Venous to Arterial Blood Gas Converter
Venous to Arterial Blood Gas Converter
A peripheral venous gas is a good screen for pH and bicarbonate and a poor substitute for an arterial pO₂ — venous oxygen tells you nothing about oxygenation. This converts the one value the evidence supports converting, and says plainly why it will not convert the others.
Venous pH → estimated arterial pH
Venous pH → arterial pHPeripheral venous pH 7.25, patient not shocked
The offset, and its provenance
- 0.034
- the weighted mean arterio-venous pH difference from Kelly’s systematic review of 2,087 paired samples, with narrow limits of agreement. Byrne’s meta-analysis of 18 studies and 1,768 subjects gives 0.033 (95% CI 0.029 to 0.038) — the two agree to the third decimal place, which is why pH is the value this page converts
- pCO₂
- NOT converted. The central estimate is about 4 to 6 mmHg higher in venous blood (Byrne 4.4, Kelly 6.2), but Kelly reports 95% limits of agreement of the order of ±20 mmHg and Byrne calls the prediction interval ‘unacceptably wide’. A correction with that much scatter is a guess wearing a decimal point
- bicarbonate
- NOT converted, because it barely needs to be. Kelly’s weighted mean difference is 1.20 mmol/L with 95% limits of agreement around ±5 mmol/L. Read the venous bicarbonate as the arterial one
- base excess
- Kelly’s mean arterio-venous difference is 0.4 mmol/L, with conflicting data on the width of the limits of agreement. Treat a venous base excess as approximately arterial
- pO₂
- NOT convertible at all. A peripheral venous pO₂ measures local tissue extraction, not pulmonary gas exchange. There is no number to convert
- which venous sample
- peripheral venous. Central venous and mixed venous samples have different relationships to arterial values and these offsets do not apply to them
Worked example
Peripheral venous pH 7.25, patient not shocked
7.25 + 0.034 = 7.284, which displays as 7.28
That is an acidaemia — below the arterial reference interval of 7.35 to 7.45
The correction is small, and that is the point: at 0.034 pH units, a venous pH is within about 0.03 of the arterial value before anything is done to it. For deciding whether a patient is acidaemic at all, the raw venous pH would have served
The venous bicarbonate from the same sample can be read as arterial: Kelly's mean difference is 1.20 mmol/L
The venous pCO₂ is NOT corrected here. A venous pCO₂ of 36 mmHg has a central estimate of about 30 mmHg arterial, but with 95% limits of agreement of roughly ±20 mmHg the true value could be anywhere from 10 to 50 — which is the difference between a profound respiratory alkalosis and impending respiratory failure
If this patient needs an arterial pO₂ — breathless, hypoxaemic on the oximeter, or being considered for ventilatory support — no arithmetic on this page substitutes for taking one
What transfers from a venous gas, and what does not
| Value | Venous versus arterial | Limits of agreement | Use it? |
|---|---|---|---|
| pH | Venous about 0.03 lower (Kelly 0.034, Byrne 0.033) | Narrow | Yes — add 0.03, or read it raw for a yes/no answer |
| Bicarbonate | Venous about 1.2 mmol/L higher | About ±5 mmol/L | Yes — read it as arterial |
| Base excess | Difference about 0.4 mmol/L | Conflicting data | Yes, with care |
| Lactate | Venous acceptable for screening | Widely used this way | Yes — a venous lactate is standard practice |
| pCO₂ | Venous about 4–6 mmHg higher | About ±20 mmHg | As a screen only. A normal venous pCO₂ makes arterial hypercapnia unlikely; an abnormal one needs an arterial sample |
| pO₂ | Arterial about 37 mmHg higher (95% CI 27–47) | Very wide, and conceptually the wrong measurement | No. Never |
The offsets in both units
| Value | Difference (mmHg) | Difference (kPa) |
|---|---|---|
| pCO₂, venous higher — Byrne | 4.4 | 0.59 |
| pCO₂, venous higher — Kelly | 6.2 | 0.83 |
| pCO₂, 95% limits of agreement | about ±20 | about ±2.7 |
| pO₂, arterial higher — Byrne | 36.9 (95% CI 27.2 to 46.6) | 4.92 (95% CI 3.63 to 6.21) |
When a venous gas is enough, and when it is not
| Situation | Venous gas adequate? |
|---|---|
| Diabetic ketoacidosis — diagnosis and monitoring of pH and bicarbonate | Yes. Widely used and endorsed in practice |
| Screening an undifferentiated unwell patient for a metabolic acidosis | Yes |
| Measuring a lactate | Yes |
| Excluding hypercapnia when the venous pCO₂ is normal | Usually. A raised venous pCO₂ needs arterial confirmation |
| Quantifying hypercapnia in type 2 respiratory failure | No. Take an arterial sample |
| Assessing oxygenation, calculating an A-a gradient or a P/F ratio | No. Never |
| Titrating non-invasive ventilation | No |
| A shocked or hypotensive patient | No — take an arterial sample. The evidence is conflicting and the stakes are not |
| Carbon monoxide poisoning (carboxyhaemoglobin) | Yes — co-oximetry works on venous blood |
A good screen for pH, a poor substitute for a pO₂
The case for the venous gas is real and it should be made first. An arterial puncture hurts, fails more often than a venous draw, carries a small risk of arterial injury and nerve damage, and needs a skill that is unevenly distributed at three in the morning. Two systematic reviews — Kelly’s, pooling 2,087 paired pH samples, and Byrne’s, pooling 18 studies and 1,768 subjects — agree to the third decimal place that peripheral venous pH runs about 0.033 to 0.034 below arterial, with narrow limits of agreement. The bicarbonate differs by about 1.2 mmol/L and the base excess by about 0.4. For the question most blood gases are actually taken to answer — is this patient acidaemic, and is it metabolic — a venous sample answers it, and in diabetic ketoacidosis that is now ordinary practice.
The case against is equally specific, and it is about gas tensions rather than about venous blood. A peripheral venous pO₂ is not a worse measurement of oxygenation than an arterial one; it is a measurement of something else. It reports how much oxygen that particular limb extracted from the blood passing through it, which depends on local flow, on a tourniquet, on temperature and on how long the sample took. Byrne’s mean arterio-venous pO₂ difference of 36.9 mmHg has a confidence interval from 27 to 47, but even a perfectly precise offset would not help, because the thing being converted is not a diluted version of the arterial value. A normal venous pO₂ is entirely compatible with severe hypoxaemia, and a low one is entirely compatible with normal lungs.
The pCO₂ sits between the two and is where most of the harm occurs, because it looks convertible. The central estimate is stable enough — venous runs 4 to 6 mmHg higher — but Kelly reports 95% limits of agreement of the order of ±20 mmHg and Byrne calls the prediction interval unacceptably wide. Applying the mean offset to an individual patient therefore produces a number with a plausible-looking decimal point and an error bar wide enough to hide type 2 respiratory failure. This page will not print that number. The defensible use of a venous pCO₂ is as a rule-out: a normal venous pCO₂ makes arterial hypercapnia unlikely, and an abnormal one means an arterial sample is needed.
Shock is where the offsets are most often reached for and least well supported, and the literature does not agree. Kelly’s review restricts interchangeability to patients who are not in shock. Rudkin and colleagues, studying hypovolaemic shock directly, found the arterio-venous differences no wider than in normotensive patients and yet concluded that the peripheral venous gas is not a surrogate for tissue acid-base status in circulatory failure. Prasad and colleagues, in hypotensive emergency department patients, concluded that it is a reasonable alternative. Three careful groups, three different answers, and a patient in front of you whose perfusion is the very thing in question. The practical position this page takes is that a shocked patient usually needs an arterial line anyway — and that the interpretation of any gas supports a clinician’s judgement rather than replacing it, and is read alongside the patient rather than instead of them.
Frequently asked questions
How much lower is venous pH than arterial?
About 0.03. Kelly’s systematic review of 2,087 paired samples gives a weighted mean arterio-venous difference of 0.034 pH units with narrow limits of agreement, and Byrne’s meta-analysis gives 0.033 (95% CI 0.029 to 0.038). The two agree closely enough that pH is the one value this page converts.
Can a venous gas replace an arterial one?
For acid-base questions in a patient who is not shocked, largely yes: venous pH and bicarbonate are close enough to arterial to be used clinically, which is why venous sampling is standard in diabetic ketoacidosis. For oxygenation, no — never. A venous pO₂ measures what a limb extracted, not how the lungs are working, so no A-a gradient and no P/F ratio can be built from it.
Why will this page not convert the venous pCO₂?
Because the scatter is too wide to be useful. The central estimate is about 4 to 6 mmHg higher in venous blood, but the 95% limits of agreement are of the order of ±20 mmHg. A converted value could read 35 when the true arterial pCO₂ is 55. A normal venous pCO₂ is a reasonable rule-out for hypercapnia; an abnormal one means taking an arterial sample.
Is the venous bicarbonate the same as the arterial one?
Close enough to be used directly. Kelly’s weighted mean difference is 1.20 mmol/L with 95% limits of agreement around ±5 mmol/L, and the base excess differs by about 0.4 mmol/L. Neither is corrected on this page because correcting by one millimole would imply a precision the limits of agreement do not support.
Do these offsets still hold in shock?
The evidence conflicts. Kelly’s review limits interchangeability to patients not in shock; Rudkin’s study of hypovolaemic shock found the differences no wider but concluded a peripheral venous gas is still not a surrogate for tissue acid-base status; Prasad’s study of hypotensive emergency patients concluded it is a reasonable alternative. In a shocked patient, take an arterial sample.
Related calculators
References
- Kelly AM. Agreement between arterial and venous blood gases in emergency medical care: a systematic review. Hong Kong J Emerg Med. 2013;20(3):166–71.
- 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):168–75.
- Rudkin SE, Anderson CL, Grogan TR, Elashoff DA, Treger RM. Assessing acid-base status in circulatory failure: relationship between arterial and peripheral venous blood gas measurements in hypovolemic shock. J Intensive Care Med. 2020;35(5):511–18.
- Prasad H, Vempalli N, Agrawal N, et al. Correlation and agreement between arterial and venous blood gas analysis in patients with hypotension — an emergency department-based cross-sectional study. Int J Emerg Med. 2023;16:18.
- 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):81–8.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.