The short answer: The CO2 value on a comprehensive metabolic panel (CMP) is mainly a measure of the amount of bicarbonate in your blood, not a measurement of the carbon dioxide you breathe out. Bicarbonate is an important base that helps keep your blood from becoming too acidic.
A low CO2/bicarbonate level can occur with metabolic acidosis, but it does not automatically mean kidney disease. A high level can have several causes as well. The result needs to be interpreted alongside your kidney function, electrolytes, medications, health conditions, and previous results. (NKF)
What Does CO2 Mean on a CMP?
On a CMP, CO2 usually represents total carbon dioxide (total CO2) in the blood. Most of this is present as bicarbonate (HCO₃⁻).
This can be confusing because the test is called “CO2,” but it is primarily being used as a convenient estimate of the blood’s bicarbonate level.
Bicarbonate helps maintain the body’s acid-base balance. The kidneys and lungs work together to keep this balance within a healthy range. (NKF)
So, when you see:
CO2: 24 mmol/L
on a CMP, it is generally reasonable to think of this as a measurement closely related to your serum bicarbonate level.
What Is a Normal CO2 or Bicarbonate Level?
A commonly used adult reference range for serum bicarbonate is approximately 22–29 mEq/L (mmol/L), although the exact range can vary between laboratories. (NKF)
Therefore:
- Low CO2/bicarbonate: may indicate too much acid in the body or another acid-base disturbance.
- Normal CO2/bicarbonate: generally suggests that this part of acid-base balance is within the laboratory’s reference range.
- High CO2/bicarbonate: can occur for several reasons, including conditions associated with increased bicarbonate or compensation for certain breathing-related problems.
The laboratory’s own reference range should be used when interpreting your report.
What Does a Low CO2 Level Mean?
A low CO2 on a CMP often means that serum bicarbonate is low.
One important possibility is metabolic acidosis, a condition in which the body has too much acid relative to its available base.
This can happen when:
- The kidneys cannot remove enough acid
- The body produces excessive acid
- Bicarbonate is lost from the body
- Certain illnesses or medications affect acid-base balance
In CKD, metabolic acidosis becomes more common as kidney function declines because damaged kidneys may have more difficulty removing acid and maintaining bicarbonate levels. ([KDIGO][3])
However, a low CO2 result does not automatically prove metabolic acidosis.
For example, low bicarbonate can also occur as part of compensation for certain respiratory conditions. When the cause is unclear, a clinician may consider additional testing, including a blood gas.
What Does a High CO2 Level Mean?
A high CO2/bicarbonate result can also have several explanations.
It may occur with conditions associated with metabolic alkalosis, such as significant vomiting or certain diuretic effects.
It can also occur when the body is compensating for chronic respiratory acidosis, in which carbon dioxide has been retained because of an underlying breathing problem.
Therefore, a high CO2 level does not automatically mean that the kidneys are working poorly.
The rest of the CMP, medical history, medications, and sometimes blood-gas measurements help determine what is happening.
How Are the Kidneys Involved?
Your kidneys play a major role in maintaining acid-base balance.
They help:
- Remove acid from the body
- Reabsorb bicarbonate
- Produce new bicarbonate when needed
- Regulate the amount of acid and base eliminated in urine
As eGFR falls, serum bicarbonate tends to decrease, particularly in more advanced CKD. KDIGO notes that reductions become more apparent in CKD stages G4 and G5. ([KDIGO][3])
This is one reason bicarbonate/CO2 is useful when evaluating people with kidney disease.
But a low bicarbonate level can occur for reasons unrelated to CKD, so it should not be used by itself to diagnose kidney disease.
Does Low CO2 Mean Kidney Failure?
No.
A low CO2/bicarbonate level does not by itself diagnose CKD or kidney failure.
Your clinician may also look at:
- Creatinine
- eGFR
- Potassium
- Sodium
- Chloride
- Glucose
- Anion gap
- Urine tests
- Previous laboratory results
- Symptoms and recent illness
For example, a person can have normal kidney function but develop a low bicarbonate level from another cause of metabolic acidosis.
Likewise, someone with CKD can have a normal bicarbonate level.
One abnormal result is not the same as one diagnosis.
Why the Anion Gap Can Matter
If bicarbonate is low, clinicians often consider the anion gap.
The anion gap is calculated from electrolyte measurements and can help determine whether certain unmeasured acids may be accumulating in the blood.
A high anion gap can occur in conditions such as:
- Lactic acidosis
- Ketoacidosis
- Advanced kidney dysfunction
- Certain poisonings or medication-related conditions
A normal-anion-gap metabolic acidosis has a different group of possible causes, including bicarbonate loss and some kidney-related disorders.
The anion gap therefore provides context, rather than a diagnosis by itself.
CO2, Bicarbonate, and CKD
Low bicarbonate is particularly relevant in people with CKD because persistent metabolic acidosis can be associated with complications such as muscle loss and bone problems. Low bicarbonate has also been associated with worse kidney outcomes, although associations do not automatically prove that low bicarbonate itself causes CKD progression. ([KDIGO][3])
KDIGO 2024 advises considering treatment for clinically important metabolic acidosis in CKD while avoiding treatment that pushes serum bicarbonate above the normal range or worsens blood pressure, potassium, or fluid status. ([KDIGO][3])
Importantly, this does not mean that everyone with a mildly low CO2 should start baking soda or another bicarbonate product.
Should You Take Baking Soda for Low CO2?
Do not self-treat a low CO2 result with baking soda.
Sodium bicarbonate can increase sodium intake and may affect blood pressure or fluid balance. Treatment decisions depend on the cause of the low bicarbonate, the severity and persistence of the abnormality, kidney function, and other medical conditions. (NKF)
A clinician may first confirm the abnormal result and determine whether true metabolic acidosis is present.
What Should You Do With an Abnormal CO2 Result?
Start by looking at the result in context.
A useful approach is:
Previous CO2 → current CO2 → size of change → time interval → kidney function → other electrolytes → medical context
Ask your healthcare professional whether the result needs to be repeated or investigated further, particularly if it is significantly outside the laboratory’s range or has changed substantially from previous tests.
If you have CKD, the result can be considered alongside your eGFR, potassium, blood pressure, medications, and other kidney-related tests.
When Does a Low CO2 Need Urgent Attention?
A mildly abnormal result without symptoms is not automatically an emergency.
However, urgent medical assessment may be appropriate when a markedly abnormal CO2/bicarbonate level occurs with symptoms such as:
- Severe or unusual breathing changes
- Severe weakness
- Confusion
- Persistent vomiting
- Significant dehydration
- Chest symptoms
- Severe illness or rapidly worsening kidney function
The urgency depends on the underlying cause and the overall clinical picture, not the CO2 number alone.
Bottom Line
The CO2 value on a CMP is primarily a convenient measure related to serum bicarbonate, which is an important part of the body’s acid-base system.
A low CO2 can point toward metabolic acidosis, including acidosis associated with more advanced CKD, but it does not prove kidney disease. A high CO2 also has multiple possible explanations.
The most useful interpretation comes from looking at the CO2 trend together with creatinine, eGFR, electrolytes, medications, symptoms, and the reason the test was ordered.
If your CO2 is abnormal, the sensible question is not simply “Is my bicarbonate high or low?” but:
“Why is it abnormal, is the finding persistent, and what does it mean alongside my other results?”