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Arterial Blood Gas (ABG) Stepwise Interpreter

Stepwise acid–base interpretation: pH < 7.35 acidemia, > 7.45 alkalemia; primary disorder from pCO₂ (35–45) and HCO₃⁻ (22–26); expected compensation (Winters pCO₂ = 1.5 × HCO₃ + 8 ± 2; metabolic alkalosis 40 + 0.7 × ΔHCO₃ ± 5; respiratory 0.1/0.35 and 0.2/0.4 per mmHg); anion gap (albumin-corrected) and delta ratio.

  • pH
  • pCO₂ (mmHg or kPa)
  • HCO₃⁻ (mmol/L or mEq/L)
  • Sodium (optional) (mmol/L or mEq/L)
  • Chloride (optional) (mmol/L or mEq/L)
  • Albumin (optional) (g/dL or g/L)
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Interpretation

ResultMeaning
pH < 7.35Acidemia
pH > 7.45Alkalemia
AG > 12 (albumin-corrected)High anion gap
Delta ratio < 0.4 / 0.4–0.8 / 0.8–2 / > 2NAGMA / mixed HAGMA+NAGMA / pure HAGMA / HAGMA + metabolic alkalosis

Formula

Winters: expected pCO₂ = 1.5 × HCO₃⁻ + 8 ± 2. Metabolic alkalosis: expected pCO₂ = 40 + 0.7 × (HCO₃⁻ − 24) ± 5. Respiratory acidosis: expected HCO₃⁻ = 24 + 0.1 (acute) or 0.35 (chronic) × (pCO₂ − 40). Respiratory alkalosis: 24 − 0.2 (acute) or 0.4 (chronic) × (40 − pCO₂). AG = Na − (Cl + HCO₃); corrected AG = AG + 2.5 × (4 − albumin g/dL). Delta ratio = (AG − 12) ÷ (24 − HCO₃).

Clinical pearls

  • Compensation never overcorrects pH; a "normal" pH with abnormal pCO₂ and HCO₃⁻ usually means a mixed disorder.
  • An anion gap ≥ 20 suggests a high-gap metabolic acidosis even when pH and HCO₃⁻ look normal.
  • Always correct the anion gap for low albumin (each 1 g/dL fall lowers the gap by ≈ 2.5); this tool applies rules only and cannot replace clinical context (e.g. osmolal gap, urine studies).

Examples

abg ph 7.25 pco2 28 hco3 12 na 140 cl 100interpret blood gas: pH 7.25, PaCO2 60, bicarb 26acid base ph 7.40 pco2 40 hco3 24 na 140 cl 96 albumin 4

FAQ

How do you interpret an ABG step by step?

Check pH, identify whether pCO₂ or HCO₃⁻ explains it, check expected compensation, calculate the anion gap (corrected for albumin), then the delta ratio for hidden disorders.

What is Winters' formula?

Expected pCO₂ in metabolic acidosis = 1.5 × HCO₃⁻ + 8 ± 2. Higher pCO₂ means a coexisting respiratory acidosis; lower means respiratory alkalosis.

Evidence

  1. Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid–base disturbances. N Engl J Med 2014;371:1434–45.
  2. Rastegar A. Use of the ΔAG/ΔHCO3− ratio in the diagnosis of mixed acid-base disorders. J Am Soc Nephrol 2007;18:2429–31.

Pulmonary & Critical Care · Reviewed 2026-10-14 · Methodology