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.
Also known as: ABG calculator, arterial blood gas calculator, ABG analyzer, arterial blood gas (ABG) calculator, blood gas calculator.
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 (ABG) represent a patient’s acid-base condition much more accurately when taken as a whole than when taken separately. An ABG calculator applies the typical 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; and then compare the other variable to a published compensation formula to determine whether the body’s reaction 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 prevalent in critically ill patients. By comparing predicted alveolar oxygen to what actually reached the artery, the A-a gradient helps this calculator pinpoint 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 swiftly work through textbook practice scenarios, check their own manual interpretation, or investigate how changing one variable affects the entire picture by using this calculator, which automatically executes every step of that reasoning as you type. Pair it with a standalone anion gap calculator when you only need that single piece.
How This Calculator Interprets Arterial Blood Gas Values
This ABG calculator adheres to the normal procedure for interpreting arterial blood gas results: categorize the pH, determine which value (PaCO₂ or HCO₃⁻) explains it, and then determine whether the body’s compensation is within the predicted range.
Step 1 — pH
7.35 to 7.45 is the typical range. Alkalemia is above that, and acidemia is below. This indicates the direction of the main issue, although if compensation is full, the underlying mechanism (metabolic or respiratory) may still be hidden at a normal pH.
Step 2 — primary disorder
Compare the pH direction with PaCO₂ (normal 35–45 mmHg) and HCO₃⁻ (normal 22–26 mEq/L). The principal disturbance is identified as the one that shifted in a way that explains the pH shift.
Step 3 — expected compensation
For example, Winter’s formula predicts the expected PaCO₂ in metabolic acidosis as 1.5 × HCO₃⁻ + 8 (±2). The body uses the other mechanism to partially adjust pH. A PaCO₂ well outside that range suggests a second, separate disorder.
Step 4 — anion gap & A-a gradient
Anion gap (Na⁺ − (Cl⁻ + HCO₃⁻)) aids in the cause-based classification of metabolic acidosis. To identify a respiratory oxygenation issue, this ABG calculator contrasts the expected and actual A-a gradient.
Arterial Blood Gas Calculator: Worked Examples
Three typical patterns worked end-to-end using the same formulas as the calculator above.
Elevated anion gap and 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?
Given inputs
- pH: 7.22
- PaCO₂: 26 mmHg
- HCO₃⁻: 10 mEq/L
- Na⁺ / Cl⁻: 138 / 100 mEq/L
Computed outputs
- 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?
Given inputs
- pH: 7.36
- PaCO₂: 60 mmHg
- HCO₃⁻: 33 mEq/L
Computed outputs
- 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 evaluation of the A-a gradient
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?
Given inputs
- pH: 7.52
- PaCO₂: 26 mmHg
- HCO₃⁻: 21 mEq/L
- PaO₂ / FiO₂ / age: 88 mmHg / 21% / 28
Computed outputs
- 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
Calculator Mistakes to Avoid
Stopping at a normal pH
An acid-base problem is not ruled out by a normal pH; full compensation can return pH to normal while PaCO₂ and HCO₃⁻ are still obviously abnormal. Never check pH alone; always check all three readings at once.
Skipping the compensation check
Identifying the fundamental disorder is not the final stage. A second, independent disorder is probably present if the “compensating” result deviates from the expected range; this confused picture alters management.
Forgetting to correct the anion gap for albumin
A true elevated-gap acidosis may be concealed by low albumin, which reduces the computed anion gap regardless of any acid-base activity. For every 1 g/dL of albumin below 4.0 g/dL, add roughly 2.5 mEq/L.
Confusing acute vs. chronic respiratory compensation
The same PaCO₂ predicts a considerably different expected HCO₃⁻ depending on whether the process is acute or chronic, since metabolic compensation for a respiratory disease develops over days; applying the wrong calculation results in a false “mixed disorder” flag.
Using the A-a gradient without adjusting for age or FiO₂
The typical A-a gradient is dependent on the proportion of inspired oxygen and widens with age. Without those adjustments, comparing a raw gradient to a predetermined “normal” number may incorrectly indicate a diffusion or shunt issue.
FAQ
What counts as a normal ABG?+
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.