[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Acid–base interpretation (step by step) [Biochemistry](https://durgical.com/en/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](https://durgical.com/en/conditions/high-anion-gap-metabolic-acidosis/) - [Normal anion gap (hyperchloraemic) metabolic acidosis](https://durgical.com/en/conditions/normal-anion-gap-metabolic-acidosis/) - [Metabolic alkalosis](https://durgical.com/en/conditions/metabolic-alkalosis/) - [Respiratory acidosis](https://durgical.com/en/conditions/respiratory-acidosis/) - [Respiratory alkalosis](https://durgical.com/en/conditions/respiratory-alkalosis/) - [Diabetic ketoacidosis](https://durgical.com/en/conditions/diabetic-ketoacidosis/) - [Lactic acidosis](https://durgical.com/en/conditions/lactic-acidosis/) ## Related - [Bicarbonate–CO₂ buffer system](https://durgical.com/en/biochemistry/topics/bicarbonate-co-buffer-system/) - [Anion gap and delta ratio](https://durgical.com/en/biochemistry/topics/anion-gap-and-delta-ratio/) - [Serum osmolality and osmolar gap](https://durgical.com/en/biochemistry/topics/serum-osmolality-and-osmolar-gap/) - [Acid–base compensation rules](https://durgical.com/en/biochemistry/topics/acid-base-compensation-rules/) - [Lactate](https://durgical.com/en/biochemistry/molecules/lactate/) - [β-Hydroxybutyrate](https://durgical.com/en/biochemistry/molecules/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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/biochemistry/topics/acid-base-interpretation/) Educational information, not medical advice. For any symptom, see a health professional. Sources: [Arterial Blood Gas (ABG) Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/arterial-blood-gas-abg-test/) (MedlinePlus, US National Library of Medicine, 2026); [Blood gases: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003855.htm) (MedlinePlus, US National Library of Medicine, 2026); [Anion Gap Blood Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/anion-gap-blood-test/) (MedlinePlus, US National Library of Medicine, 2026); [Acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001181.htm) (MedlinePlus, US National Library of Medicine, 2026); [Alkalosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001183.htm) (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). [← All of biochemistry](https://durgical.com/en/biochemistry/) ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/biochemistry/topics/acid-base-interpretation/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/topics/acid-base-interpretation/