How do you interpret an ABG for acid-base balance?

Start with pH to identify acidemia or alkalemia, then compare PaCO2 and HCO3 to determine whether the primary process is respiratory or metabolic. Finally, check compensation and anion gap when relevant.

What is the anion gap formula?

The common anion gap formula is sodium minus chloride plus bicarbonate: AG = Na - (Cl + HCO3). Some courses include potassium: AG = Na + K - (Cl + HCO3).

What is Winter's formula used for?

Winter's formula estimates the expected PaCO2 in metabolic acidosis to check whether respiratory compensation is appropriate.

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Acid-Base Balance Calculator

Interpret arterial blood gases, identify acidosis or alkalosis, check respiratory or metabolic compensation, calculate anion gap, and learn the step-by-step A&P reasoning.

Background

Acid-base balance depends mainly on blood pH, carbon dioxide controlled by the respiratory system, and bicarbonate regulated by the kidneys. This calculator helps students connect ABG numbers to the body system driving the imbalance.

Interpret acid-base balance

Choose solve mode

Start with ABG interpretation, or focus on anion gap, compensation, or a study drill.

ABG values

Typical reference ranges used here: pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L.

PaCO₂ (mmHg)

HCO₃⁻ (mEq/L)

Clinical context

Student mode or clinical mode

Student mode keeps the result focused on A&P class interpretation. Clinical mode adds expected compensation, mixed-disorder warnings, Winter's formula, and delta ratio.

Interpretation depth

Normal pH

7.35–7.45

Normal PaCO₂

35–45 mmHg

Normal HCO₃⁻

22–26 mEq/L

Typical anion gap

~8–12 mEq/L

Electrolytes for anion gap

Optional, but useful for metabolic acidosis. Potassium can be included or excluded depending on your course convention.

Na⁺ (mEq/L)

Cl⁻ (mEq/L)

K⁺ (mEq/L)

Albumin (g/dL, optional)

Anion gap formula

Learning options

Show visual ABG map

Show reference-range bars

Show interpretation table

Show step-by-step explanation

Show common mistakes panel

Quick examples (click to see result)

Result

No result yet. Enter ABG values, then click Calculate.

How to use this calculator

Enter pH, PaCO₂, and HCO₃⁻ from an arterial blood gas problem.
Add Na⁺, Cl⁻, K⁺, and albumin if you want anion gap and corrected anion gap interpretation.
Click Calculate to classify the disorder, identify the likely primary system, and see compensation logic.
Use the quick examples to practice common A&P exam patterns.

How this calculator works

The calculator first decides whether the pH shows acidemia, alkalemia, or a near-normal compensated state. Then it checks whether PaCO₂ points in the respiratory direction and whether HCO₃⁻ points in the metabolic direction. Finally, it checks expected compensation and anion gap to make the interpretation more student-friendly.

Formulas & Equations Used

Anion gap: AG = Na⁺ − (Cl⁻ + HCO₃⁻)

Anion gap with potassium: AG = Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻)

Albumin-corrected AG: Corrected AG = AG + 2.5 × (4.0 − albumin)

Winter's formula: Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2

Metabolic alkalosis compensation: Expected PaCO₂ ≈ 0.7 × (HCO₃⁻ − 24) + 40 ± 5

Delta ratio: (Corrected AG − 12) / (24 − HCO₃⁻)

Example Problems & Step-by-Step Solutions

Example 1: Respiratory acidosis

Problem: pH 7.28, PaCO₂ 55 mmHg, HCO₃⁻ 25 mEq/L.

pH is below 7.35, so the blood is acidemic.
PaCO₂ is high. CO₂ behaves as an acid in the blood, so high CO₂ supports acidosis.
HCO₃⁻ is near normal, so the kidneys have not strongly compensated yet.
Conclusion: Primary respiratory acidosis, likely uncompensated or minimally compensated.

Example 2: Metabolic acidosis with high anion gap

Problem: pH 7.22, PaCO₂ 25, HCO₃⁻ 10, Na⁺ 140, Cl⁻ 100.

pH is low, so this is acidemia.
HCO₃⁻ is low, so the primary problem is metabolic acidosis.
PaCO₂ is also low, which shows respiratory compensation by blowing off CO₂.
Anion gap = 140 − (100 + 10) = 30, which is elevated.
Conclusion: High anion gap metabolic acidosis with respiratory compensation.

Example 3: Metabolic alkalosis

Problem: pH 7.50, PaCO₂ 48, HCO₃⁻ 34.

pH is high, so this is alkalemia.
HCO₃⁻ is high, so the primary process is metabolic alkalosis.
PaCO₂ is high, which fits respiratory compensation through hypoventilation.
Conclusion: Metabolic alkalosis with respiratory compensation.

Example 4: Respiratory alkalosis

Problem: pH 7.52, PaCO₂ 28, HCO₃⁻ 23.

pH is high, so this is alkalemia.
PaCO₂ is low. Less CO₂ means less carbonic acid, so this supports alkalosis.
HCO₃⁻ is near normal, so compensation is limited.
Conclusion: Primary respiratory alkalosis.

Frequently Asked Questions

What does PaCO₂ represent in acid-base balance?

PaCO₂ reflects the respiratory side. High PaCO₂ pushes toward acidosis, while low PaCO₂ pushes toward alkalosis.

What does HCO₃⁻ represent?

HCO₃⁻ reflects the metabolic and renal side. Low bicarbonate supports metabolic acidosis, while high bicarbonate supports metabolic alkalosis.

Can pH be normal even when there is an acid-base disorder?

Yes. A near-normal pH with abnormal PaCO₂ and HCO₃⁻ often means compensation or a mixed disorder. Always check all three ABG values.