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

References

Rastegar, Asghar. "Use of the ΔAG/ΔHCO3− Ratio in the Diagnosis of Mixed Acid-Base Disorders." Journal of the American Society of Nephrology 18.9 (2007): 2429-2431.