ABG Calculator
This ABG calculator walks you step-by-step through arterial blood gas interpretation — acid-base status, primary disorder, expected compensation, anion gap, and A-a gradient — using established reference formulas.
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ABG interpretation is stepwise: classify the pH as acidemia, alkalemia, or normal; check whether PaCO₂ (respiratory) or HCO₃⁻ (metabolic) explains that direction to find the primary disorder; then compare the other value to its expected compensation formula. When measured compensation falls outside the predicted range, suspect a second, mixed disorder.
1.5 × HCO₃⁻ + 8 = expected PaCO₂Interactive step-by-step proof
- Enter values above to see the calculation unfold step by step.
Why Use an ABG Calculator for Arterial Blood Gas Interpretation
The pH, PaCO₂, and HCO₃⁻ readings from an arterial blood gas represent a patient’s acid-base condition far more accurately taken together than taken separately. An ABG calculator applies the standard teaching method, stepwise interpretation, automatically: determine whether the pH is acidemic, alkalemic, or normal; determine whether the respiratory variable PaCO₂ or the metabolic variable HCO₃⁻ explains that direction; then compare the other variable to a published compensation formula to check whether the body’s response is consistent with a single, simple disorder.
A mixed disorder, two separate acid-base processes occurring simultaneously, should be suspected when measured compensation deviates from the predicted range — this is common in critically ill patients. Comparing predicted alveolar oxygen to what actually reached the artery, the A-a gradient helps this calculator flag a respiratory oxygenation issue, and layering in the anion gap (Na⁺ − (Cl⁻ + HCO₃⁻)) helps further categorize a metabolic acidosis by cause.
Students and clinicians can work through textbook practice scenarios quickly, check a manual interpretation, or explore how changing one variable shifts the entire picture, since this calculator runs every step of that reasoning as you type.
How This Blood Gas Interpretation Calculator Works
This ABG calculator follows the standard procedure for interpreting arterial blood gas results: categorize the pH, determine which value (PaCO₂ or HCO₃⁻) explains it, then determine whether the body’s compensation falls within the predicted range.
Step 1 — pH
7.35 to 7.45 is the typical range. Alkalemia sits above that, acidemia below. This indicates the direction of the main issue, though if compensation is full, the underlying mechanism may still hide behind a normal pH.
Step 2 — primary disorder
Compare the pH direction with PaCO₂ (normal 35–45 mmHg) and HCO₃⁻ (normal 22–26 mEq/L). Whichever shifted in a way that explains the pH change is the principal disturbance.
Step 3 — expected compensation
Winter’s formula, for example, predicts expected PaCO₂ in metabolic acidosis as 1.5 × HCO₃⁻ + 8 (±2). The body partially adjusts pH using the other mechanism. A PaCO₂ well outside that range suggests a second, separate disorder.
Step 4 — anion gap & A-a gradient
Anion gap (Na⁺ − (Cl⁻ + HCO₃⁻)) helps classify metabolic acidosis by cause. This calculator compares expected vs. actual A-a gradient to flag a respiratory oxygenation issue.
ABG Calculator: Three Classic Patterns
Common patterns worked end-to-end using the same formulas as the calculator above.
Elevated anion gap, uncompensated metabolic acidosis
A patient presents with pH 7.22, PaCO₂ 26 mmHg, HCO₃⁻ 10 mEq/L, Na⁺ 138 mEq/L, Cl⁻ 100 mEq/L. What’s the interpretation?
Inputs
- pH: 7.22
- PaCO₂: 26 mmHg
- HCO₃⁻: 10 mEq/L
- Na⁺ / Cl⁻: 138 / 100 mEq/L
Result
- Status: Acidemia
- Primary disorder: Metabolic acidosis
- Expected PaCO₂ (Winter’s): 21–25 mmHg — measured 26 is just outside, borderline mixed picture
- Anion gap: 28 mEq/L (high — suggests an unmeasured acid, e.g. lactate or ketones)
Compensated respiratory acidosis
A patient with COPD has pH 7.36, PaCO₂ 60 mmHg, HCO₃⁻ 33 mEq/L. Is this acute or chronic?
Inputs
- pH: 7.36
- PaCO₂: 60 mmHg
- HCO₃⁻: 33 mEq/L
Result
- Status: Normal pH (near lower edge)
- Primary disorder: Compensated respiratory acidosis
- Acute expected HCO₃⁻: ≈26 · Chronic expected HCO₃⁻: ≈31
- Measured HCO₃⁻ of 33 tracks closest to the chronic pattern, consistent with long-standing CO₂ retention
Acute respiratory alkalosis with A-a gradient check
A panicked patient on room air has pH 7.52, PaCO₂ 26 mmHg, HCO₃⁻ 21 mEq/L, PaO₂ 88 mmHg, age 28. What’s driving the low PaCO₂, and is oxygenation impaired?
Inputs
- pH: 7.52
- PaCO₂: 26 mmHg
- HCO₃⁻: 21 mEq/L
- PaO₂ / FiO₂ / age: 88 mmHg / 21% / 28
Result
- Status: Alkalemia
- Primary disorder: Respiratory alkalosis (e.g. hyperventilation/anxiety)
- Acute expected HCO₃⁻: ≈21 — measured value fits an acute, uncompensated picture
- A-a gradient: ≈9 mmHg (expected upper limit ≈11 for age 28) — normal, no oxygenation defect
ABG Interpretation Mistakes to Avoid
Stopping at a normal pH
A normal pH doesn’t rule out an acid-base problem; full compensation can return pH to normal while PaCO₂ and HCO₃⁻ are still clearly abnormal. Always check all three readings together, never pH alone.
Skipping the compensation check
Identifying the primary disorder isn’t the final step. A second, independent disorder is likely present if the “compensating” value falls outside the expected range — this mixed picture changes management.
Forgetting to correct the anion gap for albumin
Low albumin reduces the computed anion gap regardless of any acid-base activity, which can conceal a true elevated-gap acidosis. Add roughly 2.5 mEq/L for every 1 g/dL of albumin below 4.0 g/dL.
Confusing acute vs. chronic respiratory compensation
The same PaCO₂ predicts a very different expected HCO₃⁻ depending on whether the process is acute or chronic, since metabolic compensation develops over days. Applying the wrong one produces a false “mixed disorder” flag.
Using the A-a gradient without adjusting for age or FiO₂
The typical A-a gradient depends on the fraction of inspired oxygen and widens with age. Comparing a raw gradient to a fixed “normal” number without those adjustments can wrongly suggest a diffusion or shunt problem.
ABG Calculator: Frequently Asked Questions
What counts as a normal ABG?+
Is a “blood gas calculator” the same as this ABG calculator?+
Why can pH be normal with an abnormal PaCO₂ and HCO₃⁻?+
Why does the compensation check matter?+
Why does albumin affect the anion gap?+
How do I know if respiratory compensation is acute or chronic?+
Is this calculator a substitute for clinical judgment?+
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ABG Calculator: Formula References
- Burger M, Schaller DJ. Metabolic Acidosis. StatPearls [Internet]. NCBI Bookshelf, National Library of Medicine.
- Patel S, Majmundar SH. Respiratory Acidosis. StatPearls [Internet]. NCBI Bookshelf, National Library of Medicine.
- Alkalosis. StatPearls [Internet]. NCBI Bookshelf, National Library of Medicine.
- Pandey DG, Sharma S. Biochemistry, Anion Gap. StatPearls [Internet]. NCBI Bookshelf, National Library of Medicine.
- Hantzidiamantis PJ, Amaro E. Physiology, Alveolar to Arterial Oxygen Gradient. StatPearls [Internet]. NCBI Bookshelf, National Library of Medicine.
- Winter’s Formula for Metabolic Acidosis Compensation. MDCalc.