Comparison of a new simplified acid–base tool to the original Stewart–Figge approach: a study on cardiac surgical patients

To suggest a simplified method for strong ion gap ([SIG]) calculation. To simplify [SIG] calculation, we used the following assumptions: (1) the major determinants of apparent strong ion difference ([SIDa]) are [Na+], [K+] and [Cl−] (2) [Ca2+] and [Mg2+] do not contribute significantly to [SIDa] variation and can be replaced by their reference concentrations (3) physiologically relevant pH variation is at the order of 10−2 and therefore we can assume a standard value of 7.4. In the new model, [SIDa] is replaced by its adjusted form, i.e. [SIDa,adj] = [Na+] + [K+] − [Cl−] + 6.5 and [SIG] is replaced by “bicarbonate gap”, i.e. [BICgap] = [SIDa,adj] − (0.25·[Albumin]) − (2·[Phosphate]) − [HCO3−]. The model was tested in 224 postoperative cardiac surgical patients. Strong correlations were observed between [SIDa,adj] and [SIDa] (r = 0.93, p   4 meq/l were 73.4 and 82.3% for a [BICgap] > 12.2 meq/l and 74.5 and 83.1% for a [SIG] > 12 meq/l, respectively. The [BICgap] model bears a very good agreement with the [SIG] model while being simpler and easier to apply at the bedside. [BICgap] could be used as an alternative tool for the diagnosis of unmeasured ion acidosis.