CO2 blood test
Total carbon dioxide (bicarbonate)
In short
A typical adult CO2 result is 22 to 29 mmol/L. A mildly high CO2 most often follows vomiting or water pills. Doctors read it with chloride and potassium. Always read your result against the range printed on your own report.
Mostly bicarbonate, which keeps your blood from becoming too acidic.
On most reports, CO2 is mostly bicarbonate, which neutralizes acid in the blood. Your kidneys and lungs work together to keep it steady. A result outside the range can show that the body's acid balance has shifted.
Is my CO2 high or low?
Type your result to see where it sits.
Nothing you type leaves this device.
Typical range
| For | Range | Unit |
|---|---|---|
| Adults | 22–29 | mmol/L |
Typical ranges for adults. Labs differ a little, so the range printed next to your result is the one to go by.
If your CO2 is high
A mildly high CO2 most often follows vomiting or water pills. Doctors read it with chloride and potassium.
Common reasons
- Water pills (diuretics)
- Vomiting, which loses stomach acid
- Not drinking enough
- Taking a lot of antacids
- Less often: long-term lung disease that holds in carbon dioxide
- Less often: a hormone problem that makes the body hold salt
If your CO2 is low
A mildly low CO2 is common, for example after diarrhea. Doctors check the anion gap to see what is behind it.
Common reasons
- Diarrhea, which drains bicarbonate
- Fasting or a very low-carb diet, which makes acids called ketones
- Hard exercise shortly before the test
- Kidneys that do not clear acid well
- Less often: acid buildup from uncontrolled diabetes or a serious illness
What can throw it off
A tube that is not filled all the way, or waits long before testing, can read falsely low.
Questions to take to your appointment
- 1Is this far enough off to need a repeat test?
- 2What does my anion gap show?
- 3Could my kidneys or medicines explain it?
Read together with
Doctors rarely read CO2 alone. These usually sit next to it.
Quick answers
What is the CO2 blood test?
CO2 (Total carbon dioxide (bicarbonate)): Mostly bicarbonate, which keeps your blood from becoming too acidic. On most reports, CO2 is mostly bicarbonate, which neutralizes acid in the blood. Your kidneys and lungs work together to keep it steady. A result outside the range can show that the body's acid balance has shifted.
What is a normal CO2 level?
For most adults, CO2 is typically 22 to 29 mmol/L. Labs differ a little, so the range printed next to your result is the one to go by.
What does a high CO2 mean?
A mildly high CO2 most often follows vomiting or water pills. Doctors read it with chloride and potassium. Common reasons include water pills (diuretics); vomiting, which loses stomach acid; not drinking enough; and taking a lot of antacids. A doctor reads it together with your other results and how you feel.
What does a low CO2 mean?
A mildly low CO2 is common, for example after diarrhea. Doctors check the anion gap to see what is behind it. Common reasons include diarrhea, which drains bicarbonate; fasting or a very low-carb diet, which makes acids called ketones; hard exercise shortly before the test; and kidneys that do not clear acid well.
What can affect my CO2 result?
A tube that is not filled all the way, or waits long before testing, can read falsely low.
What should I ask my doctor about my CO2?
Questions worth taking to the appointment: Is this far enough off to need a repeat test? What does my anion gap show? Could my kidneys or medicines explain it?
What this page can’t do
It reads each number on its own.
Doctors read results together — ALT with AST, ferritin with hemoglobin, TSH with free T4. A pattern says more than any single flag.
It doesn't know you.
Your age, your medicines, how you feel and your last result all change what a number means.
It doesn't remember.
Close this page and your results are gone. That's on purpose: nothing was ever sent to us.
We also make Vita: Health Coach, an app for those parts. It reads your whole report with your medicines and history in mind, answers follow-up questions in plain words, and keeps your reports in one place — for you and for the people you look after.
Vita: Health Coach on the App StoreSources
Stuck on a word? See the medical words. How we choose our sources. More about Vita Health Coach.