Carbon Dioxide (CO2): Normal 23-29, High >30, Low <22 & What It Means

Key Takeaways

• Carbon dioxide (CO2) in blood tests measures bicarbonate, which reflects your body's acid-base balance and breathing function

• Normal CO2 range: 23-29 mmol/L (may vary slightly between laboratories)

• High CO2 levels (>30 mmol/L) most commonly occur with chronic lung disease, sleep apnea, or breathing problems

• Low CO2 levels (<22 mmol/L) may indicate hyperventilation, kidney disease, or metabolic acidosis

• Doctors interpret CO2 results together with other electrolytes and kidney function markers from your metabolic panel

• Most abnormal CO2 results are temporary and resolve once the underlying cause is addressed

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What Is the CO2 Blood Test?

The carbon dioxide (CO2) blood test measures the amount of carbon dioxide in your blood, specifically in the form of bicarbonate (HCO₃⁻). This test is part of a comprehensive metabolic panel (CMP) or basic metabolic panel (BMP).

Carbon dioxide plays an essential role in maintaining your body's acid-base balance. When you breathe, your lungs remove excess CO2 from your blood. Your kidneys also help regulate CO2 levels by controlling bicarbonate.

The CO2 blood test is also referred to as:

• Total CO2

• Carbon dioxide, total

• CO2 content

• Bicarbonate (though technically measuring total CO2)

• TCO2

The test helps doctors evaluate lung function, kidney function, and your body's acid-base balance.

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CO2 Blood Test: Quick Interpretation Guide

If your blood test shows CO2, carbon dioxide total, or bicarbonate, the result represents the level of carbon dioxide in your blood.

Most laboratories report the normal adult range as:

23 – 29 mmol/L
(millimoles per liter)

Some labs may use slightly different ranges (such as 20-29 or 22-30 mmol/L), so always compare your result to your lab's specific reference range.

Interpretation Guide

CO2 Level (mmol/L)	Possible Interpretation
Below 22	Low CO2 (possible metabolic acidosis, hyperventilation, kidney disease)
23-29	Normal CO2 range
30-34	Mildly elevated (often seen with COPD, sleep apnea, or breathing issues)
Above 35	High CO2 (may require evaluation if persistent)

Mild changes in CO2 levels are common. Doctors interpret results together with:

• Other electrolytes (sodium, potassium, chloride)

• Kidney function markers (creatinine, BUN)

• Symptoms

• Breathing patterns

• Changes over time

A single abnormal CO2 result does not necessarily indicate a serious condition.

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What Does CO2 Measure in a Blood Test?

The CO2 blood test measures total carbon dioxide, which includes:

• Bicarbonate (HCO₃⁻) – makes up about 95% of total CO2

• Dissolved CO2 gas

• Carbonic acid

Since bicarbonate makes up the vast majority of total CO2, the terms are often used interchangeably in clinical practice.

What CO2 tells your doctor:

• How well your lungs are removing carbon dioxide

• Whether your kidneys are properly regulating acid-base balance

• If you have metabolic acidosis or alkalosis

• Your body's buffering capacity

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Normal Range for CO2 Blood Test

For most adults, the normal reference range is:

23 – 29 mmol/L

However, normal ranges can vary slightly between laboratories. Some common variations include:

• 20-29 mmol/L

• 22-30 mmol/L

• 23-31 mmol/L

Always compare your result to the reference range listed on your specific lab report.

Result	Meaning
Below 22-23 mmol/L	Low CO2 (hypobicarbonatemia)
23-29 mmol/L	Normal range
Above 29-30 mmol/L	Elevated CO2 (hyperbicarbonatemia)

Children may have slightly different reference ranges, particularly infants.

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What Do Specific CO2 Numbers Mean?

Many people search for the meaning of a specific CO2 value from their blood test. Below are interpretations for common CO2 levels.

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Is CO2 Level 16 Low?

Yes, significantly low. A CO2 level of 16 mmol/L is well below the normal range and indicates moderate to severe metabolic acidosis.

Common causes:

• Diabetic ketoacidosis (DKA)

• Severe diarrhea

• Kidney failure

• Lactic acidosis

• Severe dehydration

Should you worry?

• Yes, this requires medical attention

• CO2 levels this low indicate significant acid-base disturbance

• Often accompanied by rapid breathing (compensatory hyperventilation)

When to seek immediate care:

• Rapid or difficult breathing

• Confusion or altered mental state

• Severe nausea or vomiting

• If you have diabetes and feel unwell

Next steps:

• Contact your doctor immediately

• May require arterial blood gas (ABG) testing

• Treatment depends on underlying cause

• May need IV fluids and electrolyte correction

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Is CO2 Level 17 Low?

Yes, significantly low. A CO2 level of 17 mmol/L is below normal and indicates moderate metabolic acidosis.

Common causes:

• Diabetic ketoacidosis

• Chronic kidney disease

• Severe diarrhea or GI losses

• Certain medications (metformin, aspirin overdose)

• Lactic acidosis

Should you worry?

• Yes, this warrants prompt evaluation

• Indicates your body's acid-base balance is disrupted

• May be compensating with faster breathing

When to see a doctor:

• Within 24-48 hours if you feel well

• Immediately if you have diabetes, breathing changes, or feel unwell

What your doctor may do:

• Order arterial blood gas (ABG) test

• Check kidney function

• Evaluate for diabetic ketoacidosis if you have diabetes

• Test urine for ketones

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Is CO2 Level 18 Low?

Yes, low. A CO2 level of 18 mmol/L is below the normal range and indicates mild to moderate metabolic acidosis.

Common causes:

• Hyperventilation (anxiety, pain, or respiratory compensation)

• Early kidney disease

• Diabetic ketoacidosis (mild)

• Diarrhea or GI fluid loss

• Certain medications

Should you worry?

• Usually not immediately dangerous if you feel well

• May be temporary, especially if caused by hyperventilation

• Requires evaluation if persistent

When to follow up:

• Schedule appointment with your doctor within 1-2 weeks

• Sooner if you have diabetes, kidney disease, or symptoms

• May need repeat testing to confirm

Next steps:

• Repeat metabolic panel in 1-2 weeks

• May need arterial blood gas (ABG) if persistent

• Review medications and symptoms with doctor

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Is CO2 Level 19 Low?

Yes, low. A CO2 level of 19 mmol/L is below the normal range (23-29) and indicates mild metabolic acidosis or compensated respiratory alkalosis.

Common causes:

• Hyperventilation (anxiety, fever, pain)

• Metabolic acidosis (diabetic ketoacidosis, kidney disease)

• Chronic diarrhea

• Early kidney disease

• Certain medications (diuretics, metformin)

Should you worry?

• Usually not immediately dangerous if you feel well

• Often temporary, especially with hyperventilation

• May resolve within 24-48 hours for minor causes

When to follow up:

• If persistent on repeat testing in 1-2 weeks

• If accompanied by rapid breathing, confusion, or fatigue

• If you have diabetes, kidney disease, or chronic illness

Next steps:

• Review symptoms and medications with your doctor

• Repeat metabolic panel in 1-2 weeks

• May need arterial blood gas (ABG) test if symptoms present

• Check kidney function if not recently tested

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Is CO2 Level 20 Low?

Yes, borderline low. A CO2 level of 20 mmol/L is slightly below the typical normal range and may indicate mild metabolic acidosis.

Common causes:

• Hyperventilation

• Mild kidney dysfunction

• Diarrhea or GI losses

• Dehydration

• Certain medications

Should you worry?

• Usually not concerning if you feel well

• Often temporary

• May be within normal range for some laboratories (20-29 mmol/L)

When to follow up:

• If accompanied by symptoms (rapid breathing, fatigue, nausea)

• If you have diabetes or kidney disease

• If persistent on repeat testing

Next steps:

• Repeat testing in 2-4 weeks if asymptomatic

• Sooner if symptoms develop

• Review medication list with doctor

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Is CO2 Level 21 Low?

Borderline low. A CO2 level of 21 mmol/L is at the lower edge of the normal range for most labs.

Common causes:

• May be normal variation

• Mild hyperventilation

• Early metabolic acidosis

• Dehydration

• High-protein diet

Should you worry?

• No, usually not concerning

• Often within normal limits

• May represent normal variation

When to follow up:

• Only if accompanied by symptoms

• If you have underlying kidney or lung disease

• If levels drop further on repeat testing

Next steps:

• Routine follow-up sufficient for most people

• No immediate action needed if asymptomatic

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Is CO2 Level 22 Low?

Borderline normal. A CO2 level of 22 mmol/L is at the lower limit of normal for most laboratories.

Common interpretation:

• Usually considered normal

• Some labs classify as slightly low (if range is 23-29)

• Often no clinical significance

Common causes:

• Normal variation

• Mild hyperventilation

• Well-controlled breathing pattern

Should you worry?

• No, typically not concerning

• Within or very close to normal range

When to follow up:

• Only if symptomatic

• If levels continue to decrease

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Is CO2 Level 30 High?

Yes, mildly elevated. A CO2 level of 30 mmol/L is slightly above the typical normal range (23-29).

Common causes:

• Chronic obstructive pulmonary disease (COPD) – most common

• Sleep apnea

• Chronic respiratory conditions

• Obesity hypoventilation syndrome

• Medications (diuretics causing chloride loss)

• Vomiting or gastric suctioning

Should you worry?

• Usually not immediately dangerous

• Often chronic and stable in people with lung disease

• May warrant evaluation if new or accompanied by symptoms

When to follow up:

• If you have new shortness of breath or breathing changes

• If accompanied by confusion, headaches, or drowsiness

• If you don't have a known lung condition

Next steps:

• Your doctor may order pulmonary function tests

• May check arterial blood gas (ABG) to assess breathing

• Evaluate for sleep apnea if appropriate

• Monitor if you have known COPD

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Is CO2 Level 31 High?

Yes, elevated. A CO2 level of 31 mmol/L is above normal and indicates mild respiratory acidosis or metabolic alkalosis.

Common causes:

• COPD or chronic lung disease

• Sleep apnea

• Chronic bronchitis

• Long-term diuretic use

• Compensation for chronic respiratory issues

Should you worry?

• Not usually an emergency if you feel well

• Common in people with chronic lung conditions

• May be stable and well-compensated

When to see a doctor:

• If this is a new finding

• If accompanied by increased shortness of breath

• If you experience morning headaches, confusion, or excessive daytime sleepiness

What your doctor may do:

• Evaluate lung function with spirometry

• Check arterial blood gas (ABG)

• Screen for sleep apnea

• Assess oxygen levels

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Is CO2 Level 32 High?

Yes, elevated. A CO2 level of 32 mmol/L is clearly above the normal range.

Common causes:

• Chronic obstructive pulmonary disease (COPD) – most common

• Obstructive sleep apnea

• Chronic respiratory failure

• Obesity hypoventilation syndrome

• Severe asthma

• Prolonged vomiting

Should you worry?

• May require evaluation, especially if new

• Often stable in people with chronic lung disease

• Can indicate inadequate breathing or gas exchange

When to seek evaluation:

• If you don't have a known lung condition

• If accompanied by drowsiness, confusion, or morning headaches

• If oxygen levels are low

• If breathing has worsened recently

Next steps:

• Pulmonary function testing

• Arterial blood gas (ABG) test

• Sleep study if sleep apnea suspected

• May need oxygen therapy or breathing support

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Is CO2 Level 33 High?

Yes, significantly elevated. A CO2 level of 33 mmol/L is well above normal and indicates respiratory acidosis or significant metabolic alkalosis.

Common causes:

• Advanced COPD

• Severe sleep apnea

• Chronic respiratory failure

• Neuromuscular disorders affecting breathing

• Severe obesity hypoventilation

Should you worry?

• Yes, this warrants medical evaluation

• Indicates significant impairment in CO2 elimination

• May require treatment or oxygen therapy

When to seek prompt care:

• If accompanied by confusion, severe drowsiness, or difficulty staying awake

• If you have new or worsening shortness of breath

• If oxygen saturation is low

What your doctor may recommend:

• Arterial blood gas (ABG) testing

• Oxygen therapy assessment

• Pulmonary rehabilitation

• CPAP or BiPAP for sleep apnea

• Possible hospitalization if acute

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Is CO2 Level 34 High?

Yes, significantly elevated. A CO2 level of 34 mmol/L is well above normal and indicates substantial respiratory acidosis.

Common causes:

• Severe COPD

• Acute respiratory failure

• Severe sleep apnea

• Neuromuscular disease

• Drug overdose (sedatives, opioids)

Should you worry?

• Yes, requires prompt medical attention

• Indicates your body is retaining too much CO2

• May indicate respiratory failure

When to seek immediate care:

• Confusion or altered mental status

• Severe drowsiness or difficulty staying awake

• Rapid or labored breathing

• Bluish skin or lips (cyanosis)

Recommended evaluation:

• Emergency room visit if symptomatic

• Arterial blood gas (ABG) testing

• Chest X-ray

• Oxygen therapy may be needed

• Possible hospitalization

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Is CO2 Level 35 High?

Yes, significantly elevated. A CO2 level of 35 mmol/L is well above normal and indicates severe respiratory acidosis.

Common causes:

• Severe COPD exacerbation

• Acute-on-chronic respiratory failure

• Severe untreated sleep apnea

• Neuromuscular respiratory failure

• Central nervous system depression

Should you worry?

• Yes, this is concerning and requires immediate evaluation

• Indicates severe impairment in breathing or gas exchange

• May be life-threatening if acute

When to seek emergency care:

• Any confusion, disorientation, or altered consciousness

• Severe shortness of breath

• Inability to complete sentences

• Extreme fatigue or inability to stay awake

Immediate next steps:

• Seek emergency medical care

• Arterial blood gas testing

• Oxygen therapy

• Possible mechanical ventilation support

• Hospitalization likely required

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Is CO2 Level 36 or Higher Dangerous?

Yes, very concerning. CO2 levels of 36 mmol/L or higher indicate severe respiratory acidosis and potential respiratory failure.

Possible causes:

• Severe COPD exacerbation

• Acute respiratory distress syndrome (ARDS)

• Drug overdose (opioids, benzodiazepines)

• Severe neuromuscular disease

• Brain injury affecting breathing

Immediate danger:

• This is a medical emergency

• Risk of CO2 narcosis (altered consciousness from high CO2)

• May progress to respiratory arrest

• Can be life-threatening

Seek emergency care immediately if:

• Any level of confusion or drowsiness

• Difficulty breathing

• Bluish skin tone

• Severe fatigue

Emergency treatment typically includes:

• Oxygen therapy

• Possible non-invasive ventilation (BiPAP)

• May require intubation and mechanical ventilation

• ICU admission often necessary

• Treatment of underlying cause

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What Does CO2 Mean in a Blood Test?

CO2 in a blood test reflects your body's acid-base balance and is influenced by both your lungs and kidneys.

The CO2 test helps doctors:

• Evaluate lung function and breathing efficiency

• Assess kidney function and acid-base regulation

• Diagnose metabolic acidosis or alkalosis

• Monitor chronic lung or kidney conditions

• Guide treatment for breathing disorders

How CO2 is regulated:

Lungs: Remove excess CO2 through breathing. When you breathe faster (hyperventilation), CO2 levels drop. When you breathe slower or less efficiently, CO2 levels rise.

Kidneys: Control bicarbonate levels to maintain pH balance. Healthy kidneys can compensate for some breathing problems by adjusting bicarbonate.

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What Is the Difference Between CO2 and Bicarbonate?

The CO2 blood test measures total CO2, which is primarily bicarbonate (HCO₃⁻).

Measurement	What It Includes	Typical Use
CO2 (Total CO2)	Bicarbonate (~95%) + dissolved CO2 gas + carbonic acid	Standard on metabolic panels
Bicarbonate	Only the bicarbonate ion (HCO₃⁻)	Arterial blood gas tests

In clinical practice, the terms are often used interchangeably because bicarbonate makes up the vast majority of total CO2.

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What Causes High CO2 Levels in Blood?

High CO2 levels occur when your body retains too much carbon dioxide or when your kidneys compensate for chronic breathing problems.

Common Causes of High CO2

Chronic lung diseases (most common)

• Chronic obstructive pulmonary disease (COPD)

• Chronic bronchitis

• Emphysema

• Severe asthma

• Cystic fibrosis

Sleep-related breathing disorders

• Obstructive sleep apnea

• Central sleep apnea

• Obesity hypoventilation syndrome

Medications and substances

• Long-term diuretic use (causes chloride loss)

• Sedatives or opioids (suppress breathing)

• Excessive antacid use

Other respiratory conditions

• Neuromuscular disorders affecting breathing muscles

• Severe obesity

• Chest wall abnormalities

Metabolic causes

• Prolonged vomiting (loss of stomach acid)

• Nasogastric suctioning

• Severe dehydration

• Cushing's syndrome

In many cases, high CO2 is chronic and stable in people with known lung disease.

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What Causes Low CO2 Levels in Blood?

Low CO2 levels occur when your body eliminates too much carbon dioxide through breathing or when metabolic acidosis develops.

Common Causes of Low CO2

Hyperventilation

• Anxiety or panic attacks

• Pain

• Fever

• High altitude

• Pregnancy

Metabolic acidosis

• Diabetic ketoacidosis (DKA)

• Lactic acidosis

• Kidney failure

• Severe diarrhea

• Starvation

Kidney disease

• Chronic kidney disease

• Renal tubular acidosis

• Kidney failure

Medications

• Metformin

• Aspirin overdose

• Some diuretics

• Topiramate

Other causes

• Severe dehydration

• Shock or sepsis

• Liver disease

• Methanol or ethylene glycol poisoning

Many cases of mild low CO2 are temporary and related to hyperventilation.

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What CO2 Level Is Dangerous?

Most abnormal CO2 levels are not immediately dangerous, but certain thresholds require urgent evaluation.

Low CO2 Danger Thresholds

CO2 Level	Risk Level	Action Needed
20-22 mmol/L	Mild, usually not dangerous	Monitor, follow up with doctor
16-19 mmol/L	Moderate acidosis	Evaluation within 24-48 hours
12-15 mmol/L	Severe acidosis	Prompt medical evaluation needed
Below 12 mmol/L	Critical, life-threatening	Emergency care required

High CO2 Danger Thresholds

CO2 Level	Risk Level	Action Needed
30-32 mmol/L	Mildly elevated	Evaluation if new or symptomatic
33-35 mmol/L	Moderate elevation	Medical evaluation recommended
36-40 mmol/L	Severe, concerning	Urgent evaluation needed
Above 40 mmol/L	Critical, risk of CO2 narcosis	Emergency care required

Warning Symptoms Requiring Immediate Care

Low CO2 warning signs:

• Rapid, deep breathing

• Confusion or altered mental state

• Severe nausea or vomiting

• If you have diabetes and feel unwell

• Seizures

High CO2 warning signs:

• Confusion, disorientation, or drowsiness

• Severe headache (especially morning headaches)

• Difficulty staying awake

• Shortness of breath or rapid breathing

• Bluish skin or lips (cyanosis)

If you experience any of these symptoms with an abnormal CO2 level, seek medical attention immediately.

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Can Dehydration Cause High CO2 Levels?

Yes, but indirectly. Severe dehydration can cause high CO2 through a process called contraction alkalosis.

How dehydration affects CO2:

• Severe fluid loss concentrates bicarbonate in the blood

• Loss of stomach acid from vomiting raises bicarbonate

• Kidney compensation for volume loss may increase bicarbonate retention

When dehydration causes high CO2:

• Prolonged vomiting

• Severe diarrhea combined with vomiting

• Excessive diuretic use

• Heat exhaustion with inadequate fluid replacement

Typical pattern:

• Mild dehydration: usually no significant effect

• Moderate to severe dehydration: may raise CO2 by 2-5 mmol/L

• Usually resolves with rehydration

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Can Dehydration Cause Low CO2 Levels?

Yes. Dehydration, particularly from diarrhea, can cause low CO2 levels through bicarbonate loss.

How dehydration causes low CO2:

• Diarrhea causes direct loss of bicarbonate

• Metabolic acidosis develops from bicarbonate depletion

• Kidneys may be unable to compensate adequately

When dehydration causes low CO2:

• Severe or prolonged diarrhea

• Cholera or other severe GI infections

• Certain types of kidney disease with dehydration

Typical pattern:

• Diarrhea-related dehydration: lowers CO2

• Vomiting-related dehydration: may raise CO2

• Mixed (both): effect depends on which is predominant

Rehydration with appropriate electrolyte replacement typically normalizes CO2 levels within 24-48 hours.

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What Should You Do If Your CO2 Level Is Abnormal?

Mildly High CO2 (30-32 mmol/L)

Typical approach:

• Review symptoms and breathing patterns

• Check for known lung disease

• Evaluate sleep quality (screen for sleep apnea)

• Repeat testing in 2-4 weeks

• May order pulmonary function tests if new

High CO2 (33-35 mmol/L)

Recommended evaluation:

• Pulmonary function testing (spirometry)

• Arterial blood gas (ABG) test

• Sleep study if sleep apnea suspected

• Chest X-ray

• Oxygen saturation monitoring

• Possible referral to pulmonologist

Severely High CO2 (>35 mmol/L)

Urgent evaluation needed:

• Arterial blood gas (ABG)

• Chest imaging

• Oxygen therapy assessment

• Possible hospitalization

• Treatment of underlying condition

Mildly Low CO2 (20-22 mmol/L)

Typical approach:

• Assess for hyperventilation

• Review medications

• Repeat testing in 1-2 weeks

• Check kidney function if not recent

Low CO2 (18-19 mmol/L)

Recommended evaluation:

• Arterial blood gas (ABG) if symptomatic

• Comprehensive metabolic panel

• Kidney function assessment

• Screen for diabetes complications if diabetic

• Evaluate for metabolic acidosis

Severely Low CO2 (<18 mmol/L)

Prompt medical care:

• Immediate evaluation if symptomatic

• Arterial blood gas (ABG) testing

• Kidney function tests

• Blood sugar and ketone testing if diabetic

• May require hospitalization

Persistent abnormalities may require referral to a nephrologist (kidney specialist) or pulmonologist (lung specialist).

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How Long Does It Take for CO2 to Return to Normal?

The timeline depends on the underlying cause:

Hyperventilation-related low CO2: 30 minutes to 2 hours after calming breathing

Dehydration-related changes: 24-48 hours with adequate rehydration

Diarrhea-related low CO2: 2-5 days after diarrhea resolves

Vomiting-related high CO2: 2-4 days after vomiting stops

Medication-related changes: 1-2 weeks after stopping or adjusting medication

Diabetic ketoacidosis: 12-24 hours with appropriate treatment

Chronic lung disease: May remain elevated long-term but stabilizes with treatment

Chronic kidney disease: May remain abnormal; requires ongoing management

Timeline varies based on the underlying cause and individual health factors.

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When Is a CO2 Test Ordered?

The CO2 blood test is usually measured as part of:

Comprehensive Metabolic Panel (CMP) – includes:

• Electrolytes (sodium, potassium, chloride, CO2)

• Kidney function (creatinine, BUN)

• Liver function (ALT, AST, bilirubin)

• Blood sugar (glucose)

• Protein levels (albumin, total protein)

Basic Metabolic Panel (BMP) – includes:

• Electrolytes (sodium, potassium, chloride, CO2)

• Kidney function (creatinine, BUN)

• Blood sugar (glucose)

Doctors may order these tests to:

• Evaluate symptoms like shortness of breath, confusion, or fatigue

• Monitor chronic lung or kidney disease

• Assess acid-base balance

• Check electrolyte levels

• Evaluate response to treatment

• Screen for metabolic disorders

• Before surgery or as part of routine health screening

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Key Takeaway

The CO2 blood test measures bicarbonate levels, which reflect your body's acid-base balance and the function of your lungs and kidneys.

Most abnormal CO2 results are caused by breathing disorders, kidney disease, dehydration, or metabolic conditions. Doctors interpret CO2 levels together with other electrolytes, symptoms, and changes over time to determine whether further evaluation is needed.

Normal range: 23-29 mmol/L
Mildly low: 20-22 mmol/L (often temporary)
Significantly low: <20 mmol/L (requires evaluation)
Mildly high: 30-32 mmol/L (common in lung disease)
Significantly high: >33 mmol/L (requires evaluation)

If your CO2 level is abnormal, discuss results with your healthcare provider for personalized interpretation based on your symptoms and medical history.