Bicarbonate, actual (blood gas HCO₃⁻)

What it measures

Blood gas bicarbonate is calculated from measured pH and pCO₂ using the Henderson–Hasselbalch equation. It reflects the metabolic component of acid–base balance and the degree of renal compensation.

Specimen and preparation

• Specimen: Arterial blood in a heparinised (balanced lithium heparin) syringe
• Tube: Other top
• Preparation: Expel air bubbles, cap immediately, mix well and analyse within 30 minutes (ideally 15) at room temperature in a plastic syringe; do not ice plastic syringes, and use a glass syringe on ice only if longer delay is unavoidable. Record FiO₂ and temperature.

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Metabolic alkalosis (vomiting, diuretics)
• Compensation for chronic respiratory acidosis
• Bicarbonate therapy

Causes of low values

• Metabolic acidosis (DKA, lactic acidosis, renal failure, diarrhoea)
• Compensation for chronic respiratory alkalosis

Method

Calculated from pH and pCO₂ (Henderson–Hasselbalch)

Good to know

Should agree within about 2–3 mmol/L with serum total CO₂ taken at the same time. Standard bicarbonate removes the respiratory effect.

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