CO₂ (Bicarbonate): What Your Blood Test Means

What Is CO₂ (Bicarbonate)

CO₂ in a blood test refers to bicarbonate (HCO₃⁻) — the main buffering substance that keeps blood pH stable. The body constantly produces carbon dioxide (CO₂), which is converted into bicarbonate in the blood. The kidneys and lungs together regulate bicarbonate levels — the lungs remove CO₂ through breathing, while the kidneys regulate bicarbonate reabsorption.

This marker reflects metabolic acid-base balance and is often measured alongside electrolytes.

Reference Range

The normal CO₂ (bicarbonate) range in blood is 22–29 mmol/L. This applies to adults. The range stated by your laboratory always takes priority.

Bicarbonate levels are interpreted in context — alongside sodium, potassium, chloride and, if needed, arterial blood gas analysis. A single number without context rarely gives a complete picture.

Why CO₂ May Be High

Elevated bicarbonate (> 29 mmol/L) indicates metabolic alkalosis or compensation for respiratory acidosis. Possible causes:

• Prolonged vomiting — stomach acid is lost, leaving more bicarbonate in the blood.
• Diuretic use — especially thiazide and loop diuretics can cause metabolic alkalosis.
• High intake of antacids or sodium bicarbonate (baking soda).
• Chronic obstructive pulmonary disease (COPD) — the lungs cannot exhale enough CO₂, so the kidneys compensate by retaining bicarbonate.
• Hyperaldosteronism — excess adrenal hormone.

Why CO₂ May Be Low

Low bicarbonate (< 22 mmol/L) indicates metabolic acidosis or compensation for respiratory alkalosis. Possible causes:

• Diabetic ketoacidosis — the body produces ketone bodies that consume bicarbonate.
• Kidney disease — the kidneys cannot regenerate bicarbonate.
• Lactic acidosis — due to severe infection, shock, or intense physical exertion.
• Diarrhoea — bicarbonate is lost through the intestines.
• Intoxication: methanol, ethylene glycol, aspirin overdose.
• Hyperventilation (respiratory alkalosis) — the lungs remove too much CO₂, and the kidneys compensate by reducing bicarbonate.

What to Do Next

A bicarbonate deviation is always evaluated in context with other markers. Your doctor may calculate the anion gap (anion gap = Na − Cl − HCO₃), which helps differentiate types of metabolic acidosis. If the anion gap is elevated, the search focuses on toxins, ketoacidosis, or lactic acidosis.

If the deviation is small and symptom-free, observation and a repeat test may be sufficient. If the deviation is significant, a more thorough investigation is needed.

How to Track Changes Over Time

Monitoring CO₂ is especially important for patients with chronic kidney disease, diabetes, COPD, or those taking diuretics. A regularly repeated electrolyte panel shows whether the trend is stable.

On the QbiLabs platform, CO₂ is displayed alongside other electrolytes — you can track the full acid-base picture on a single timeline.

Questions for Your Doctor

1. Does my bicarbonate level indicate an acid-base disturbance? 2. Do I need additional tests — such as arterial blood gases? 3. What is my anion gap and what does it indicate? 4. Could this be related to my lung or kidney function? 5. Could my medications have affected this result?

*This is not medical advice. Consult your doctor for decisions about your health.*