Arterial blood gas (ABG) interpretation tells you how well a patient is oxygenating, ventilating, and buffering acid. Memorizing values is not enough. Use the same three steps every time.
Normal Values
Parameter | Normal Range | What It Tells You |
|---|---|---|
pH | 7.35 – 7.45 | Overall acid-base state |
PaCO₂ | 35 – 45 mmHg | Respiratory component |
HCO₃⁻ | 22 – 26 mEq/L | Metabolic component |
PaO₂ | 80 – 100 mmHg | Oxygenation |
SaO₂ | > 95% | Oxygen saturation |
Step 1: Check the pH
- pH < 7.35: acidemia
- pH > 7.45: alkalemia
A normal pH does not rule out a problem. It can mean a fully compensated disorder or a mixed disorder.
Step 2: Find the Primary Driver
Ask which value explains the pH.
Disorder | pH | Driver |
|---|---|---|
Respiratory acidosis | ↓ | PaCO₂ ↑ (hypoventilation) |
Respiratory alkalosis | ↑ | PaCO₂ ↓ (hyperventilation) |
Metabolic acidosis | ↓ | HCO₃⁻ ↓ |
Metabolic alkalosis | ↑ | HCO₃⁻ ↑ |
Step 3: Check Compensation
The opposite system tries to bring the pH back toward normal.
- Lungs (fast, minutes to hours): change PaCO₂ in response to metabolic disorders.
- Kidneys (slow, 24 to 72 hours): change HCO₃⁻ in response to respiratory disorders.
Stages:
- Uncompensated: pH abnormal, compensating value still normal.
- Partially compensated: compensating value abnormal, pH still abnormal.
- Fully compensated: pH back in the normal range.
Tip: If the pH is normal, look at which side of 7.40 it sits on. A pH of 7.36 means the primary problem was acidosis.
In metabolic acidosis, use Winter’s formula to check the respiratory response: expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2. If the measured PaCO₂ is higher, a respiratory acidosis is also present. If lower, a respiratory alkalosis is also present.
Anion Gap in Metabolic Acidosis
Whenever you see metabolic acidosis, calculate the anion gap.
Anion Gap = Na⁺ − (Cl⁻ + HCO₃⁻)
Normal: 8 – 12 mEq/L
High Anion Gap (acid added): MUDPILES
- Methanol
- Uremia
- DKA
- Propylene glycol
- Isoniazid / Iron
- Lactic acidosis
- Ethylene glycol
- Salicylates
Normal Anion Gap (bicarbonate lost, chloride rises)
- Diarrhea (GI bicarbonate loss)
- Renal tubular acidosis
- Large-volume normal saline
Quick Summary
- pH: acidemia or alkalemia?
- Driver: PaCO₂ or HCO₃⁻?
- Compensation: none, partial, or full?
- If metabolic acidosis: calculate the anion gap.
References
Albert, M. S., Dell, R. B., & Winters, R. W. (1967). Quantitative displacement of acid-base equilibrium in metabolic acidosis. Annals of Internal Medicine, 66(2), 312-322. https://doi.org/10.7326/0003-4819-66-2-312
Berend, K., de Vries, A. P. J., & Gans, R. O. B. (2014). Physiological approach to assessment of acid-base disturbances. New England Journal of Medicine, 371(15), 1434-1445. https://doi.org/10.1056/NEJMra1003327
Kraut, J. A., & Madias, N. E. (2007). Serum anion gap: Its uses and limitations in clinical medicine. Clinical Journal of the American Society of Nephrology, 2(1), 162-174. https://doi.org/10.2215/CJN.03020906
Kraut, J. A., & Madias, N. E. (2012). Differential diagnosis of nongap metabolic acidosis: Value of a systematic approach. Clinical Journal of the American Society of Nephrology, 7(4), 671-679. https://doi.org/10.2215/CJN.09450911