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: