Venous to Arterial pH Estimator
Venous blood gases can approximate arterial pH in stable patients. Enter venous pH (and optionally PCO₂) to estimate arterial values.
Used for interpretation only. Does not alter the pH estimation.
Estimated arterial pH
7.34
Difference (arterial − venous)
+0.02
How to Use This Calculator
Obtain venous blood gas
Collect venous sample while ensuring minimal delay in analysis. Record pH and, if available, PCO₂.
Enter venous values
Input pH (mandatory) and PCO₂ (optional). The calculator adds ~0.02 to venous pH, reflecting average veno-arterial gradient.
Interpret alongside clinical status
Use the estimate for trend monitoring or screening. Obtain arterial blood gas when precise oxygenation or acid-base data are required.
Formula
Estimated arterial pH ≈ Venous pH + 0.02
Arterial PCO₂ ≈ Venous PCO₂ − 5 mm Hg (contextual reference)
These relationships apply best in haemodynamically stable patients without severe shock.
Full Description
Venous blood gases correlate closely with arterial pH in most clinical settings, allowing clinicians to assess acid-base status without arterial puncture. However, the venous-to-arterial pH gradient varies with perfusion, anaerobic metabolism, and sampling site. This calculator applies an average offset of +0.02, suitable for emergency and critical care screening but not a substitute for arterial analysis when precise data or oxygenation assessment is required.
Frequently Asked Questions
How accurate is venous pH compared to arterial?
Venous pH is usually 0.02–0.04 lower than arterial in stable patients. Accuracy decreases in shock or severe circulatory failure.
Can I rely on venous pH for ventilation assessment?
Venous PCO₂ correlates reasonably but is less reliable in extremes. Obtain arterial blood gas to titrate ventilation precisely.
Does sampling site matter?
Central venous samples correlate better with arterial values than peripheral samples. Document sampling site for interpretation.
When must I confirm with arterial blood gas?
Whenever oxygenation assessment is required, in severe shock, or when acid-base status guides critical interventions.