Step-by-step reading of an arterial blood gas: pH, primary disorder, expected compensation, anion gap and delta gap, oxygenation (Habib et al. 2025)
pH
PaCO₂
HCO₃⁻ (mmol/L)
Optional — each value adds a step
Na⁺ (mmol/L) — for the anion gap
Cl⁻ (mmol/L) — for the anion gap
Albumin — corrects the anion gap
PaO₂ — for oxygenation
FiO₂ (fraction, e.g. 0.21) — for the expected PaO₂
Enter pH, PaCO₂ and HCO₃⁻ to interpret
About this score
When to use
Reads an arterial blood gas in five steps: oxygenation, pH, primary disorder with expected compensation, anion gap with albumin correction, and the delta gap. [1]
Pearls and pitfalls
• Complex, coexisting acid–base disorders can be present even when the pH looks normal. [1]
• An anion gap of 20 mmol/L or more means a metabolic acidosis, whatever the pH or bicarbonate. [1]
• In metabolic acidosis, the expected PaCO₂ is 1.5 × HCO₃⁻ + 8 (±2). [2]
Summarised from the numbered sources under Evidence. Check the source before relying on a statement.
Evidence
[1] Original publication
Habib T, Nair A, Murphy S, Saeed H, Ishaya N. Mastering blood gas interpretation: a practical guide for primary care providers. Five-step approach used here: oxygenation, pH, primary disorder with expected compensation (Table 2), anion gap with albumin correction, and the delta gap.
South African Family Practice, 2025
Open source
[2] Validation
Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Origin of the expected PaCO₂ in metabolic acidosis (1.5 × HCO₃⁻ + 8 ± 2).
Annals of Internal Medicine, 1967
Open source
Last reviewed: 2026-10
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MedScore is an educational and clinical decision-support tool. It does not provide a diagnosis, a treatment, or a drug-dosing recommendation, and it does not replace clinical judgement or local guidelines. Confirm every result against the original source before acting on it. The treating clinician remains responsible for all decisions.