Euglycemic diabetic ketoacidosis (EDKA) can be difficult to recognize during surgery, particularly when the anion gap (AG) is within the normal range. A 73-year-old female patient with type 2 diabetes mellitus underwent surgical adhesiolysis and enterostomy for intestinal obstruction. The patient showed a normal serum glucose level and hyperchloremic acidosis but had an accumulation of unmeasured anions (UA) detected by partitioning of the standard base excess (SBE). This prompted testing for serum ketones, which confirmed the presence of EDKA during surgery. The calculation of the hyperchloremic base deficit as a percentage of the total acidifying base deficit revealed a significant contribution of hyperchloremic acidosis to the severity of mixed metabolic acidosis. The AG approach can misdiagnose high AG metabolic acidosis (HAGMA) with hyperchloremic acidosis as hyperchloremic acidosis only. Partitioning of the SBE can be useful for detecting UA when both HAGMA and hyperchloremic acidosis are present, and it may help determine the necessity of treating hyperchloremia during EDKA.