Anion gap calculator (albumin-corrected, delta ratio)
Enter your values. The answer and the full worked solution update as you type.
Result
Reference ranges differ between laboratories; use the normal value given in the question. For exam practice only. This is a study aid, never a clinical decision: always follow the prescription, local protocols and the product information.
Calculate the serum anion gap from sodium, chloride and bicarbonate, with or without potassium, as in acid–base and ABG interpretation questions. Add the albumin to correct the gap for hypoalbuminaemia, which otherwise hides a high anion gap acidosis. When the gap is raised, the delta ratio shows whether a second acid–base disorder is present.
Step-by-step solution
Anion gap
Correct for albumin (Figge): +2.5 per g/dL below 4.0
Compare with the normal gap
Delta ratio = rise in the gap ÷ fall in bicarbonate
Key formulas
- Normal ranges
- pH 7.35–7.45, PaCO₂ 35–45 mmHg (4.7–6.0 kPa), HCO₃⁻ 22–26 mmol/L
- Anion gap
- AG = Na⁺ − (Cl⁻ + HCO₃⁻) ≈ 8–12
- Albumin-corrected gap
- AG + 2.5·(4.0 − alb g/dL) = AG + 0.25·(40 − alb g/L)
- Delta ratio
- (AG − 12)/(24 − HCO₃⁻)
How to calculate the anion gap
- Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻): 140 − (100 + 14) = 26 mmol/L. Some labs add K⁺, which raises the normal value by about 4.
- Correct for albumin: add 2.5 mmol/L for every 1 g/dL that albumin is below 4.0 g/dL (divide g/L by 10 first).
- Compare with the normal gap, often taken as 12 mmol/L without K⁺ in exam questions; above it is a high anion gap.
- Delta ratio = (AG − 12) ÷ (24 − HCO₃⁻). From 0.8 to 2, the high anion gap acidosis explains the fall in bicarbonate on its own.
- Below 0.4 a normal anion gap acidosis is also present; above 2 there is also a metabolic alkalosis or a high bicarbonate before the acidosis.
Questions and answers
What is a normal anion gap?
Commonly 8 to 12 mmol/L without potassium (about 12 to 16 with it), but modern analysers often give lower values, around 3 to 11. Use the reference range in the question or from the lab.
Why correct for albumin?
Albumin is the main unmeasured anion. When it is low, the gap falls by about 2.5 mmol/L per g/dL, which can hide a high anion gap acidosis. The correction comes from Figge and colleagues (Critical Care Medicine, 1998).
What causes a high anion gap acidosis?
Acids that add unmeasured anions: lactic acidosis, ketoacidosis, kidney failure and toxins such as methanol, ethylene glycol or salicylates. Mnemonics such as MUDPILES or GOLDMARK help to remember them.
How reliable is the delta ratio?
It is a rule of thumb that assumes a normal gap of 12 and a normal bicarbonate of 24. Values near the band edges are hard to interpret, so use it together with the history and the other results.
Practise it yourself
Practise Arterial blood gas interpretation
A calculator gives the answer. In the exam you need the method: practise it with new problems, hints and worked solutions.