The base excess gap is not a valid tool for the quantification of unmeasured ions in cardiac surgical patients A retrospective observational study

BACKGROUNDThe base excess gap (BEgap) method is commonly used for the quantification of unmeasured ions in critically ill patients. However, it has never been validated against the standard quantitative acid-base approach.OBJECTIVETo compare the BEgap as a tool for the prediction of the excess of unmeasured ions with the offset of strong ion gap (SIG) from its reference value.DESIGNA retrospective observational study.SETTINGAdult ICU in a tertiary hospital.PATIENTSOne hundred and thirty-five cardiac surgical patients admitted for postoperative care.INTERVENTIONS None.MAIN OUTCOME MEASURESBE(gap) was calculated as BEgap=SBE-BEsi-BEwa, where SBE is the standard base excess, BEsi is the partition due to strong ions ([Na+]-[Cl-]-[lactate(-)]-30.5) and BEwa is the partition due to weak acids [0.25x{42-(albumin)}]. The deviation of the observed SIG (SIG(ob)) from its reference value was calculated as deltaSIG=2.85-SIG(ob). We used Bland-Altman and concordance correlation analysis to compare BEgap with deltaSIG. A bias of 1meql(-1) with limits of agreement of 2meql(-1) and a concordant correlation coefficient of more than 0.9 were considered to indicate a strong agreement.RESULTSThe concordant correlation coefficient between BEgap and deltaSIG was 0.702. The mean bias between the two variables was 1.8meql(-1), with a lower limit of agreement of -0.9meql(-1) and an upper limit of agreement of 4.4meql(-1).CONCLUSIONThe BE gap method cannot reliably quantify the unmeasured ion excess in cardiac surgical patients. Clinicians should use the full Stewart-Figge model for quantitative acid-base assessments.