Reference ranges used. pH 7.35–7.45 · PaCO₂ 4.7–6.0 kPa (35–45 mmHg) · HCO₃⁻ 22–26 mmol/L · PaO₂ on air 10.6–14.0 kPa (80–105 mmHg), falling with age. Conversions: 1 kPa = 7.50062 mmHg.

Step 1 — pH. Under 7.35 is acidaemia, over 7.45 is alkalaemia. A normal pH does not exclude a disorder; it may mean full compensation or two disorders cancelling.

Step 2 — which side moved. A high PaCO₂ drives acidosis, a low one alkalosis; a low HCO₃⁻ drives acidosis, a high one alkalosis. The component that moves in the direction that explains the pH is the primary disorder. If both move in a way that could each explain the pH, the tool reports a mixed disorder rather than picking one.

Step 3 — is the compensation right? Metabolic acidosis: Winter's formula, expected PaCO₂ = 1.5 × HCO₃⁻ + 8, ± 2 mmHg. Actual above the range means an added respiratory acidosis (often tiring respiratory muscles — the important one to catch); below it means an added respiratory alkalosis. Metabolic alkalosis: expected PaCO₂ ≈ 40 + 0.7 × (HCO₃⁻ − 24) mmHg. Respiratory acidosis: HCO₃⁻ rises about 1 mmol/L per 10 mmHg of PaCO₂ acutely, about 4 mmol/L chronically once the kidney has had 3–5 days. Respiratory alkalosis: HCO₃⁻ falls about 2 mmol/L per 10 mmHg acutely, about 5 mmol/L chronically.

Step 4 — oxygenation. Alveolar gas equation: PAO₂ = FiO₂ × (760 − 47) − PaCO₂ ÷ 0.8 in mmHg, with a respiratory quotient of 0.8. A-a gradient = PAO₂ − PaO₂; expected is roughly (age ÷ 4) + 4 mmHg. P/F ratio is PaO₂ in mmHg ÷ FiO₂ as a fraction: over 400 is normal, and the Berlin ARDS definition puts mild at 200–300, moderate at 100–200 and severe under 100, all requiring PEEP of at least 5 cmH₂O. Type 1 respiratory failure is a low PaO₂ with a normal or low PaCO₂; type 2 adds a PaCO₂ above 6.5 kPa (50 mmHg).

Worked example. pH 7.25, PaCO₂ 3.2 kPa (24 mmHg), HCO₃⁻ 12, PaO₂ 12 kPa (90 mmHg) on air, age 40. Acidaemia with a low bicarbonate → metabolic acidosis. Winter's: 1.5 × 12 + 8 = 26, range 24–28 mmHg; the actual 24 mmHg sits at the lower edge, so respiratory compensation is appropriate — a simple metabolic acidosis, no second disorder. PAO₂ = 0.21 × 713 − 24 ÷ 0.8 = 149.7 − 30 = 119.7; A-a = 119.7 − 90 = 29.7 mmHg against an expected 14, so the gradient is widened — the hypoxaemia is not explained by the low PaCO₂ alone. P/F = 90 ÷ 0.21 = 429, normal. Next step for this gas is an anion gap: use the Anion Gap & Corrected Electrolytes tool.

Where the rules break. The compensation formulas are population averages with wide confidence intervals; a value inside the predicted range does not prove there is only one disorder, and a value outside it does not prove there are two. Mixed acid-base disorders are common in critical illness and no formula detects them reliably without the anion gap, the delta ratio and the clinical picture. The A-a gradient assumes a steady state, a respiratory quotient of 0.8 and an accurate FiO₂ — on high-flow oxygen or a non-rebreather mask the delivered FiO₂ is not the number on the flowmeter, so P/F and A-a are only as good as that estimate. Venous gases are not arterial gases: pH and HCO₃⁻ track reasonably, PaCO₂ is about 0.6 kPa higher and PvO₂ tells you nothing about oxygenation. Carboxyhaemoglobin and methaemoglobin make a co-oximeter SpO₂ and a calculated oxygen content disagree. The bicarbonate on a gas machine is calculated from pH and PaCO₂ using the Henderson-Hasselbalch equation, so it is not an independent measurement of the metabolic component.

Disclaimer. A structured prompt for qualified clinicians — not medical advice and not a substitute for looking at the whole gas, the patient and the trend. Sources: Winter's formula (Albert et al., NEJM 1967) · Berlin ARDS definition (JAMA 2012) · standard compensation rules as tabulated in Oh's Intensive Care Manual. Last reviewed: 2026-09-21.