Anion Gap Calculator (Albumin-Corrected, Delta Ratio)

Overview

The anion gap (AG) is the difference between the routinely measured cations and anions in a blood chemistry panel, and it serves as the entry point for working up an acid-base disturbance because it reflects how much unmeasured anion — lactate, ketoacids, phosphate — has accumulated. Once metabolic acidosis is established, a high gap points toward added acid (lactic acidosis, ketoacidosis, toxic ingestion) while a normal gap points toward lost bicarbonate (diarrhea, renal tubular acidosis). Albumin is the largest negatively charged protein in plasma, so hypoalbuminemia makes the gap read falsely low; adding 2.5 mEq/L for every 1 g/dL that albumin falls below 4.0 restores it. This calculator returns the standard gap, the potassium-inclusive variant, the albumin-corrected gap, and the delta ratio used to unmask mixed disorders.

Formula

Anion gap (mEq/L) = Na⁺ − (Cl⁻ + HCO₃⁻), normal 8–12 (8–16 in some labs) | Potassium-inclusive gap = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻), normal 10–20 | Albumin-corrected gap = AG + 2.5 × (4.0 − measured albumin in g/dL) | Delta ratio = (corrected AG − 12) ÷ (24 − HCO₃⁻) — below 0.4 normal-gap acidosis, 0.4–0.8 mixed, 0.8–2.0 pure high-gap acidosis, above 2.0 suggests a coexisting metabolic alkalosis | Example: Na 140, Cl 104, HCO₃ 20, albumin 3.2 → AG = 140 − 124 = 16.0, corrected = 16.0 + 2.5 × 0.8 = 18.0, delta ratio = (18.0 − 12) ÷ (24 − 20) = 1.50

How to Use

1. 1Copy sodium, chloride, and bicarbonate (or TCO₂) straight from the electrolyte panel. Values in mmol/L are identical to mEq/L for these monovalent ions.
2. 2Enter serum albumin in g/dL from the same panel. Leaving it at 4.0 means no correction is applied.
3. 3Choose the potassium-inclusive formula if your lab or textbook uses it, then enter K⁺; the normal reference band shifts to 10–20.
4. 4Compare the raw and albumin-corrected gaps. If the verdict flips from normal to high after correction, hypoalbuminemia was masking an acidosis.
5. 5When the corrected gap exceeds 12 and bicarbonate is below 24, the delta ratio is calculated — read it against the band table to screen for a mixed disorder.

Examples

Background

The anion gap exists because plasma must be electrically neutral: the difference between the measured cation (mainly Na⁺) and the measured anions (Cl⁻ and HCO₃⁻) is not empty space but the share held by anions routine panels never report — albumin, phosphate, sulfate, and organic acids. Albumin alone accounts for roughly three quarters of the normal gap, which is why hypoalbuminemia depresses it and why the correction factor of 2.5 mEq/L per 1 g/dL, proposed by Figge and colleagues in 1998, became standard in critical care. Reference ranges drifted from 8–16 in older textbooks to 8–12 and, in some modern labs, as low as 3–11, because ion-selective electrodes measure chloride higher than the colorimetric methods they replaced. The delta ratio rests on the buffering principle that each unit of added acid should raise the gap by one and consume one bicarbonate; when that one-to-one relationship breaks down, more than one acid-base process is at work.

Tips

• ✔The gap does not tell you whether an acid-base disorder exists — it classifies one that has already been identified. Always read it alongside pH and PaCO₂.
• ✔In inpatients or ICU patients with albumin below 3.0 g/dL, the uncorrected gap is close to useless. Judge from the corrected value.
• ✔If the panel reports only TCO₂ rather than HCO₃⁻, entering it is acceptable; it typically runs 1–2 mEq/L higher.
• ✔A gap above 20 can hide a serious metabolic acidosis even when pH looks normal because an alkalosis is offsetting it.
• ✔A low gap (under 5) is uncommon; suspect a lab artifact first, but consider screening for multiple myeloma if it is reproducible.
• ✔Pairing the gap with an osmolal gap helps with toxic alcohol screening — a high gap acidosis plus a raised osmolal gap is the classic methanol or ethylene glycol clue.

FAQ