Health
ABG Calculator (Arterial Blood Gas Interpreter)
Last updated: May 31, 2026
Written by Blake Boege
An ABG (Arterial Blood Gas) calculator is an educational interpreter for acid-base physiology. It classifies pH as acidemic or alkalemic, identifies the primary disorder (respiratory vs. metabolic), calculates respiratory compensation using Winter's formula or metabolic alkalosis standards, assesses renal compensation, and computes the anion gap when electrolytes are provided.
Interpret arterial blood gas results to evaluate acid-base disorders, check for respiratory or metabolic compensation, and calculate the anion gap using electrolytes.
Quick Answer
Interpret arterial blood gas (ABG) values. Enter pH, PaCO₂, and HCO₃⁻ to determine acidosis, alkalosis, compensation, and anion gap status.
Arterial Blood Gas Values
Normal: 7.35 - 7.45 · e.g. 7.40
Normal: 35 - 45 mmHg · e.g. 40
Normal: 22 - 26 mEq/L · e.g. 24
Oxygenation (Optional)
Normal: 80 - 100 mmHg
Anion Gap Electrolytes (Optional)
Normal: 135 - 145
Normal: 98 - 107
Interpretation
Normal Acid-Base Status
No mixed respiratory/metabolic compensation required
Reference values: pH: 7.35–7.45; PaCO₂: 35–45 mmHg; HCO₃⁻: 22–26 mEq/L.
Disclaimer: This calculator is for educational and training purposes only. It must not be used to direct patient care or substitute for clinical judgment.
Step-by-Step Diagnostic Steps
Examples
Metabolic acidosis with respiratory compensation: pH 7.20, PaCO2 25, HCO3 10
Acidosis · Metabolic · Expected PaCO2 = 1.5(10)+8 = 23 ± 2 (Actual 25 is compensated)
Respiratory acidosis: pH 7.28, PaCO2 55, HCO3 26
Acidosis · Respiratory · HCO3 is near normal (indicative of acute decompensation)
Mixed metabolic & respiratory alkalosis: pH 7.55, PaCO2 30, HCO3 30
Alkalosis · Combined Respiratory Alkalosis (low PaCO2) and Metabolic Alkalosis (high HCO3)
How it works
The interpretation of an ABG sample follows a structured diagnostic sequence:
- Look at the pH: Determine if there is acidemia (<7.35) or alkalemia (>7.45).
- Identify the primary driver: If pH is acidotic, look for a high PaCO₂ (respiratory) or a low HCO₃⁻ (metabolic). If alkalotic, look for a low PaCO₂ (respiratory) or a high HCO₃⁻ (metabolic).
- Evaluate compensation: Calculate the expected secondary response. For metabolic acidosis, we use Winter's formula.
- Check the anion gap (optional): An elevated anion gap indicates the presence of unmeasured anions (HAGMA).
Clinical Context & Anion Gap
For patient care, an elevated anion gap (above 12 mEq/L) is a vital clue in metabolic acidosis, pointing to causes like lactic acidosis, diabetic ketoacidosis, or ingestion of toxic substances (MUDPILES). Normal anion gap metabolic acidosis (NAGMA) is typically caused by bicarbonate loss, such as through diarrhea or renal tubular acidosis. Always correlate blood gas values with the patient's clinical presentation, history, and physical exam.
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Frequently asked questions
An Arterial Blood Gas (ABG) test measures the levels of oxygen, carbon dioxide, and bicarbonate in the blood, as well as blood pH. It is a critical tool used to evaluate respiratory function and identify metabolic and renal disturbances in acid-base balance.
Standard reference ranges for arterial blood are: pH: 7.35 – 7.45; PaCO₂: 35 – 45 mmHg; HCO₃⁻: 22 – 26 mEq/L; PaO₂: 80 – 100 mmHg. Values outside these ranges indicate potential acid-base or oxygenation abnormalities.
When a primary respiratory or metabolic disturbance occurs, the body attempts to return pH toward normal. Respiratory disorders are compensated by the kidneys adjusting HCO₃⁻ excretion (takes 2–5 days). Metabolic disorders are compensated quickly by the lungs adjusting ventilation to change PaCO₂ (takes minutes to hours).
Winter's formula is a mathematical model used in metabolic acidosis to calculate the expected PaCO₂ needed for adequate respiratory compensation: Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2. If the actual PaCO₂ is higher than expected, a primary respiratory acidosis is also present; if lower, a primary respiratory alkalosis is present.
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