Blood gas interpretation follows a sequence: is the pH acidaemic or alkalaemic, is the cause respiratory or metabolic, is compensation appropriate, and – in metabolic acidosis – what is the anion gap? Inappropriate compensation means a second, mixed disorder.
How to use the Acid–Base Calculator
- pH
- PaCO₂
- Bicarbonate
- Sodium
- Chloride
- If respiratory: duration
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter pH, PaCO₂ and bicarbonate, plus sodium and chloride for the anion gap, and say whether a respiratory problem is acute or chronic.
Inputs explained
| Field | What to enter |
|---|---|
| pH | A number |
| PaCO₂ | In mmHg |
| Bicarbonate | In mEq/L |
| Sodium | In mEq/L |
| Chloride | In mEq/L |
| If respiratory: duration | Choose one: Acute, Chronic (more than 3–5 days) |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Metabolic acidosis: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 (Winters)
Metabolic alkalosis: expected PaCO₂ = 0.7 × HCO₃⁻ + 21 ± 2
Respiratory acidosis: HCO₃⁻ rises 1 (acute) or 3.5 (chronic) per 10 mmHg PaCO₂
Respiratory alkalosis: HCO₃⁻ falls 2 (acute) or 5 (chronic) per 10 mmHg
Worked example
Inputs — pH: 7.3; PaCO₂: 30 mmHg; Bicarbonate: 15 mEq/L; Sodium: 140 mEq/L; Chloride: 105 mEq/L; If respiratory: duration: Acute.
Results — Primary disorder = Metabolic acidosis; Expected compensation = 30.5; Compensation = Appropriate; Anion gap = 20 mEq/L.
Key terms
- ABG
- Arterial blood gas.
- Compensation
- The body's attempt to bring pH back toward normal.
Frequently asked questions
What are normal blood gas values?
pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L.
How do I tell metabolic from respiratory?
If pH and bicarbonate move the same way, it is metabolic; if pH and PaCO₂ move opposite ways, it is respiratory.
What is a mixed disorder?
Two or more primary disorders at once, revealed by inappropriate compensation.