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ABG interpretation in 6 steps
A repeatable 6-step method for reading arterial blood gases on the RT board exam, with worked examples. Pair it with our free ABG interpretation calculator to check your answers.
Normal values
| Value | Normal range |
|---|---|
| pH | 7.35–7.45 |
| PaCO2 | 35–45 mm Hg |
| HCO3− | 22–26 mEq/L |
| PaO2 (room air, adult) | 80–100 mm Hg |
Reference ranges vary slightly by source and lab.[1]
The 6 steps
- pH: below 7.35 is acidemia, above 7.45 is alkalemia.[1]
- Respiratory or metabolic? Find the value that explains the pH. High PaCO2 causes acidosis and low PaCO2 alkalosis (respiratory); low HCO3 causes acidosis and high HCO3 alkalosis (metabolic).[2]
- Compensation? The other system moves in the same direction as the primary change to pull the pH back. pH still abnormal = partially compensated; pH normal = fully compensated (judge by which side of 7.40 it sits on).[3]
- Is the compensation appropriate? For metabolic acidosis use Winter's formula: expected PaCO2 = 1.5 × HCO3 + 8 (± 2). Higher than expected suggests an added respiratory acidosis; lower suggests an added respiratory alkalosis.[4]
- Metabolic acidosis? Check the anion gap: Na − (Cl + HCO3). A high gap points to added acids (for example lactate or ketones).[5]
- Oxygenation: assess PaO2 against FiO2 (P/F ratio, A–a gradient) and SpO2 vs SaO2. Use our P/F and A–a gradient calculators.
Practice ABGs free
Worked examples
pH 7.32, PaCO2 60, HCO3 30. Acidemia; high CO2 explains it (respiratory); HCO3 is elevated, moving the same direction to compensate; pH still abnormal. Partially compensated respiratory acidosis.
pH 7.25, PaCO2 28, HCO3 12. Acidemia; low HCO3 explains it (metabolic). Winter's: 1.5 × 12 + 8 = 26 (24–28). Measured 28 is within range: metabolic acidosis with appropriate respiratory compensation.
pH 7.50, PaCO2 30, HCO3 23. Alkalemia; low CO2 explains it; HCO3 normal. Uncompensated (acute) respiratory alkalosis.
Study content only, not clinical advice.
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