- Enter sodium, chloride, and bicarbonate from the same basic metabolic panel — these three are required to compute the anion gap.
- Optionally enter albumin (choose g/dL or g/L) to obtain the albumin-corrected anion gap; the delta-ratio logic then uses the corrected gap.
- Optionally enter the measured PaCO₂ from an ABG to check respiratory compensation against Winter's expected value.
- Results update live: the anion gap (with corrected AG when albumin is supplied), the delta ratio with its mixed-disorder pattern, Winter's expected PaCO₂ ±2, and a plain-language interpretation flagging any concurrent respiratory disorder.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use this tool whenever you have a basic metabolic panel showing a low bicarbonate, an unexplained acid–base disturbance, or any critically ill patient in whom a mixed disorder is possible. It computes the serum anion gap, corrects it for albumin, applies the delta ratio to detect a coexisting normal-gap or metabolic-alkalosis process, and uses Winter's formula to confirm that respiratory compensation is appropriate.
Appropriate population
Any adult with a measured Na⁺, Cl⁻, and HCO₃⁻ and a suspected metabolic acidosis — diabetic ketoacidosis, lactic acidosis, toxic ingestions, uremic acidosis of CKD, or diarrheal/renal-tubular (normal-gap) acidosis. Adding albumin is essential in CKD, sepsis, and the chronically ill, where hypoalbuminemia lowers the baseline gap and can hide a true high-gap acidosis. Add an ABG PaCO₂ to test compensation.
When NOT to rely on it
These are screening formulas, not a diagnosis. The anion gap varies with the local reference interval and assay; a "normal" gap does not exclude acidosis, and the delta ratio is an approximation that assumes a starting bicarbonate of 24 and a normal gap of 12. Winter's formula validates compensation only for a primary metabolic acidosis — it does not apply to primary respiratory or metabolic-alkalosis disorders. Always interpret alongside a full ABG, lactate, ketones, glucose, and the clinical picture.
Pearls & Pitfalls
Always correct the gap for albumin
In hypoalbuminemic patients — the rule in CKD and critical illness — the uncorrected anion gap routinely reads "normal" despite a real high-gap acidosis. Correcting for albumin (≈2.5 mEq/L per 1 g/dL below 4.0) restores sensitivity and frequently changes the diagnosis.
Use the delta ratio to find the second disorder
When the gap is elevated, the delta ratio tells you whether the bicarbonate fall fully accounts for the gap rise. A ratio under 1 points to a coexisting normal-gap acidosis; a ratio over 2 points to a coexisting metabolic alkalosis or chronic respiratory acidosis — both invisible on bicarbonate alone.
Pitfalls
(1) Winter's formula validates compensation for a primary metabolic acidosis only — do not apply it to a primary respiratory or alkalotic disorder. (2) The normal gap is assay- and lab-dependent; calibrate the thresholds to your reference interval. (3) The delta ratio assumes a baseline HCO₃⁻ of 24 and AG of 12 and is unreliable at extremes or with rapid fluid shifts. (4) These bedside calculations never replace a full ABG, lactate, ketone, and toxicology workup with physician interpretation.
Why Use It
Mixed acid–base disorders are common and easily missed. A patient can have a high-gap acidosis and a concurrent normal-gap acidosis or metabolic alkalosis simultaneously — and the bicarbonate alone will not reveal it. Correcting the anion gap for albumin unmasks high-gap acidosis in hypoalbuminemic patients (a 1 g/dL fall in albumin lowers the apparent gap by ~2.5 mEq/L). The delta ratio compares the rise in anion gap to the fall in bicarbonate to flag a second metabolic process, and Winter's formula confirms whether the lungs are compensating or whether a separate respiratory disorder is also present. Bundling these steps prevents both over- and under-treatment.
Acid-Base & Anion Gap Calculator — Corrected AG, Delta Ratio & Winter's Formula
Enter your sodium, chloride, and bicarbonate to compute the anion gap and screen for the type of metabolic acidosis. Add albumin to correct the gap (low albumin hides a real gap), and add PaCO₂ to check whether your breathing is compensating appropriately using Winter's formula.
⚕ AG = Na − (Cl + HCO₃). Corrected AG = AG + 2.5 × (4.0 − albumin). Delta ratio = (AG − 12) / (24 − HCO₃), interpreted only when AG > 12. Winter's expected PaCO₂ = 1.5 × HCO₃ + 8 (±2). These bedside formulas screen for mixed disorders but never replace a full ABG with clinical correlation by a physician.
Next Steps
Use the result to support — not replace — clinical judgment.
- Interpret the value against the targets shown in the calculator and the Evidence section below, in the context of the full clinical picture.
- Trend serial measurements rather than acting on a single result; confirm abnormal or unexpected values before changing management.
- Apply the relevant KDIGO / specialty-guideline threshold and document the indication.
- Escalate or refer to nephrology when results are out of range, rapidly changing, or discordant with the clinical picture — and discuss the implications with the patient.
Evidence & References
Formula & Equations
| Quantity | Equation |
|---|---|
| Anion gap (mEq/L) | Na⁺ − (Cl⁻ + HCO₃⁻) |
| Albumin-corrected AG (mEq/L) | Measured AG + 2.5 × (4.0 − albumin in g/dL) |
| Delta ratio (Δ/Δ) | (AG − 12) ÷ (24 − HCO₃⁻), computed only when AG > 12 |
| Winter's expected PaCO₂ (mmHg) | 1.5 × HCO₃⁻ + 8, ± 2 |
Interpreting the delta ratio
| Δ/Δ ratio | Suggested pattern |
|---|---|
| < 0.4 | Pure normal-gap (hyperchloremic) acidosis |
| 0.4–1.0 | Mixed high-gap + normal-gap metabolic acidosis |
| 1.0–2.0 | Pure high-gap metabolic acidosis |
| > 2.0 | High-gap acidosis plus a metabolic alkalosis or chronic respiratory acidosis |
The normal anion gap (≈8–12 mEq/L) is laboratory-specific; lower reference gaps are reported with modern ion-selective electrodes. The corrected-AG coefficient of 2.5 per g/dL of albumin is the Figge factor. Winter's formula applies to primary metabolic acidosis only.
Evidence & References
Winter's formula derives from the empirical relationship between bicarbonate and the expected respiratory compensation described by Albert, Dell, and Winters. The albumin correction of the anion gap was quantified by Figge and colleagues, who showed that hypoalbuminemia substantially lowers the measured gap and can conceal a high-gap acidosis. Acid–base targets and the management of metabolic acidosis in CKD follow KDIGO and KDOQI guidance.
- Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med. 1967;66(2):312–322.
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807–1810.
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. 2024;105(4S):S117–S314.
- National Kidney Foundation. K/DOQI Clinical Practice Guidelines for Nutrition in Chronic Renal Failure (serum bicarbonate maintenance ≥22 mEq/L). Am J Kidney Dis. 2000;35(6 Suppl 2):S1–S140.
