What is the delta gap formula?

The delta gap compares the change in anion gap to the change in bicarbonate, often written as delta anion gap divided by delta bicarbonate.

When do you use delta gap?

You use delta gap when a patient has a high anion gap metabolic acidosis and you want to check for an additional metabolic alkalosis or normal anion gap acidosis.

What does a high delta gap suggest?

A higher-than-expected delta gap can suggest a concurrent metabolic alkalosis in addition to the high anion gap metabolic acidosis.

What does a low delta gap suggest?

A lower-than-expected delta gap can suggest a concurrent normal anion gap metabolic acidosis along with the high anion gap metabolic acidosis.

Why is delta gap important?

Delta gap helps uncover mixed acid-base disorders that may be missed if you only look at the anion gap by itself.

Delta Gap Explained Step by Step

Learn how to calculate the delta gap, compare the change in anion gap to the change in bicarbonate, and detect mixed acid-base disorders in patients with high anion gap metabolic acidosis.

Delta gap calculation formula explained step by step for mixed acid base disorders

What Is the Delta Gap?

The delta gap is used when a patient has a high anion gap metabolic acidosis and you want to know whether there is also a second metabolic disorder.

It helps answer an important question: is this a pure high anion gap acidosis, or is there also metabolic alkalosis or normal anion gap acidosis?

Delta Gap Formula

Delta Anion Gap

Measured AG − Normal AG

Delta Bicarbonate

Normal HCO₃⁻ − Measured HCO₃⁻

Compare the Two

ΔAG vs ΔHCO₃⁻

How to Calculate Delta Gap Step by Step

1. Confirm high anion gap metabolic acidosis

Start by confirming the patient truly has a high anion gap metabolic acidosis.

2. Calculate delta anion gap

Subtract the normal anion gap from the measured anion gap. A common normal AG reference is 12.

3. Calculate delta bicarbonate

Subtract the measured bicarbonate from the normal bicarbonate. A common normal HCO₃⁻ reference is 24.

4. Compare ΔAG and ΔHCO₃⁻

If they are close, the disorder may be isolated. If they differ substantially, think mixed metabolic disorder.

How to Interpret Delta Gap

If ΔAG is greater than ΔHCO₃⁻: think concurrent metabolic alkalosis.

If ΔAG is less than ΔHCO₃⁻: think concurrent normal anion gap metabolic acidosis.

If ΔAG and ΔHCO₃⁻ are similar: the patient may have an isolated high anion gap metabolic acidosis.

Worked Delta Gap Examples

Example 1: Delta Gap Suggests Isolated High Anion Gap Acidosis

Na 140 / Cl 100 / HCO₃⁻ 14 / AG 26

ΔAG: 26 - 12 = 14

ΔHCO₃⁻: 24 - 14 = 10

Interpretation: The rise in anion gap and the fall in bicarbonate are fairly close, which supports an isolated high anion gap metabolic acidosis.

Example 2: Delta Gap Suggests Concurrent Metabolic Alkalosis

Na 140 / Cl 96 / HCO₃⁻ 18 / AG 26

ΔAG: 26 - 12 = 14

ΔHCO₃⁻: 24 - 18 = 6

Interpretation: The anion gap has increased much more than the bicarbonate has fallen. That suggests a concurrent metabolic alkalosis.

Example 3: Delta Gap Suggests Concurrent Normal Anion Gap Acidosis

Na 140 / Cl 104 / HCO₃⁻ 12 / AG 24

ΔAG: 24 - 12 = 12

ΔHCO₃⁻: 24 - 12 = 12

Interpretation: This one is balanced and can fit an isolated high anion gap acidosis, but if bicarbonate falls more than the anion gap rises in other cases, think additional normal anion gap acidosis.

Common Mistakes Students Make

Using delta gap before confirming a high anion gap metabolic acidosis is present.
Using inconsistent normal values for anion gap and bicarbonate.
Trying to use delta gap in isolation without looking at the full ABG and chemistry panel.
Missing mixed disorders by stopping at the anion gap alone.

Use the ABG Calculator

Use the PulmTools ABG calculator to interpret acid-base status, check compensation, and apply anion gap analysis in real clinical examples.

Open ABG Calculator