Delta Gap Calculator
Compute the delta gap (AG − 12) − (24 − HCO₃⁻) to unmask mixed acid–base disorders hidden behind a high-anion-gap metabolic acidosis. Positive = concurrent metabolic alkalosis; negative = concurrent NAGMA.
What the delta gap measures
The delta gap compares how much the anion gap has risen above normal against how much the bicarbonate has fallen below normal. In a pure high-anion-gap metabolic acidosis (HAGMA), every extra unmeasured anion consumes one bicarbonate, so the rise in AG and the fall in HCO₃⁻ should match — the delta gap should be zero. When the two deltas do not match, a second acid–base disorder is hiding underneath the obvious one. That is the entire purpose of the calculation: detecting mixed disorders from a single blood gas and chemistry panel.
The delta gap formula is (AG − 12) − (24 − HCO₃⁻), using the conventional normal AG of 12 mEq/L and normal bicarbonate of 24 mEq/L. The first term (ΔAG) measures how far the gap is elevated; the second term (ΔHCO₃⁻) measures how far the bicarbonate is depressed. Subtracting one from the other leaves any mismatch — the "hidden" deviation that signals a second disorder. A patient with a high anion gap may look straightforwardly acidotic, but the delta gap tells you whether something else is going on at the same time.
Worked examples
Consider a DKA patient with an anion gap of 24 and bicarbonate of 12 mEq/L. ΔAG = 24 − 12 = 12; ΔHCO₃⁻ = 24 − 12 = 12. The delta gap is 12 − 12 = 0 mEq/L — pure HAGMA, the bicarbonate fell exactly as much as the gap rose, no second disorder. Now picture the same gap of 24 but with a bicarbonate of 22: ΔAG = 12, ΔHCO₃⁻ = 2, delta gap = 12 − 2 = +10 mEq/L. The bicarbonate did not fall nearly as much as the gap rose — there is a concurrent metabolic alkalosis lifting the bicarbonate back up (classic in a patient who has been vomiting). Finally, a gap of 24 with a bicarbonate of 8: ΔAG = 12, ΔHCO₃⁻ = 16, delta gap = 12 − 16 = −4 mEq/L. The bicarbonate fell further than the gap rose, revealing a concurrent normal-anion-gap (hyperchloremic) metabolic acidosis — for example, a patient in ketoacidosis who is also losing bicarbonate through diarrhea.
Interpreting the delta gap
The standard cut-offs are straightforward. A delta gap in the range of −6 to +6 mEq/L is consistent with a pure HAGMA — the small deviation is within biological noise. A delta gap above +6 mEq/L indicates a concurrent metabolic alkalosis (or chronic respiratory acidosis with renal compensation), because something is holding the bicarbonate up. A delta gap below −6 mEq/L indicates a concurrent normal-anion-gap metabolic acidosis, because something extra is pulling the bicarbonate down. The delta gap is a difference of differences, so it carries the same units (mEq/L) as the gap itself.
The delta gap is a subtraction; the delta ratio (ΔAG ÷ ΔHCO₃⁻) is the division of the same two quantities. They carry the same information, but the ratio is more commonly quoted in modern references because its cut-offs (1, 2, 0.4–0.8) are easier to remember. Use whichever your team prefers — both detect the same mixed disorders.
This calculator runs entirely in your browser — no values are transmitted or stored. The delta gap formula and interpretation logic are reviewed against StatPearls and LITFL acid–base references. Results are informational only and should always be interpreted in full clinical context by a qualified clinician.