Carbon Dioxide (CO₂)
Other names: Carbon Dioxide, Total, Bicarbonate (HCO₃⁻), Total CO₂, CO₂ (Serum), CO₂, Carbon Dioxide (Blood), TCO₂, Serum Bicarbonate, Plasma CO₂
What Is Carbon Dioxide (CO₂) on a Blood Test?
Carbon dioxide (CO₂) is a metabolic waste product produced when the body converts nutrients into energy. In the blood, CO₂ is transported primarily as bicarbonate (HCO₃⁻), which plays a central role in maintaining blood pH within a narrow, healthy range.
Rather than measuring gaseous CO₂ directly, a standard blood test reflects total bicarbonate concentration, making CO₂ an important marker of acid–base balance and electrolyte regulation.
What Does a CO₂ Blood Test Measure?
A CO₂ blood test—also called a bicarbonate or total CO₂ test—measures the amount of bicarbonate in the blood. This helps evaluate whether the body is trending toward acidosis (too acidic) or alkalosis (too alkaline) and whether the lungs or kidneys are contributing to the imbalance.
Common alternate names include:
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Bicarbonate (HCO₃⁻)
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Total CO₂ (TCO₂)
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CO₂ content
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CO₂ serum test
Why CO₂ Levels Matter
Bicarbonate acts as a buffering system that stabilizes blood pH. Abnormal levels may signal:
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Kidney dysfunction (impaired acid removal)
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Respiratory disorders (altered CO₂ exhalation)
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Dehydration or fluid imbalance
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Metabolic disturbances
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Acid–base disorders (acidosis or alkalosis)
For this reason, CO₂ is routinely assessed as part of electrolyte and metabolic panels.
When Is CO₂ Testing Used?
CO₂ testing may be ordered when symptoms suggest acid–base imbalance, including:
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Shortness of breath
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Nausea or vomiting
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Fatigue
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Confusion
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Rapid or shallow breathing
It is also commonly monitored in individuals with kidney disease, lung disease, diabetes, dehydration, or critical illness.
CO₂ vs. Bicarbonate: Are They the Same?
Clinically, yes:
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Bicarbonate (HCO₃⁻) is the primary form of CO₂ in the blood.
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The CO₂ test reflects total bicarbonate concentration.
This is why the terms are often used interchangeably in lab reports.
Normal CO₂ (Bicarbonate) Levels
Typical adult reference range:
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20–29 mEq/L
Ranges may vary slightly by laboratory. Results outside this range should be interpreted in context and do not automatically indicate disease.
Additional Testing: Arterial Blood Gases (ABG)
If more precise evaluation is needed, an arterial blood gas (ABG) test may be ordered. ABGs directly measure:
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Blood pH
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Partial pressure of CO₂ (PaCO₂)
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Oxygen levels
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Bicarbonate
ABGs are especially useful in respiratory and critical care settings.
Conditions Associated With Abnormal CO₂ Levels
Abnormal bicarbonate levels may be linked to:
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Metabolic acidosis or alkalosis
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Chronic kidney disease
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Liver dysfunction
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Prolonged vomiting or diarrhea
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COPD or other chronic lung diseases
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Diabetic ketoacidosis
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Severe infections (sepsis)
Key Takeaway
The CO₂ (bicarbonate) blood test is a core marker of acid–base balance and metabolic health. While abnormal results can indicate serious conditions, many cases are temporary or reversible. Interpretation alongside other electrolytes, kidney markers, respiratory status, and symptoms is essential for accurate diagnosis and management.
What does it mean if your Carbon Dioxide (CO₂) result is too high?
Elevated CO₂ levels reflect excess bicarbonate or impaired CO₂ elimination, most often due to metabolic alkalosis or chronic respiratory retention.
Key contributors include:
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Dehydration
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Chronic lung disease (e.g., COPD)
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Prolonged vomiting
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Diuretic overuse
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Kidney compensation for respiratory disorders
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Endocrine conditions such as Cushing’s syndrome
High CO₂ should always be interpreted with chloride, potassium, blood pH, and clinical context before drawing conclusions.
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What does it mean if your Carbon Dioxide (CO₂) result is too low?
What Causes Low Carbon Dioxide (CO₂) or Bicarbonate Levels?
And What You Can Do About It
A low carbon dioxide (CO₂) level on a blood test—most commonly reflecting low bicarbonate (HCO₃⁻)—signals a disruption in the body’s acid–base balance. Depending on blood pH and clinical context, this typically points to either metabolic acidosis or respiratory alkalosis.
Identifying the underlying cause is essential, as low bicarbonate is not a diagnosis itself, but rather a marker of an underlying metabolic, respiratory, renal, or systemic issue.
Common Causes of Low CO₂ (Bicarbonate)
Low bicarbonate levels may result from increased acid production, impaired acid excretion, or excessive loss of CO₂. Common causes include:
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Hyperventilation (excessive CO₂ loss)
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Prolonged diarrhea or dehydration
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Uncontrolled diabetes (diabetic ketoacidosis)
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Kidney dysfunction (reduced acid excretion)
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Liver disease
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Severe infections or sepsis
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Aspirin (salicylate) toxicity or alcohol overdose
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Major cardiac events (e.g., large myocardial infarction)
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Hyperthyroidism
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Severe malnutrition
Interpreting Low Bicarbonate in the Context of Blood pH
1. Metabolic Acidosis
Low CO₂ + Low pH (< 7.35)
Occurs when excess acid accumulates or the kidneys cannot remove acid efficiently.
Common causes:
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Chronic kidney disease
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Diabetic ketoacidosis (DKA)
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Lactic acidosis (from shock, hypoxia, or severe illness)
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Severe or prolonged diarrhea
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Liver failure
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Sepsis or severe infection
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Prolonged hypoxia
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Seizures or major metabolic stress
2. Respiratory Alkalosis
Low CO₂ + High pH (> 7.45)
Results from excessive CO₂ loss due to rapid or deep breathing.
Common triggers:
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Anxiety or panic attacks
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Fever
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Severe pain
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Acute illness causing hyperventilation
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High-altitude exposure
Medications That May Lower Bicarbonate Levels
Certain medications can contribute to reduced bicarbonate or acid–base disturbances, including:
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Thiazide diuretics
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Potassium-sparing diuretics (e.g., triamterene)
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Certain antibiotics (e.g., tetracyclines, nitrofurantoin)
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Salicylates (especially at toxic doses)
Medication effects should always be evaluated in the context of kidney function, hydration status, and electrolyte balance.
Actionable Steps to Address Low CO₂ (Bicarbonate)
1. Confirm the Cause
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Request comprehensive testing, including:
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Serum electrolytes
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Kidney and liver function tests
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Blood glucose and ketones (if diabetes is suspected)
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Arterial or venous blood gases (ABG/VBG)
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Determine whether the disturbance is metabolic or respiratory in origin.
2. Correct Hydration and Fluid Loss
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Replace fluids if dehydration or diarrhea is present.
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Oral rehydration may be sufficient for mild cases.
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Severe dehydration may require intravenous fluids under medical supervision.
3. Treat the Underlying Condition
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Diabetic ketoacidosis: Requires urgent medical treatment with insulin and electrolyte replacement.
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Kidney disease: Long-term management may include dietary modification, alkali therapy, or specialist care.
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Liver disease: Address contributing factors such as infection, alcohol use, or metabolic dysfunction.
4. Address Breathing-Related Causes
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For hyperventilation-related alkalosis:
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Practice slow, diaphragmatic breathing
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Address pain, fever, or anxiety triggers
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Consider stress management or behavioral therapy when appropriate
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5. Review Medications
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Review current prescriptions with a healthcare provider.
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Adjust dosing or consider alternatives if medications are contributing to acid–base imbalance.
6. Support Nutritional and Electrolyte Balance
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Address malnutrition with adequate caloric and protein intake.
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Ensure sufficient intake of potassium and magnesium, which are closely linked to acid–base regulation.
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Consider dietitian support if nutritional deficiencies are present.
7. Monitor and Reassess
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Repeat bicarbonate, electrolyte, and pH testing as recommended.
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Track trends rather than relying on a single value.
When to Seek Immediate Medical Care
Low CO₂ levels can become life-threatening when associated with severe acidosis or systemic illness. Seek urgent care if symptoms include:
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Rapid or deep breathing
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Confusion or altered mental status
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Severe weakness or fatigue
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Chest pain or palpitations
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Signs of shock (cold, clammy skin; rapid heartbeat)
Key Takeaway
Low bicarbonate (CO₂) levels indicate a disruption in the body’s acid–base balance—not a standalone diagnosis. Determining whether the cause is metabolic, respiratory, renal, or systemic is critical. With timely evaluation and targeted treatment, many causes are reversible, and early intervention can prevent serious complications.
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