🧬 Anion Gap

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 1 independent reference calculation · how PhDino checks its numbers

A calculated value from basic blood electrolytes used to help categorize the cause of a metabolic acid-base disturbance.

Blood contains many charged particles (ions) whose total positive and negative charges must balance — but routine lab panels only directly measure a few of them. The anion gap is calculated from the ones that are measured (sodium as the main positive ion; chloride and bicarbonate as the main routinely measured negative ions), and the resulting "gap" represents the unmeasured anions that are also present but not directly reported by a basic panel.

A significantly elevated anion gap is a useful clue in evaluating certain metabolic acid-base disorders, since it points toward the accumulation of an unmeasured acid (as happens in several distinct clinical conditions) rather than a loss of bicarbonate alone.

Key formula

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)

Notes & limitations

• A "normal" anion gap range is often cited as roughly 8–12 mEq/L, though the exact reference range depends on the specific lab and measurement method used — always check the reporting lab's own reference range rather than a single universal number.
• The anion gap is one piece of a larger diagnostic picture, not a standalone diagnosis — interpreting it clinically requires the broader clinical context and other lab values, which is exactly the kind of judgment call this calculator does not make.