Purpose: To suggest a simplified method for strong ion gap ([SIG]) calculation.
Patients and methods: 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.
Results: Strong correlations were observed between [SIDa,adj] and [SIDa] (r = 0.93, p < 0.0001) and between [BICgap] and [SIG] (r = 0.95, p < 0.0001). The mean bias (limits of agreement) of [SIDa,adj] - [SIDa] and of [BICgap]-[SIG] was - 0.6 meq/l (- 2.7 to 1.5) and 0.2 meq/l (- 2 to 2.4), respectively. The intraclass correlation coefficients between [SIDa,adj] and [SIDa] and between [BICgap] and [SIG] were 0.90 and 0.95, respectively. The sensitivities and specificities for the prediction of a [lactate-] > 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.
Conclusions: 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.
Keywords: Anion gap; Base excess; Metabolic acidosis; Stewart–Figge acid–base model.