Calculate the delta gap and delta ratio from the anion gap and bicarbonate to check a high anion gap metabolic acidosis for a hidden second acid-base disorder.
Delta Gap and Delta Ratio Formula
The delta gap and delta ratio both compare how far the anion gap has risen against how far the bicarbonate has fallen. They start from the anion gap:
AG = Na - (Cl + HCO3)
The delta gap is the absolute difference between those two changes:
Delta Gap = (AG - 12) - (24 - HCO3)
The delta ratio expresses the same comparison as a ratio:
Delta Ratio = (AG - 12) / (24 - HCO3)
A corrected bicarbonate adds the extra anion gap back onto the measured bicarbonate to reveal a hidden disorder:
Corrected HCO3 = HCO3 + (AG - 12)
When albumin is low, correct the anion gap before you read the ratio:
Corrected AG = AG + 2.5 * (4.0 - Albumin)
Variables:
- AG is the anion gap in mEq/L, with a common reference value of 12
- Na is the serum sodium in mEq/L
- Cl is the serum chloride in mEq/L
- HCO3 is the serum bicarbonate in mEq/L, with a common reference value of 24
- Albumin is the serum albumin in g/dL, with a reference value of 4.0
- The numbers 12 and 24 are the assumed normal anion gap and normal bicarbonate; you can change both in the calculator
The anion gap measures the unmeasured anions in blood, so a raised gap points to an acid such as lactate or a ketoacid. The term (AG – 12) is the rise in the gap, and the term (24 – HCO3) is the fall in bicarbonate. In a clean high anion gap metabolic acidosis each unit of acid consumes about one unit of bicarbonate, so the two changes should be close and the ratio should sit near 1. The delta gap says the same thing on an absolute scale: a value near zero means the anion gap rise and the bicarbonate fall cancel out. The corrected bicarbonate takes the measured bicarbonate and adds back the bicarbonate that the high gap acidosis consumed, so the result estimates what the bicarbonate would be if only that acidosis were present.
Delta Ratio Ranges and Typical Values by Cause
The first table reads the delta ratio and the matching delta gap together to name the likely acid-base picture.
| Delta ratio | Delta gap | Likely acid-base picture |
|---|---|---|
| Below 0.4 | Strongly negative | Normal anion gap (hyperchloremic) metabolic acidosis |
| 0.4 to 1.0 | Negative | Combined high and normal anion gap acidosis, also seen in renal failure |
| 1.0 to 2.0 | Near zero | Pure high anion gap metabolic acidosis |
| Above 2.0 | Strongly positive (above +6) | High anion gap acidosis plus metabolic alkalosis or chronic respiratory acidosis |
The second table lists the delta ratio you tend to see with specific causes, which helps you judge whether a result fits the suspected diagnosis.
| Cause | Typical delta ratio | Why |
|---|---|---|
| Lactic acidosis | About 1.6 | Half of the acid is buffered inside cells while lactate stays in plasma, so the gap rises more than bicarbonate falls |
| Diabetic ketoacidosis | About 1.0, often lower | Ketone anions are lost in urine, which trims the gap, especially when the patient is well hydrated |
| Uremic acidosis of renal failure | Below 1 | Retained acids sit alongside a tubular loss of bicarbonate |
| Toxic alcohol (methanol, ethylene glycol) | Variable, often 1 to 2 | Depends on how much of the acid anion the kidney clears |
| Acidosis with vomiting or diuretics | Above 2 | A superimposed metabolic alkalosis lifts the baseline bicarbonate |
Example Problems
Example 1: a pure high anion gap metabolic acidosis.
Sodium is 140, chloride is 104, and bicarbonate is 10 mEq/L. First find the anion gap:
AG = 140 – (104 + 10) = 26 mEq/L.
The gap has risen by 26 – 12 = 14, and the bicarbonate has fallen by 24 – 10 = 14. The delta gap is 14 – 14 = 0 and the delta ratio is 14 / 14 = 1.0. The corrected bicarbonate is 10 + 14 = 24, which is normal. A ratio of 1.0, a delta gap of 0, and a normal corrected bicarbonate all agree on a single, pure high anion gap metabolic acidosis.
Example 2: a high anion gap acidosis hiding a metabolic alkalosis.
Sodium is 140, chloride is 88, and bicarbonate is 16 mEq/L.
AG = 140 – (88 + 16) = 36 mEq/L.
The gap has risen by 36 – 12 = 24, but the bicarbonate has only fallen by 24 – 16 = 8. The delta gap is 24 – 8 = 16 and the delta ratio is 24 / 8 = 3.0. The corrected bicarbonate is 16 + 24 = 40, well above normal. A ratio above 2, a strongly positive delta gap, and a high corrected bicarbonate all point to a metabolic alkalosis sitting on top of the acidosis, which the raw bicarbonate of 16 would have missed.
Frequently Asked Questions
What is a normal delta gap and delta ratio?
A delta gap near zero, roughly -6 to +6 mEq/L, and a delta ratio between about 1.0 and 2.0 both fit a single, pure high anion gap metabolic acidosis. Values outside those ranges suggest a second acid-base process. A ratio below 1 hints at an added normal anion gap acidosis, and a ratio above 2 hints at an added metabolic alkalosis. The numbers only carry meaning when the anion gap is genuinely raised and the bicarbonate is below normal.
When should you use the delta ratio?
Use it after you have confirmed a high anion gap metabolic acidosis. Its only job is to tell you whether that one diagnosis explains the whole picture or whether a normal anion gap acidosis or a metabolic alkalosis is hidden underneath. If the anion gap is normal, the ratio is not informative, which is why the calculator flags results where the gap is not elevated.
Do you need to correct the anion gap for albumin first?
Yes, whenever albumin is low. Albumin is the largest unmeasured anion, so every 1 g/dL that albumin sits below 4.0 g/dL lowers the measured gap by about 2.5 mEq/L. Reading the ratio from an uncorrected gap in a patient with low albumin understates the anion gap rise and can hide a real high anion gap acidosis. Add 2.5 mEq/L to the gap for each 1 g/dL the albumin is below 4.0 before you read the ratio, or enter the albumin in the calculator and let it apply the correction.