | 20–29 mmol/l |
According to a 2018 study in the Clinical Journal of American Society of Nephrology, HCO3 levels in women are approximately 1 mEq/l lower than levels in men.
An abnormal CO2 test result could mean that there are either high or low CO2 levels in the body.
Changes in CO2 levels may suggest that someone is losing or retaining normal body fluids. This points to an imbalance in the body’s electrolyte system.
Lower-than-normal CO2 levels could indicate:
- Addison’s disease
- diarrhea
- poisoning with ethylene glycol, such as from antifreeze poisoning
- ketoacidosis, wherein the body produces too many blood acids
- kidney disease
- salicylate toxicity, such as from an aspirin overdose
- lactic acidosis, or a buildup of lactate in the body
Higher-than-normal CO2 levels could indicate:
- breathing disorders
- Cushing’s syndrome
- hyperaldosteronism, a condition that affects the adrenal glands
- respiratory tract infection
As with any simple blood draw, people may experience minor side effects from the CO2 test. The risks are minor and may include:
- lightheadedness
- dizziness or feeling faint
- bleeding at the site of the needle prick
- formation of a hematoma, or a lump of blood under the skin’s surface
If these symptoms persist for longer than a day, inform a doctor.
CO2 is a naturally occurring gas in the body. Levels of CO2 may increase or decrease due to infections, medicines, or medical conditions.
Undergoing a CO2 blood test may shed light on any underlying medical conditions that are present, which the doctor will be able to diagnose and treat.