Enter pH, PaCO₂ and HCO₃⁻ to identify the primary acid-base disorder, check the expected respiratory or metabolic compensation, and (optionally) compute the anion gap and delta-delta from sodium, chloride and albumin. Designed as a quick reference for clinicians and students — always interpret in clinical context.
Created by
Daniel Whitman
Reviewed by
Hannah Brooks
Normal 7.36–7.44
mm Hg · 36–44
mEq/L · 22–26
Educational tool only. Always interpret arterial blood gases in the full clinical context — history, exam, oxygenation, electrolytes, lactate and trend over time. Not a substitute for clinical judgment.
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The calculator follows the standard four-step ABG approach: (1) classify the pH as acidemia, alkalemia or normal; (2) decide whether PaCO₂ or HCO₃⁻ is moving in the same direction as the pH (that's the primary disorder); (3) calculate the expected compensation and compare it to what's measured; (4) if Na⁺ and Cl⁻ are provided, compute the anion gap and the delta-delta ratio to detect mixed metabolic disorders.
Compensation rules are the standard ones: Winter's formula for metabolic acidosis (PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2), 0.7 × HCO₃⁻ + 21 ± 2 for metabolic alkalosis, and the per-10-mm-Hg HCO₃⁻ shifts for acute and chronic respiratory disorders. When the measured value falls outside the expected band, the result is flagged as a likely mixed disorder.
The anion gap uses AG = Na⁺ − (Cl⁻ + HCO₃⁻) with optional albumin correction (AG + 2.5 × (4 − albumin)). For high-AG metabolic acidosis, the delta-delta ratio (ΔAG / ΔHCO₃⁻) helps reveal a coexisting normal-AG metabolic acidosis (< 1) or metabolic alkalosis (> 2).
Educational use only. ABG interpretation must integrate clinical history, exam findings, oxygenation, electrolytes, lactate and the trajectory of values. Do not use this calculator as the sole basis for treatment decisions.
Quick-glance reference for normal ranges, the four primary disorders, expected compensation formulas, and the classic mnemonics for metabolic acidosis causes.
| Parameter | Normal range | Critical |
|---|---|---|
| pH | 7.36 – 7.44 | < 7.20 or > 7.60 |
| PaCO₂ | 36 – 44 mm Hg | < 20 or > 60 |
| HCO₃⁻ | 22 – 26 mEq/L | < 15 or > 35 |
| Base excess | −2 to +2 mEq/L | < −10 or > +10 |
| PaO₂ | 80 – 100 mm Hg (room air) | < 60 |
| SaO₂ | 95 – 100 % | < 90 |
| Anion gap | 8 – 12 mEq/L | > 20 |
| Disorder | pH | PaCO₂ | HCO₃⁻ | Common causes |
|---|---|---|---|---|
| Respiratory acidosis | ↓ | ↑ | ↑ (compensation) | COPD, opioids, sedation, neuromuscular failure |
| Respiratory alkalosis | ↑ | ↓ | ↓ (compensation) | Anxiety, pain, sepsis, PE, high altitude |
| Metabolic acidosis | ↓ | ↓ (compensation) | ↓ | DKA, lactic acidosis, renal failure, diarrhea |
| Metabolic alkalosis | ↑ | ↑ (compensation) | ↑ | Vomiting, NG suction, diuretics, hyperaldosteronism |
| Disorder | Formula | Notes |
|---|---|---|
| Metabolic acidosis | PaCO₂ = 1.5 × HCO₃⁻ + 8 (± 2) | Winter's formula |
| Metabolic alkalosis | PaCO₂ = 0.7 × HCO₃⁻ + 21 (± 2) | Or +0.7 per mEq HCO₃⁻ > 24 |
| Acute resp. acidosis | ΔHCO₃⁻ = +1 per 10 mm Hg PaCO₂ > 40 | Minutes – hours |
| Chronic resp. acidosis | ΔHCO₃⁻ = +3.5 per 10 mm Hg PaCO₂ > 40 | ≥ 3 days |
| Acute resp. alkalosis | ΔHCO₃⁻ = −2 per 10 mm Hg PaCO₂ < 40 | Minutes – hours |
| Chronic resp. alkalosis | ΔHCO₃⁻ = −5 per 10 mm Hg PaCO₂ < 40 | ≥ 3 days |
PaCO₂ = 1.5 × HCO₃⁻ + 8 (± 2) (Winter's formula). Metabolic alkalosis: PaCO₂ = 0.7 × HCO₃⁻ + 21 (± 2). If measured PaCO₂ falls outside the expected band, suspect a mixed disorder.AG = Na⁺ − (Cl⁻ + HCO₃⁻). Normal 8–12 mEq/L. High AG = added acids (lactate, ketones, toxins — MUDPILES). Normal AG = bicarbonate loss or hyperchloremia.Comments are stored locally in your browser. Be kind and stay on topic.