Venous Blood pH Calculator (estimate arterial pH from a VBG)

Understanding the venous blood pH calculator — concept and principle

This calculator estimates arterial pH from a venous blood gas (VBG) and shows the range in which arterial PCO₂ could lie. A peripheral venous sample is easier and less painful to take than an arterial one, and is often enough in emergency care.

Venous pH + small offset: with the default inputs the result is ≈ 7.34 (below 7.35 · acidaemia).

How it is calculated

Estimated arterial pH = venous pH + 0.03 (likely range venous + 0.029 to + 0.038)

Arterial PCO₂ range = venous PCO₂ − 10.7 to venous PCO₂ + 2.4 mmHg

pH below 7.35 = acidaemia; above 7.45 = alkalaemia.

The principle

Venous blood has passed through the tissues, picked up carbon dioxide and become slightly more acidic. A 2014 meta-analysis of 18 studies with 1,768 paired samples found arterial pH on average 0.03 higher than venous pH, with a narrow confidence interval, so pH can be estimated well. For PCO₂ the differences between individuals were wide, so only a range is reliable.

Worked example

Default inputs: venous pH 7.31, venous PCO₂ 48 mmHg.

Estimated arterial pH = 7.31 + 0.03 = 7.34 → Below 7.35 (acidaemia).
Likely range = 7.31 + 0.029 to 7.31 + 0.038 = 7.339 to 7.348.
Arterial PCO₂ could be as low as 48 − 10.7 = 37.3 or as high as 48 + 2.4 = 50.4 mmHg.

How to read the result

Where does your result fall?

The estimated pH is a population average and works well in most stable patients. Because PCO₂ agreement is poor, a venous PCO₂ cannot replace an arterial value when precise ventilation assessment is needed, although a normal venous PCO₂ makes high arterial CO₂ unlikely. In shock or cardiac arrest, venous and arterial values can differ much more.

Common mistakes

Using a central venous or mixed venous sample with peripheral offsets.
Applying the conversion in shock or cardiac arrest.
Treating the PCO₂ range as a single value.

Keep in mind: This is an estimate for clinicians, not a diagnosis. Acid–base disorders need full clinical assessment.

Frequently asked questions

Can a venous blood gas replace an arterial one?

For pH and bicarbonate often yes; for PCO₂ and oxygen usually not.

Why is venous pH lower?

Venous blood carries more carbon dioxide from the tissues.

Is venous PO₂ useful?

No; it does not reflect arterial oxygenation.

Sources: Bloom BM, Grundlingh J, Bestwick JP, Harris T, "The role of venous blood gas in the emergency department: a systematic review and meta-analysis", Eur J Emerg Med 2014;21(2):81–88

Checked 9 Nov 2026. Byrne AL, Bennett M, Chatterji R, Symons R, Pace NL, Thomas PS, “Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis”, Respirology 2014;19(2):168–175 (abstract: pH difference and PCO₂ prediction interval); acidaemia/alkalaemia limits from MSD Manual Professional Edition, “Acid-Base Disorders” (Lewis JL III, reviewed 2025).

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