The Δ Anion Gap/Δ Bicarbonate Ratio in Lactic Acidosis: Time for a New Baseline?

Background: The ratio of delta anion gap and delta bicarbonate (ΔAG/ΔHCO3) is used to detect co-existing acid-base disorders in patients with high anion gap metabolic acidosis. The ΔAG/ΔHCO3 ratio of 1.6-1.8:1 in lactic acidosis is derived from limited data using mean normal values for AG and serum HCO3. The objective of this study was to be the first to examine the ΔAG/ΔHCO3 using each patient's individual baseline AG and serum HCO3.

Methods: This was a retrospective cohort study of adult ICU patients with sepsis. Lab data from simultaneously drawn chemistry panel, including anion gap and serum lactate on admission to the ICU was obtained. Baseline AG, HCO3 and albumin measurements were obtained 1-24 months prior to ICU admission. The ΔAG/ΔHCO3 was calculated using an albumin-corrected anion gap and each patient's individual baseline AG and serum HCO3.

Results: 344 patients were included. 128 patients had normal serum lactate levels (≤1.9 mmol/L) and 216 patients had elevated serum lactate levels (>1.9 mmol/L). ΔAG/ΔHCO3 was calculated for the 216 patients who had elevated serum lactate levels (>1.9 mmol/L). The mean ΔAG/ΔHCO3 for all patients with elevated serum lactate levels was 1.20 (SD 1.50).

Conclusions: The mean ΔAG/ΔHCO3 calculated using an albumin-corrected anion gap and each patient's individual baseline AG and serum HCO3 was 1.20. The ΔAG/ΔHCO3 reported in prior literature which used mean normal AG and serum HCO3 was 1.6-1.8, highlighting that use of mean normal values affects the calculation of the ΔAG/ΔHCO3 and subsequent conclusions about underlying pathophysiology. The use of these mean normal values can result in misdiagnosis of complex acid-base disorders and inappropriate treatment. Our analysis indicates that the elevated ΔAG/ΔHCO3 is likely due to unmeasured anions contributing to an elevation in AG.