Anion gap and delta analysis
Use albumin correction and a declared reference before interpreting the relationship between the gap and bicarbonate.
Calculate a consistent anion gap
Corrected AG = AG + 0.25 × [40 − albumin (g/L)]
Potassium is excluded throughout this app. Sodium, chloride and bicarbonate should represent a compatible set of measurements. When available, contemporaneous chemistry total CO₂ is used for the chemistry-based gap; gas-derived bicarbonate is labelled as an estimate.
The RCPA no-potassium reference is 4–13 mmol/L. A local method-matched interval takes priority. The default delta anchor of 8.5 is the midpoint of 4–13, not a measured personal baseline or a published population mean.
Correct for albumin when it is measured
Lower albumin reduces the baseline contribution to the gap. Correction makes this effect visible without pretending to identify the remaining anions. With albumin in g/dL, the correction coefficient is 2.5 instead of 0.25.
AG 12.5; albumin 20 g/L
Corrected AG = 12.5 + 0.25 × (40 − 20) = 17.5 mmol/L. The raw result is inside 4–13; the corrected result is above it.
Open worked exampleIf albumin is missing, show the uncorrected AG and state that correction is unavailable. Do not substitute a normal albumin.
Compare the gap rise with the bicarbonate fall
ΔHCO₃⁻ = 24 − HCO₃⁻
Delta ratio = ΔAG / ΔHCO₃⁻
Delta-adjusted HCO₃⁻ = HCO₃⁻ + ΔAG
| Delta ratio | Question to investigate |
|---|---|
| Below 1 | Is there an additional normal-gap metabolic acidosis? |
| 1 to 2 | The pattern is compatible with high-gap acidosis without a second metabolic process being established. |
| Above 2 | Is metabolic alkalosis present, or was bicarbonate already raised before this illness? |
These are interpretive prompts. The relationship varies with the underlying disorder, timing, renal handling, fluids and the chosen baseline; the ratio cannot prove a mixed disorder by itself.
When a delta ratio is not useful
- The gap is not established as high, albumin correction is unavailable, or the primary process remains uncertain.
- Bicarbonate is not reduced: a zero denominator cannot produce a ratio. A small denominator makes the ratio unstable.
- Gas and chemistry samples are incompatible in time or show an unresolved discrepancy.
- A respiratory disorder already explains the bicarbonate change.
The app restricts ratio interpretation to a confirmed high-gap pattern with bicarbonate below 22 mmol/L and compatible measurements. Delta-adjusted bicarbonate may still be informative when a raised gap coexists with a near-normal gas; it remains a screen.
Sources
RCPA Manual — Anion gap · Figge et al. — Anion gap and hypoalbuminaemia (1998) · American Thoracic Society — Interpretation of arterial blood gases · Paulson & Gadallah — Mixed disorders in diabetic ketoacidosis (1993) · Wrenn — The delta gap and mixed disorders (1990)
Adult educational interpretation at 37°C. Apply arterial rules to arterial samples, use the laboratory’s reference intervals, and review the result alongside the clinical history.