pH (blood gas)

What it measures

Blood pH measures the hydrogen-ion activity of arterial blood. It shows whether acidaemia or alkalaemia is present and is the starting point for interpreting any acid–base disorder.

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)
• Respiratory alkalosis (hyperventilation, anxiety, PE)
• Air bubble in sample (artefact)

Causes of low values

• Metabolic acidosis (DKA, lactic acidosis, kidney failure)
• Respiratory acidosis (COPD, sedation, neuromuscular weakness)
• Delayed analysis at room temperature

Method

Potentiometric glass pH electrode on a blood gas analyser

Good to know

Critical values are usually <7.20 or >7.60. Check pCO₂ and HCO₃⁻ to identify the primary process and compensation.

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