The Delta Gap (is different from delta-delta or delta ratio)

ACID-BASE DISTURBANCES

The delta gap is different from the delta ratio (delta-delta) but is used for similar purposes. The advantage of using the Delta Gap over the Delta Ratio is that the delta gap equation can be simplified to be used with limited data and without a bicarbonate level.B

Delta Ratio (delta-delta)=ΔAG/ΔHCO3–

Delta Gap=ΔAG-ΔHCO3–

Delta Gap=ΔAG-ΔHCO3–=Na+-(Cl++HCO3–)-12-(24-HCO3–)=Na+-Cl–-36

Delta Gap allows the clinician to evaluate for a mixed metabolic acid-based disorder without having the serum bicarbonate concentration. In patients with a known AG metabolic acidosis, the values of serum bicarbonate (HCO3–) in the delta gap equation cancel out. Thus, one only requires the serum Na+ and Cl– to assess for concomitant metabolic acid-base disturbances.  To do the same with the delta-delta, the ΔHCO3–  value would be required.

“If the DG is significantly positive (>+6), a metabolic alkalosis is usually present because the rise in AG is more than the fall in HCO3–. Conversely, if the DG is significantly negative (<-6), then a hyperchloremic acidosis is usually present because the rise in AG is less than the fall in HCO3–.”B If it is between those two, then only the AG metabolic acidosis is present.

** Note that the modified delta gap equation (Na+-Cl–-36) can only be used if the clinical and other evidence of the underlying AG metabolic acidosis is still present.

“The anion gap (AG) is a helpful, yet underused, clinical tool. Not only does the presence of a high AG suggest a certain differential, but knowledge of the relationship between the rise in AG (delta AG) and the fall in bicarbonate (delta HCO3) is important in understanding mixed acid-base disorders. Simple arithmetic converts this relationship into a numerical value, the delta gap (delta gap). The delta gap = delta AG – delta HCO3. If the delta gap is significantly positive (greater than +6), a metabolic alkalosis is usually present because the rise in AG is more than the fall in HCO3. Conversely, if the delta gap is significantly negative (less than -6), then a hyperchloremic acidosis is usually present because the rise in AG is less than the fall in HCO3. Familiarity with the relationship between the changes in AG and HCO3 can be useful in unmasking occult metabolic disorders.”A

Interpreting the Delta Gap

  • <-6 = Mixed high and normal anion gap acidosis
  • -6 to 6 = Only a high anion gap acidosis exists
  • >+6 = Mixed high AG acidosis and metabolic alkalosis

References

  1. Ann Emerg Med. 1990 Nov;19(11):1310-3. The delta (delta) gap: an approach to mixed acid-base disorders. https://www.ncbi.nlm.nih.gov/pubmed/2240729
  2. Tsapenko MV. Modified delta gap equation for quick evaluation of mixed metabolic Acid-base disorders. Oman Med J. 2013;28(1):73–74. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3562975/
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