Blood Gas
pH (blood)
Blood pH is a direct measure of the acidity or alkalinity of whole blood, reported on a logarithmic scale and measured potentiometrically by an ion-selective electrode in point-of-care blood gas analysers. It is a core component of the arterial or venous blood gas panel, interpreted alongside pCO2 and bicarbonate to assess acid-base balance.
Why it is measured
pH is measured to detect and classify acidaemia and alkalaemia and to help distinguish respiratory from metabolic acid-base disturbances. It is a time-critical parameter in emergency, critical care, anaesthetic and resuscitation settings where rapid acid-base assessment guides clinical decisions.
| Typical range | Indicative adult arterial range approximately 7.35 to 7.45 (pH units, dimensionless). Venous samples typically run slightly lower, around 7.32 to 7.43. Ranges vary by sample type, method and analyser, so always apply the reporting laboratory or device range. |
|---|---|
| Sample | Heparinised whole blood, most commonly arterial, also venous or capillary depending on the clinical question. Analyse promptly and avoid air bubbles, as delay and air exposure alter the result. |
| Turnaround | Typically under 1 to 2 minutes from sample insertion at the point of care, with results available at the bedside. |
Point of care devices that report it
- Abbott i-STAT (CG4+, CG8+, EG7+ and similar cartridges)
- Siemens epoc Blood Analysis System (BGEM test card)
- Radiometer ABL90 FLEX and ABL90 FLEX PLUS
- Radiometer ABL800 FLEX
- Siemens RAPIDPoint 500e
- Werfen GEM Premier 5000
Questions, answered
What is the difference between arterial and venous blood pH?
Arterial samples are the reference standard for acid-base assessment and oxygenation. Venous pH usually reads a little lower than arterial, often by around 0.02 to 0.04 pH units, so the sample type should be recorded and the appropriate reference range applied when reviewing a result.
Why can a point-of-care pH differ from the main laboratory result?
Differences can arise from sample type, pre-analytical factors such as air bubbles, delay before analysis or temperature, and from method and calibration differences between devices. Following correct sampling and prompt analysis, and ensuring quality control is in date, helps keep point-of-care and laboratory results aligned.
How should samples be handled to keep the pH result reliable?
Use the correct heparinised collection device, expel air bubbles, mix gently and analyse promptly, ideally within minutes. Ongoing cellular metabolism in a delayed or warm sample can lower the measured pH, so timely analysis and adherence to local sample-handling protocols are important.