ABG / Acid-Base Interpreter

About the ABG / Acid-Base Interpreter

An arterial blood gas (ABG) reports the pH, the partial pressure of carbon dioxide (PaCO₂) and the bicarbonate (HCO₃⁻), often with the oxygen. Reading it is a sequence of steps: is the blood acidic or alkaline, which value explains that, is the body compensating as expected, and is there a second disorder hiding behind the first?

This interpreter walks through those steps with your numbers and shows each rule it uses: the primary disorder, the expected compensation (Winter’s formula for metabolic acidosis, the acute and chronic rules for respiratory disorders), the anion gap and albumin-corrected gap if you add sodium, chloride and albumin, the delta ratio for a high-anion-gap acidosis, the PaO₂/FiO₂ ratio, and a Henderson–Hasselbalch check that the three values belong together. Gas pressures can be entered in mmHg or kPa. It is an educational tool for students and clinicians; nothing you type leaves your browser.

Without a pass, the free preview shows the primary disorder in full with what it means and every warning, and the first step of the working; a Pro pass unlocks every finding, the rest of the steps, Copy result and Download .txt.

Common uses

• Working through an ABG step by step while learning acid–base physiology.
• Checking whether the compensation fits a single disorder or points to a mixed one.
• Adding the anion gap and delta ratio to a metabolic acidosis from the same day’s chemistry.
• Calculating the PaO₂/FiO₂ ratio from a blood gas on oxygen.

The steps and rules

1. pH: below 7.35 is acidaemia, above 7.45 alkalaemia.
2. Primary disorder: the value that moves the pH the way it has gone — a PaCO₂ above 45 mmHg (6.0 kPa) is respiratory acidosis, below 35 mmHg (4.7 kPa) respiratory alkalosis; a bicarbonate below 22 mmol/L is metabolic acidosis, above 26 metabolic alkalosis. With a normal pH and both values abnormal, the side of 7.40 the pH leans to points to the primary disorder.
3. Compensation: metabolic acidosis — Winter’s formula, expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2; metabolic alkalosis — the PaCO₂ rises 0.6–0.75 mmHg per mmol/L of bicarbonate above 24, and not above 55 mmHg; respiratory acidosis — the bicarbonate rises 1–2 mmol/L per 10 mmHg of PaCO₂ above 40 when acute and 3–4 when chronic; respiratory alkalosis — it falls 1–2 per 10 mmHg when acute and 4–5 when chronic. Outside Winter’s range, or more than 10% outside the other ranges, a second disorder is likely.
4. Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), corrected for albumin with + 2.5 × (normal albumin − albumin in g/dL); above the normal gap (12 unless you change it) a metabolic acidosis is a high-anion-gap one.
5. Delta ratio = (anion gap − 12) ÷ (24 − HCO₃⁻): below 0.4 a normal-anion-gap acidosis, 0.4–0.8 both kinds, 0.8–2 a high-anion-gap acidosis alone, above 2 a metabolic alkalosis as well.
6. Oxygenation: PaO₂ ÷ FiO₂ in mmHg, read against the Berlin definition’s bands. Arterial oxygen cannot be higher than alveolar oxygen, which the alveolar gas equation gives as FiO₂ × (760 − 47) − PaCO₂ ÷ 0.8 mmHg at sea level (less at altitude), so a PaO₂ well above it is flagged as a unit or FiO₂ error.
7. Check: pH = 6.1 + log₁₀(HCO₃⁻ ÷ (0.03 × PaCO₂)) — if the measured pH differs by more than 0.05, the values probably do not come from one sample.

Why the rules are ranges

Compensation is a physiological response, not a fixed equation: the kidneys need days to raise the bicarbonate in chronic respiratory acidosis, and the published rules give ranges rather than single values. Compensation never overshoots and cannot bring the pH all the way back to normal, which is why a normal pH with an abnormal PaCO₂ and bicarbonate usually means two opposing disorders. A measured value just outside an expected range can still be a single disorder, so this interpreter calls a second disorder only outside Winter’s ± 2 mmHg or more than 10% outside the other ranges, and it always shows the range so you can judge.

Sources

• Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis — Ann Intern Med
• Acid-base disorders — Merck Manual Professional Version: the compensation rules
• Breathing and balance: clinical insights and management strategies of respiratory acid-base disorders — Nutr Clin Pract: reference values, the 10% rule and the delta ratio
• Kraut JA, Nagami GT. The serum anion gap in the evaluation of acid-base disorders — Clin J Am Soc Nephrol
• Figge J et al. Anion gap and hypoalbuminemia — Crit Care Med
• ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin Definition — JAMA
• Matthay MA et al. A new global definition of acute respiratory distress syndrome — Am J Respir Crit Care Med
• Alveolar gas equation — StatPearls: the alveolar oxygen check
• Arterial blood gas (ABG) — Cleveland Clinic: normal values
• Blood gases — MedlinePlus: normal PaO₂ at sea level

Limitations

• Educational only: an ABG is interpreted with the patient’s history, examination and other tests by a clinician.
• The rules are for arterial blood; venous values (higher PCO₂, slightly lower pH) read on them can mislead.
• The compensation rules and delta ratio bands are approximations; values near a boundary can be read either way.
• A P/F ratio alone never diagnoses ARDS: the definition also needs PEEP or CPAP of at least 5 cmH₂O, onset within a week and bilateral opacities not explained by heart failure.
• Accepts pH 6.80–7.80, PaCO₂ 8–150 mmHg (1–20 kPa), bicarbonate 2–60 mmol/L and PaO₂ 20–700 mmHg (2.5–95 kPa).

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