The delta gap and delta ratio, advantages and disadvantages
The delta ratio (or the equivalent delta gap) are diagnostic and analytical tools used to determine the contribution from extra anions to the acidaemia of any given acidosis. A much more detailed overview of the delta ratio is available in a dedicated chapter from the Acid-Base Disturbances series. Even though the calculation of a delta ratio is expected in every ABG interpretation question, the college has never asked any questions regarding its validity, or its limitations as a diagnostic instrument.
Calculation of the delta ratio is performed using the following equation:
Delta ratio= (change in anion gap) / (change in bicarbonate)
Applied to metabolic acidosis to determine the contribution to acidosis from the unmeasured anions, the delta ratio suggests the following distinctions:
- Less than 0.4 = pure normal anion gap acidosis
- 0.4-0.8 = mixed high and normal anion gap acidosis
- 0.8-2.0 = pure high anion gap acidosis
- More than 2.0= high anion gap acidosis and a pre-existing metabolic alkalosis
Advantages:
- Easy to calculate
- Stoichiometric method of acid-base assessment
- Quantifies the relationship between normal and high anion gap metabolic acidosis
- Assesses the contribution of chloride to acidosis
- thus, informs the use of sodium bicarbonate
Disadvantages:
- Assumes all buffering is by bicarbonate (it is not)
- Assumes all buffering is extracellular (it is not)
- Possible measurement error of all electrolytes used to calculate the anion gap
- The "normal" values against which change is measured may not be applicable
