Blood gases & acid-base
ABG interpreter
Enter three numbers off the gas and walk the classic stepwise method — disorder, compensation, gap, delta — with the mixed pictures unmasked.
A patient in DKA: pH 7.25, pCO₂ 3.3 kPa (24.8 mmHg), HCO₃ 10 mmol/L, sodium 140 and chloride 100 mmol/L, PaO₂ 8.0 kPa (60 mmHg) on room air.
Anion gap 30 mmol/L, expected pCO₂ 23 mmHg, delta ratio 1.29 — high-anion-gap metabolic acidosis, compensation appropriate
A pure high-anion-gap acidosis: the measured pCO₂ sits where Winter's formula predicts, so there is no hidden respiratory disorder — a pCO₂ drifting above the expected value would flag a tiring patient about to decompensate.
Who this is for
For clinicians interpreting arterial blood gases in the emergency centre, ICU, or ward — from DKA and sepsis to chronic CO₂ retention. Enter pH, pCO₂, and HCO₃ (kPa first, as South African analysers report) and the stepwise method runs: primary disorder and compensation checking. Add sodium, chloride, and albumin for the corrected anion gap and delta ratio, and oxygen values for the A–a gradient and P/F ratio.
How it works
What it does
A stepwise arterial blood gas interpretation: identifies the primary acid-base disorder, checks whether compensation is appropriate (Winter's formula and its counterparts), computes the anion gap with albumin correction, the delta ratio, and — when oxygen values are supplied — the A–a gradient and P/F ratio.
Why compensation checking matters
The compensation rules are what unmask mixed disorders: a DKA patient whose pCO₂ is higher than Winter's formula predicts has a concurrent respiratory acidosis, often a tiring patient about to decompensate.
Units
Defaults to kPa as South African blood gas analysers report; toggle to mmHg. The compensation formulas run in mmHg internally.
Built on published evidence
- Compensation: Winter's (metabolic acidosis), 0.7×ΔHCO₃ (metabolic alkalosis), 1/4–5 per 10 mmHg pCO₂ (respiratory). References: Winter 1967; Berend et al., NEJM 2014
Codification last reviewed 2026-08-28, and checked against published reference values in automated tests on every release — see the full evidence ledger.
Questions clinicians ask
Does it take kPa or mmHg?
Both — kPa is the default, matching South African blood gas analysers, with mmHg one tap away. Computed outputs such as the expected pCO₂ and A–a gradient report in mmHg by convention (divide by 7.5 for kPa); the compensation formulas run in mmHg internally.
What does the expected pCO₂ actually tell me?
Whether compensation is appropriate — Winter's formula in metabolic acidosis, with counterparts for the other disorders (Berend, NEJM 2014). A measured pCO₂ off the expected value unmasks a second, concurrent disorder: in DKA, a pCO₂ above Winter's prediction means a respiratory acidosis is hiding in the picture.
Why correct the anion gap for albumin?
Hypoalbuminaemia lowers the apparent gap — the correction adds 0.25 mmol/L per g/L of albumin below 40, so a significant acidosis is not concealed behind an 'unremarkable' gap in an unwell, low-albumin patient.
Is the A–a gradient valid at altitude?
The calculation assumes sea level (760 mmHg), so on the Highveld at around 1600 m it reads roughly 20–25 mmHg high — interpret it against local norms rather than sea-level cut-offs.
Related calculators
One calculator is the door. The ward is the house.
Every result can land on a patient card. Paracelsus follows your whole ward — guided work-ups, trends, observation timers, and a one-tap handover — free, offline, with no patient data ever leaving your device.
Open ParacelsusInformation tool for healthcare professionals — not medical advice. Terms of use