Step 1: Check pH
Always start with pH. If pH < 7.35 = acidosis. If pH > 7.45 = alkalosis.
Step 2: Identify Primary Disorder
Compare pH with PaCO2 (respiratory) and HCO3- (metabolic). The component that moves in the same direction as pH is the primary disorder.
Step 3: Check Compensation
Check if the compensation is appropriate. Winter's formula is key for metabolic acidosis compensation.
Step 4: Calculate Anion Gap
Anion Gap = Na+ - (Cl- + HCO3-). Normal: 8-12 mEq/L.
Step 5: Delta Gap / Delta Ratio
Delta gap = AG - 12. Delta HCO3 = 24 - measured HCO3-. Delta ratio = delta gap / delta HCO3.
Clinical Case Walkthroughs
Practice with real ABG cases from UPSC CMS previous year papers to master this framework.
References & further reading
Every clinical claim in this article is anchored to a primary source. Click through to verify, and use the same habit in your revision.
Revision history
- 19 September 2026 — Initial publication
Frequently asked questions
What is the first step in acid-base interpretation?
Always start with pH. If pH < 7.35 = acidosis. If pH > 7.45 = alkalosis. Then look at PaCO2 (respiratory component) and HCO3- (metabolic component).
What is the anion gap and how is it calculated?
Anion Gap = Na+ - (Cl- + HCO3-). Normal range is 8-12 mEq/L. High anion gap metabolic acidosis has causes: MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates).
What is Winter's formula?
Winter's formula: Expected PaCO2 = (1.5 x HCO3-) + 8 +/- 2. Used to check if respiratory compensation is appropriate in metabolic acidosis. If measured PaCO2 is higher than expected = concurrent respiratory acidosis.
Related reading
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