How the analyser works
- Check the sample: the Henderson equation (H⁺ = 24 × PaCO₂ ÷ HCO₃⁻), agreement between ABG and serum bicarbonate, and plausible ranges.
- Identify the primary disorder from the pH, PaCO₂ and HCO₃⁻.
- Check compensation: Winters formula for metabolic acidosis, the metabolic alkalosis rule, and acute or chronic respiratory rules.
- Calculate the anion gap, corrected for albumin, and the delta gap and delta ratio to uncover a second metabolic process.
- Calculate the osmolal gap when sodium, glucose, BUN and measured osmolality are available.
- Apply DKA and HHS criteria (2024 hyperglycemic crises consensus) and, for normal anion gap acidosis, the RTA work-up with urine studies, kidney function and potassium.
- List likely causes and the next tests to confirm or exclude each diagnosis.
Key formulas
- Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻); albumin-corrected AG = AG + 2.5 × (4.0 − albumin g/dL)
- Delta ratio = (AG − 12) ÷ (24 − HCO₃⁻): < 0.4 normal-gap acidosis dominates, 0.4–0.8 combined high- and normal-gap acidosis, 0.8–2.0 uncomplicated high-gap acidosis, > 2.0 coexisting metabolic alkalosis (or chronic respiratory acidosis)
- Winters formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 (± 2)
- Metabolic alkalosis: expected PaCO₂ ≈ 0.7 × HCO₃⁻ + 21 (± 2)
- Respiratory acidosis: HCO₃⁻ rises 1 (acute) or 4 (chronic) mEq/L per 10 mmHg rise in PaCO₂ (± 2)
- Respiratory alkalosis: HCO₃⁻ falls 2 (acute) or 4 (chronic) mEq/L per 10 mmHg fall in PaCO₂ (± 2)
- Urine anion gap = urine Na⁺ + K⁺ − Cl⁻: negative (or urine osmolal gap > 100) means ammonium is being excreted, as in GI loss or proximal RTA; positive (or urine osmolal gap < 40) means impaired ammonium excretion, as in distal or type 4 RTA or CKD
Frequently asked questions
- How do you interpret an ABG step by step?
- Check the sample is internally consistent, decide whether the pH is acidemic or alkalemic, identify the primary process from PaCO₂ and bicarbonate, check whether compensation is appropriate, then calculate the anion gap and delta ratio to look for mixed disorders.
- What is Winters formula?
- Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 in metabolic acidosis. A PaCO₂ above the range means a coexisting respiratory acidosis; below it, a coexisting respiratory alkalosis.
- Why correct the anion gap for albumin?
- Albumin is the main unmeasured anion. Each 1 g/dL fall in albumin lowers the anion gap by about 2.5 mEq/L, which can hide a high anion gap acidosis in sick or malnourished patients.
- What does the delta ratio show?
- It compares the rise in anion gap with the fall in bicarbonate. A low ratio suggests an added normal anion gap acidosis; a high ratio suggests an added metabolic alkalosis or chronic respiratory acidosis.
- Can I enter values in SI units?
- Yes. Switch between US and SI units at the top of the analyser; only pH and PaCO₂ are required.
Related workups
Diabetic ketoacidosisHyperosmolar hyperglycemic state (HHS)Acute kidney injuryHyperkalemiaHypokalemiaSepsisCOPD exacerbationRhabdomyolysis