How to Interpret ABGs: A Step by Step Method for Nursing Students
To interpret an ABG, look at the pH first, then decide whether the PaCO2 or the bicarbonate explains it, then check for compensation. Learn the normal ranges by heart and practice on real values until the steps are automatic.
The ranges below are the ones most nursing textbooks use. Your program may teach slightly different ones, so check your own course materials and use those on your exam.
Step 1: Learn the normal ranges
- pH: 7.35 to 7.45
- PaCO2: 35 to 45 mmHg
- HCO3: 22 to 26 mEq/L
A pH below 7.35 is acidosis and above 7.45 is alkalosis. PaCO2 is the respiratory part, and it acts as an acid. HCO3 is the metabolic part, and it acts as a base.
Step 2: Read the pH
Decide whether the pH is acidic, alkaline or normal. If it is normal, note which side of 7.40 it sits on, because that tells you where the problem started in a fully compensated result.
Step 3: Find the cause
Check the PaCO2 and the HCO3 one at a time. The value that moved in the direction that explains the pH is the cause.
A memory aid many students use is ROME: Respiratory Opposite, Metabolic Equal. In a respiratory problem, the pH and the PaCO2 move in opposite directions. In a metabolic problem, the pH and the HCO3 move in the same direction.
Step 4: Check for compensation
Look at the value you did not pick as the cause.
- If it is normal, the result is uncompensated.
- If it has moved the other way and the pH is still abnormal, it is partially compensated.
- If it has moved the other way and the pH is back in the normal range, it is fully compensated.
Step 5: Practice until it is automatic
Work through a set of practice values every day in the week before your exam, writing out all four steps each time. Mistakes usually come from skipping step 2 or from reading HCO3 backwards, and writing the steps makes both easy to spot.
Practice questions
Try these before you look at the answers.
- pH 7.30, PaCO2 50, HCO3 24
- pH 7.50, PaCO2 40, HCO3 30
- pH 7.36, PaCO2 30, HCO3 16
Answers: 1. Respiratory acidosis, uncompensated, because the PaCO2 is high and the HCO3 is normal. 2. Metabolic alkalosis, uncompensated, because the HCO3 is high and the PaCO2 is normal. 3. Metabolic acidosis, fully compensated, because the pH is normal on the acid side, the HCO3 is low and the PaCO2 has dropped to make up for it.