Delta Ratio Calculator
Find the anion gap and compare its rise with the fall in bicarbonate to classify a metabolic acidosis.
What it does
The delta ratio is a step beyond the anion gap. The anion gap tells you whether a metabolic acidosis is driven by unmeasured anions such as lactate, ketones or toxins, or by a loss of bicarbonate with a normal gap. The delta ratio then asks whether the size of the gap rise matches the size of the bicarbonate fall, which is a way to spot a second acid base process hiding behind the first.
The arithmetic has two parts. First, anion gap equals sodium minus chloride minus bicarbonate, with a normal value of about 12 mmol/L in this tool. Second, the delta ratio equals the gap rise, anion gap minus 12, divided by the bicarbonate fall, 24 minus bicarbonate. A patient with a gap of 22 and a bicarbonate of 14 has a rise of 10 and a fall of 10, so the ratio is 1.00.
The bands are the useful part. A ratio near 1 means the gap rise and the bicarbonate fall are matched, which fits a pure high anion gap acidosis. A ratio below 1 means the bicarbonate fell more than the gap rose, so a normal anion gap acidosis is also present. A ratio above 2 means the bicarbonate fell less than the gap rose, which points to a concurrent metabolic alkalosis or a chronic respiratory acidosis.
The test depends on a normal baseline. If the patient already had a high anion gap before the illness, or a low albumin, the arithmetic changes. Albumin is the main unmeasured anion, and a low albumin lowers the gap, so a gap that looks normal can still hide a high anion gap process. Correct the gap for albumin when the value is low, and read the ratio with the clinical story, the lactate, the ketones and the renal function.
This tool uses the common US constants of 12 for the normal gap and 24 for the normal bicarbonate. Some laboratories and textbooks use a normal gap of 8 to 12 or a bicarbonate of 22 to 26, which shifts the ratio. Pick one convention and stay with it, and do not compare a ratio built with one set of constants against a reference range built with another.
How to use
- Enter the serum sodium, chloride and bicarbonate from the same blood panel.
- Check that all three values are in mmol/L.
- Read the anion gap first. If it is 12 mmol/L or less, the delta ratio is not applicable.
- Read the delta ratio and the pattern.
- Compare the ratio with the clinical picture, the lactate and the albumin.
- Repeat the panel after treatment rather than relying on a single set of values.
Formula
- Sodium
- Serum sodium in mmol/L.
- Chloride
- Serum chloride in mmol/L.
- Bicarbonate
- Serum bicarbonate or total CO2 in mmol/L.
- 12
- The normal anion gap used by this tool, in mmol/L.
- 24
- The normal bicarbonate used by this tool, in mmol/L.
The ratio is only meaningful when the anion gap is above the normal value and the bicarbonate is below it. If the gap is 12 or less, or the bicarbonate is 24 or more, the tool reports not applicable rather than dividing by zero or by a negative number. A ratio under 1 can also occur when a high anion gap acidosis is resolving and the bicarbonate has not caught up, so the timing of the sample matters. Albumin correction uses the measured gap plus 2.5 times the difference between 4 and the albumin in g/dL.
Worked example
Example: sodium 140, chloride 104 and bicarbonate 14
Defaults: sodium 140 mmol/L, chloride 104 mmol/L and bicarbonate 14 mmol/L.
- Subtract chloride from sodium: 140 - 104 = 36.
- Subtract bicarbonate: 36 - 14 = 22 mmol/L anion gap.
- Find the gap rise above 12: 22 - 12 = 10.
- Find the bicarbonate fall below 24: 24 - 14 = 10.
- Divide the rise by the fall: 10 / 10 = 1.00.
- A ratio of 1.00 fits a pure high anion gap acidosis, and the tool shows 22 mmol/L, 1.00 and Pure high anion gap acidosis.
Reference table
| Delta ratio | Pattern | What it suggests |
|---|---|---|
| Below 1.0 | Mixed high and normal anion gap acidosis | The bicarbonate fell more than the gap rose, so a normal gap acidosis is also present |
| 1.0 to 2.0 | Pure high anion gap acidosis | The gap rise and the bicarbonate fall are roughly matched |
| Above 2.0 | High anion gap acidosis with metabolic alkalosis | The bicarbonate fell less than the gap rose, or a chronic respiratory acidosis is present |
| Not applicable | No high anion gap or no bicarbonate deficit | The ratio needs a gap above 12 and a bicarbonate below 24 |
Limits
- Uses fixed normal values of 12 for the anion gap and 24 for bicarbonate, so other conventions give different ratios.
- A low albumin lowers the anion gap and can hide a high anion gap process unless the gap is corrected.
- The ratio is hard to interpret while an acidosis is resolving, because the gap and the bicarbonate do not change at the same rate.
- Does not identify the cause of the acidosis; lactate, ketones, toxins and renal failure need their own tests.
Common mistakes
- Using the anion gap without checking the albumin, which can make a high gap look normal.
- Applying the ratio when the gap is 12 or less or the bicarbonate is 24 or more.
- Mixing mmol/L and mEq/L values without checking that they are numerically equivalent for these ions.
- Reading a ratio below 1 as a pure high anion gap acidosis when it points to a mixed picture.
- Forgetting that a ratio above 2 can reflect a chronic respiratory acidosis as well as a metabolic alkalosis.
Try it yourself
Quick checks. Answers sit tucked under each question.
Sodium 135, chloride 100 and bicarbonate 10. What are the anion gap and delta ratio?
The gap is 135 - 100 - 10 = 25 mmol/L. The rise is 13 and the fall is 14, so the ratio is 13 / 14 = 0.93, which fits a Mixed high and normal anion gap acidosis.
Sodium 140, chloride 110 and bicarbonate 18. What does the tool report?
The gap is 140 - 110 - 18 = 12 mmol/L, which is not above the normal value. The tool reports the gap as 12 mmol/L, the delta ratio as Not applicable, and the pattern as No high anion gap.
Sodium 145, chloride 100 and bicarbonate 12. What are the anion gap and delta ratio?
The gap is 145 - 100 - 12 = 33 mmol/L. The rise is 21 and the fall is 12, so the ratio is 21 / 12 = 1.75, which fits a Pure high anion gap acidosis.
Questions
What is the delta ratio?
It is the change in the anion gap above normal divided by the change in bicarbonate below normal. It helps show whether a second acid base process is present alongside a high anion gap acidosis.
What is a normal delta ratio?
A value from 1 to 2 fits a pure high anion gap acidosis. Below 1 suggests a mixed high and normal anion gap acidosis, and above 2 suggests a concurrent metabolic alkalosis.
Why does the tool say not applicable?
The ratio needs a gap above 12 mmol/L and a bicarbonate below 24 mmol/L. If either condition is missing, there is no rise or fall to compare.
Does a low albumin matter?
Yes. Albumin is an unmeasured anion, so a low albumin lowers the gap. Add about 2.5 times the fall in albumin below 4 g/dL to the measured gap before interpreting it.
Can the ratio be below 1 in a pure high anion gap acidosis?
It can during recovery, when the gap closes faster than the bicarbonate rises. The timing of the sample and the clinical trend matter more than one value.
Is this a diagnosis?
No. It is a bedside pattern tool. The cause still comes from the history, the lactate, the ketones, the renal function and the toxin screen.
Sources
- Reddy and Mooradian: clinical utility of the anion gap in acid base disorders, PubMed
- Huggins et al: diabetic ketoalkalosis and the delta ratio, PubMed