ICCN Articles

How to Read an Arterial Blood Gas (ABG): A Step-by-Step Guide for Clinicians

By Javier Castaneda ·

The arterial blood gas (ABG) is one of the most information-dense tests in critical care, yet it intimidates many clinicians at the bedside. This guide breaks ABG interpretation into a repeatable, five-step method you can apply to almost any result in under a minute. It is written for nurses, respiratory therapists, students, and any clinician who wants a clear, practical framework.

What an ABG Measures

A standard ABG reports several values. The four you will use most are pH (how acidic or alkaline the blood is), PaCO2 (the respiratory component, reflecting ventilation), HCO3 (bicarbonate, the metabolic component), and PaO2 (the partial pressure of oxygen, reflecting oxygenation). You will also see SaO2 (oxygen saturation) and base excess.

Normal Values

  • pH: 7.35 to 7.45
  • PaCO2: 35 to 45 mmHg
  • HCO3: 22 to 26 mEq/L
  • PaO2: 80 to 100 mmHg on room air
  • SaO2: 95 to 100 percent

Step 1: Look at the pH

Start with the pH. A pH below 7.35 means acidemia; above 7.45 means alkalemia. Even when compensation has pulled the pH back toward normal, the side of 7.40 it sits on usually points to the primary problem.

Step 2: Identify the Primary Disturbance

Decide whether the problem is respiratory or metabolic by comparing PaCO2 and HCO3 against the pH. If PaCO2 moves in the opposite direction to the pH (pH down, PaCO2 up), the disturbance is respiratory. If HCO3 moves in the same direction as the pH (pH down, HCO3 down), it is metabolic. This yields four primary patterns: respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis.

Step 3: Check for Compensation

The body defends the pH. In a metabolic problem, the lungs compensate within minutes to hours by adjusting ventilation and therefore PaCO2. In a respiratory problem, the kidneys compensate over hours to days by retaining or excreting bicarbonate. Compensation moves the pH toward, but rarely fully back to, normal. For a metabolic acidosis, Winter’s formula predicts the expected respiratory compensation: expected PaCO2 = (1.5 x HCO3) + 8, plus or minus 2. A measured PaCO2 higher than predicted signals a concurrent respiratory acidosis; lower signals a concurrent respiratory alkalosis.

Step 4: Calculate the Anion Gap

Whenever you find a metabolic acidosis, calculate the anion gap: Anion Gap = Na minus (Cl + HCO3), normally about 8 to 12 mEq/L. A high anion gap points to added acids (mnemonic MUDPILES: methanol, uremia, diabetic ketoacidosis, propylene glycol, isoniazid or iron, lactic acidosis, ethylene glycol, salicylates). A normal anion gap points to bicarbonate loss, such as diarrhea or renal tubular acidosis.

Step 5: Assess Oxygenation

Finally, evaluate oxygenation using PaO2 and the patient’s inspired oxygen. The P/F ratio (PaO2 divided by FiO2) standardizes this: a value below 300 suggests acute hypoxemic respiratory failure, and below 200 indicates moderate-to-severe disease in the ARDS framework. Always interpret oxygenation in the context of how much supplemental oxygen the patient is receiving.

A Worked Example

Consider: pH 7.22, PaCO2 28 mmHg, HCO3 11 mEq/L, Na 140, Cl 100. The pH is low (acidemia). HCO3 is low and moves with the pH, so this is a metabolic acidosis. Winter’s formula predicts a PaCO2 of (1.5 x 11) + 8 = 24.5, plus or minus 2, so 22.5 to 26.5; the measured 28 is slightly high, suggesting an additional mild respiratory acidosis. The anion gap is 140 minus (100 + 11) = 29, a high anion gap. The differential now focuses on MUDPILES causes such as lactic acidosis or diabetic ketoacidosis.

Common Pitfalls

  • Treating the number instead of the patient; always correlate with the clinical picture.
  • Forgetting to calculate the anion gap in every metabolic acidosis.
  • Missing mixed disorders by skipping the compensation check.
  • Interpreting PaO2 without noting the FiO2.

The Bottom Line

A disciplined five-step approach (pH, primary disturbance, compensation, anion gap, oxygenation) turns the ABG from a wall of numbers into a clear clinical story. For deeper, evidence-based breakdowns of acid-base physiology and the rest of critical care, explore more from the Interprofessional Critical Care Network.