Arterial blood gases look intimidating because they hand you six numbers at once. The fix is not memorizing more — it is reading them in the same order, every time. Here is the sequence I teach.
1. Look at the pH first
The pH tells you which way the patient has gone overall.
- pH < 7.35 — acidemia
- pH > 7.45 — alkalemia
- 7.35–7.45 — normal, or a mixed disorder that happens to cancel out
Do not skip that last point. A normal pH with abnormal PaCO₂ and HCO₃⁻ is a mixed disorder until proven otherwise.
2. Identify the primary process
Ask which value explains the pH.
| Disorder | pH | Primary change |
|---|---|---|
| Respiratory acidosis | ↓ | PaCO₂ ↑ |
| Respiratory alkalosis | ↑ | PaCO₂ ↓ |
| Metabolic acidosis | ↓ | HCO₃⁻ ↓ |
| Metabolic alkalosis | ↑ | HCO₃⁻ ↑ |
3. Check whether compensation is appropriate
For a metabolic acidosis, use Winter’s formula:
Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2
If the measured PaCO₂ is higher than expected, there is a concurrent respiratory acidosis. If it is lower, a concurrent respiratory alkalosis.
4. Calculate the anion gap
In any metabolic acidosis, calculate the gap:
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)
A typical reference range is roughly 8–12 mmol/L, but check your own lab. Hypoalbuminemia hides a raised gap — add about 2.5 for every 1 g/dL that albumin falls below 4.
5. If the gap is high, check the delta ratio
The delta ratio compares the rise in the gap to the fall in bicarbonate:
Δ ratio = (AG − 12) ÷ (24 − HCO₃⁻)
- < 1 — a coexisting normal-gap acidosis
- 1–2 — a pure high-gap acidosis
- > 2 — a coexisting metabolic alkalosis
The point of a system
None of these steps is difficult on its own. The value comes from running them in order, so that a second disorder never hides behind the first.
This article is for education only and is not a substitute for clinical judgment or local protocols.