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Blood gas sample handling and pH — FRCA Primary MCQ

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ModeratePhysics and Clinical MeasurementBlood gas sample handling and pHFRCA Primary

An arterial blood gas analyser measures pH using a glass electrode. The sample is left uncapped at room temperature before analysis and PaCO2 falls. What change in pH is most likely?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: C — pH decreases because CO2 leaves the sample

Loss of CO2 from a blood gas sample reduces carbonic acid and increases pH. The tempting error is to focus on bicarbonate alone, but the CO2-bicarbonate system determines pH. Delayed analysis can also be affected by ongoing cellular metabolism. The pearl is to analyse blood gas samples promptly and handle them correctly.

Reference: RCoA Primary FRCA Syllabus 2026; Davis and Kenny Basic Physics and Measurement in Anaesthesia