ABG Interpretation Guide
Stepwise interpretation of arterial blood gas. Identifies primary acid-base disorder and degree of compensation.
Score interpretation
pH 7.35–7.45, PaCO₂ 4.7–6.0 kPa and HCO₃⁻ 22–26 mmol/L.
→ No acid–base disturbance. Assess oxygenation separately: PaO₂ < 8 kPa = respiratory failure (type 2 if PaCO₂ > 6.0 kPa).
pH within range but PaCO₂ and/or HCO₃⁻ abnormal.
→ Interpret with the history: chronic compensated disorder or two opposing disorders. Assess oxygenation separately: PaO₂ < 8 kPa = respiratory failure (type 2 if PaCO₂ > 6.0 kPa).
Low pH with raised PaCO₂ and HCO₃⁻ not raised — acute respiratory acidosis.
→ Identify cause: COPD exacerbation, acute respiratory failure, sedation, chest wall disorder. NIV if appropriate. Target PaCO₂ 5.5–6.5 kPa in chronic CO₂ retainers.
Low pH with raised PaCO₂ and raised HCO₃⁻ — chronic or acute-on-chronic respiratory acidosis.
→ Identify cause: COPD exacerbation, acute respiratory failure, sedation, chest wall disorder. NIV if appropriate. Target PaCO₂ 5.5–6.5 kPa in chronic CO₂ retainers.
Low pH with low HCO₃⁻ and PaCO₂ not low.
→ Calculate the anion gap. Identify cause using MUDPILES (methanol, uraemia, DKA, propylene glycol, INH, lactic acidosis, ethylene glycol, salicylates). Treat underlying cause.
Low pH with low HCO₃⁻ and low PaCO₂.
→ Calculate the anion gap. Identify cause using MUDPILES (methanol, uraemia, DKA, propylene glycol, INH, lactic acidosis, ethylene glycol, salicylates). Treat underlying cause.
High pH with low PaCO₂ and HCO₃⁻ not low.
→ Identify cause: anxiety, pain, sepsis, PE, CNS disorder, pregnancy, salicylate toxicity. Treat underlying cause.
High pH with low PaCO₂ and low HCO₃⁻.
→ Identify cause: anxiety, pain, sepsis, PE, CNS disorder, pregnancy, salicylate toxicity. Treat underlying cause.
High pH with raised HCO₃⁻ and PaCO₂ not raised.
→ Identify cause: vomiting, diuretics, hypokalaemia, Bartter syndrome. Chloride-responsive vs resistant. Treat the cause; correct volume and potassium depletion.
High pH with raised HCO₃⁻ and raised PaCO₂.
→ Identify cause: vomiting, diuretics, hypokalaemia, Bartter syndrome. Chloride-responsive vs resistant. Treat the cause; correct volume and potassium depletion.
Low pH with raised PaCO₂ and low HCO₃⁻ — both processes are lowering the pH.
→ Treat both components: ventilatory support as needed and the cause of the metabolic acidosis. Assess oxygenation separately: PaO₂ < 8 kPa = respiratory failure (type 2 if PaCO₂ > 6.0 kPa).
High pH with low PaCO₂ and raised HCO₃⁻ — both processes are raising the pH.
→ Identify and treat both causes. Assess oxygenation separately: PaO₂ < 8 kPa = respiratory failure (type 2 if PaCO₂ > 6.0 kPa).
The pH is abnormal but the PaCO₂ and HCO₃⁻ do not account for it.
→ Check the values were entered correctly and repeat the sample if needed.
Interpretation bands for the ABG Interpretation. Apply clinical judgement and local guidance.
References
- Kellum JA. Determinants of blood pH in health and disease. Crit Care. 2000;4(1):6-14.
- Berend K et al. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434-1445.
Related
Curated clinical cross-links plus same-class fallbacks.
- AIR Score for Appendicitis · Diagnostic
- Spontaneous Bacterial Peritonitis (SBP) Diagnosis · Diagnostic
- Alvarado Score (MANTRELS) for Appendicitis · Diagnostic
- New Orleans Head CT Decision Rule · Diagnostic
- MS Phenotype Classification (McDonald / Phenotype) · Diagnostic
- CHALICE Rule for Childhood Head Injury · Diagnostic
- Noradrenaline (Norepinephrine) · Vasopressor (Alpha-1 and Beta-1 Agonist)
- Tranexamic Acid (ICU/Trauma/Surgical) · Antifibrinolytic
- Doxapram hydrochloride · Respiratory stimulant
- Trimetazidine · Metabolic Anti-Anginal
- Selexipag · Pulmonary Arterial Hypertension
- Macitentan · Pulmonary Arterial Hypertension
- Difficult Airway Algorithm (DAS) · DAS 2015; Royal College of Anaesthetists
- Major Haemorrhage Protocol · NICE NG24; UK MHP guidelines
- New-Onset Atrial Fibrillation · ESC 2020 AF Guidelines; NICE NG196
- Hypertensive Emergency · ESC/ESH 2018 Hypertension Guidelines; NICE NG136
- Bradycardia Management · Resuscitation Council UK ABCDE; ESC 2021 Pacing Guidelines
- Ventricular Tachycardia / Fibrillation · Resuscitation Council UK ACLS; ESC 2022 Ventricular Arrhythmia Guidelines
Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.