ABG Interpretation Pearls
A small set of high-yield rules and traps to turn an arterial blood gas from a wall of numbers into a story.
- Published
- 8 May 2026
- Read time
- 3 min read
A blood gas tells you three stories at once: ventilation, acid-base, and oxygenation. Read them in that order and the numbers fall into place.
Read in this order
- pH — acid or alkali? (Normal 7.35-7.45)
- PaCO2 — respiratory contribution? (Normal 4.7-6.0 kPa)
- HCO3- — metabolic contribution? (Normal 22-26 mmol/L)
- PaO2 — oxygenation, in the context of FiO2
- Lactate, base excess, anion gap — the modifiers
If pH is low and PaCO2 is high — respiratory acidosis.
If pH is low and HCO3- is low — metabolic acidosis.
If both are abnormal in the same direction — a mixed picture.
The anion gap pearl
For any metabolic acidosis, calculate the anion gap:
AG = (Na+ + K+) - (Cl- + HCO3-)
Normal ≈ 12 ± 2 mmol/L. A raised anion gap narrows the differential sharply.
Clinical pearl
Raised anion gap acidosis — think MUDPILES: Methanol, Uraemia, DKA, Paraldehyde / Propylene glycol, Iron / Isoniazid / INH, Lactate, Ethylene glycol, Salicylates.
Normal anion gap (hyperchloraemic) acidosis — think HARDUP: Hyperalimentation, Addison's, Renal tubular acidosis, Diarrhoea, Ureteric diversion, Pancreatic fistula.
The compensation rule
For every primary disturbance, the body partially compensates with the opposite system. Quick checks:
| Primary disorder | Expected compensation |
|---|---|
| Metabolic acidosis | PaCO2 ≈ (1.5 × HCO3-) + 1.0 (kPa) |
| Metabolic alkalosis | PaCO2 rises ~0.1 kPa per 1 mmol/L HCO3- rise |
| Acute respiratory acidosis | HCO3- rises 1 mmol/L per 1.3 kPa PaCO2 rise |
| Chronic respiratory acidosis | HCO3- rises ~3.5 mmol/L per 1.3 kPa PaCO2 rise |
If compensation is significantly off the predicted value, a second disturbance is hiding. This is the most powerful single trick in blood gas reading.
Oxygenation — use the A-a gradient, not just PaO2
A "normal" PaO2 on 60% oxygen is not normal. Calculate (or estimate) the A-a gradient:
A-a = (FiO2 × (Patm - PH2O)) - (PaCO2 / 0.8) - PaO2
A raised A-a gradient points to V/Q mismatch, shunt, or diffusion failure (pneumonia, PE, pulmonary oedema, ARDS). A normal A-a gradient with hypoxia points to hypoventilation or low FiO2.
Pitfalls
- Venous gases are not arterial. A venous pH runs ~0.03 lower, venous PaCO2 ~0.7 kPa higher, and venous PaO2 is meaningless. Use VBG for trends, ABG when oxygenation matters.
- "Normal" PaO2 on supplemental oxygen. Always note the FiO2.
- Type 2 respiratory failure with normal pH — chronic COPD with HCO3- in the 30s. Don't aggressively correct.
- Lactate from a tourniquet. Prolonged stasis at venepuncture spuriously raises lactate by 1-2 mmol/L.
Key points
- Read pH first, then ventilation, then metabolic, then oxygenation.
- Calculate the anion gap for every metabolic acidosis.
- Use the compensation rules to find hidden second disturbances.
- Use the A-a gradient, not raw PaO2, when oxygen is on.
References
- [1]Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434-1445. Link
About the author
Dr. Theo LindqvistVerifiedVerified medical practitioner
MBChB, FRCEM
Emergency medicine consultant with a teaching role for foundation and core trainees. Interests in resuscitation, sepsis, and time-critical decisions.