Biochemistry · Topic
Acid–base interpretation (step by step) !
A fixed order for reading a blood gas: pH, primary cause, compensation, anion gap and mixed disorders.
Updated 2026-11-01
Key facts
- Always the same order: pH → is the primary disorder respiratory (pCO₂) or metabolic (HCO₃⁻)? → is compensation as expected? → anion gap → delta ratio → osmolar gap when relevant.
- Low pH with low HCO₃⁻ = metabolic acidosis; low pH with high pCO₂ = respiratory acidosis; high pH with high HCO₃⁻ = metabolic alkalosis; high pH with low pCO₂ = respiratory alkalosis.
- Compensation never fully corrects the pH or overshoots it: a normal pH with abnormal pCO₂ and HCO₃⁻ means a mixed disorder.
- Every metabolic acidosis needs an anion gap, corrected for albumin.
- Reference values for pH, pCO₂ and HCO₃⁻ vary by laboratory and analyser.
In detail
- Step 1. Consistency and context: check that the sample is arterial and that the pH fits the pCO₂ and HCO₃⁻ (the Henderson–Hasselbalch relation); read the history, because the cause is nearly always there (vomiting, diarrhoea, diabetes, COPD, sepsis, drugs).
- Step 2. pH: below the laboratory's range = acidaemia; above it = alkalaemia. A normal pH does not rule out a disorder: it may be mixed.
- Step 3. Primary disorder: find the variable that explains the pH. If HCO₃⁻ moves in the same direction as the pH, the origin is metabolic; if pCO₂ moves opposite to the pH, it is respiratory.
- Step 4. Expected compensation: in metabolic acidosis apply Winter's formula (expected pCO₂ = 1.5 × HCO₃⁻ + 8 ± 2); in respiratory disorders tell acute from chronic by how much HCO₃⁻ has changed. If the measured pCO₂ or HCO₃⁻ falls outside the expected value, a second disorder is present.
- Step 5. Anion gap (AG = Na⁺ − [Cl⁻ + HCO₃⁻]) in every metabolic acidosis, corrected for low albumin. High AG: unmeasured acids have been added (lactate, ketones, toxins, uraemia). Normal AG: bicarbonate has been lost or chloride retained (hyperchloraemic).
- Step 6. If the AG is high, the delta ratio (ΔAG/ΔHCO₃⁻) shows whether a normal-AG acidosis (low ratio) or a metabolic alkalosis (high ratio) is also present.
- Step 7. If the AG is high without a clear cause (not enough ketones, lactate or uraemia), calculate the osmolar gap to look for toxic alcohols (methanol, ethylene glycol).
- Step 8. In normal-AG acidosis the urine anion gap (urine Na⁺ + K⁺ − Cl⁻) separates gut loss (negative) from renal tubular acidosis (positive); in metabolic alkalosis, urine chloride separates volume-responsive from non-responsive causes.
- Also called
- Arterial blood gas interpretation, ABG reading, Acid–base algorithm
Clinical notes
- A venous gas is enough for pH and HCO₃⁻ in many stable patients, but it does not replace an arterial sample when oxygenation or an exact pCO₂ is needed.
- A high-AG metabolic acidosis and a metabolic alkalosis can coexist (for example ketoacidosis with vomiting) and leave the HCO₃⁻ almost normal: only the AG and delta ratio reveal it.
- Salicylate poisoning in adults typically gives an early respiratory alkalosis plus a high-AG metabolic acidosis.
Related conditions
- High anion gap metabolic acidosis
- Normal anion gap (hyperchloraemic) metabolic acidosis
- Metabolic alkalosis
- Respiratory acidosis
- Respiratory alkalosis
- Diabetic ketoacidosis
- Lactic acidosis
Related
- Bicarbonate–CO₂ buffer system
- Anion gap and delta ratio
- Serum osmolality and osmolar gap
- Acid–base compensation rules
- Lactate
- β-Hydroxybutyrate
Test yourself
Think of the answer, then tap the question to see it.
What does a normal pH with high pCO₂ and high HCO₃⁻ indicate?
A mixed disorder (respiratory acidosis plus metabolic alkalosis), because compensation alone never brings the pH back to normal.
What is Winter's formula used for?
It predicts the pCO₂ in metabolic acidosis (1.5 × HCO₃⁻ + 8 ± 2); a higher measured value means added respiratory acidosis, a lower one added respiratory alkalosis.
What is calculated in every metabolic acidosis, and why is it corrected?
The anion gap; it is corrected for albumin because hypoalbuminaemia lowers it and can hide unmeasured acids.
When is the osmolar gap calculated?
In a high-anion-gap acidosis without a clear explanation, to look for methanol or ethylene glycol.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Arterial Blood Gas (ABG) Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Blood gases: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Anion Gap Blood Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Acidosis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Alkalosis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.