A simplified quantitative acid–base approach for patients with acute respiratory diseases

  • Michalis AgrafiotisEmail author
  • Maria Papathanassiou
  • Christos Karachristos
  • Eleni Kerezidou
  • Stavros Tryfon
  • Evangelia Serasli
  • Diamantis Chloros
Original Research


The Stewart-Figge acid–base model has been criticized for being mathematically complex. We aimed to develop simpler formalisms, which can be used at the bedside. The following simplifications were used: (1) [Ca2+] and [Mg2+] are replaced by their mid-reference concentrations (2) pH is set to 7.4. In the new model [SIDa] is replaced by its adjusted form, [SIDa, adj] = [Na+] + [K+] − [Cl] + 6.5 and [SIG] is replaced by “bicarbonate gap”, [BICgap] = [SIDa, adj] − (0.28⋅[Albumin]) − (1.82⋅[Phosphatei])- [HCO3̄]. The diagnostic performance of the model was tested in 210 patients with acute respiratory diseases and 17 healthy volunteers. [BICgap] was also compared to albumin-corrected anion gap ([AGc]). The concordant correlation coefficient between [SIDa, adj] and [SIDa] and between [BICgap] and [SIG] was 0.98 in both comparisons. The mean bias (limits of agreement) of [SIDa, adj] − [SIDa] and of [BICgap] − [SIG] were 0.53 meq/l (− 0.46 to 1.53) and 0.50 meq/l (− 0.70 to 1.70), respectively. A [SIDa, adj] < 50.4 meq/l had an accuracy of 0.995 (p < 0.001) for the diagnosis of strong ion (SI) acidosis, while a [SIDa, adj] > 52.5 meq/l had an accuracy of 0.997 (p < 0.001) for the diagnosis of SI alkalosis. A [BICgap] > 11.6 meq/l predicted unmeasured ion (UI) acidosis with an accuracy of 0.997 (p < 0.001), while an [AGc] > 19.88 meq/l predicted UI acidosis with an accuracy of 0.994 (p < 0.001). The “[BICgap] model” is a reliable tool for the assessment of acid–base disorders in patients with acute respiratory diseases. [BICgap] is not inferior to [AGc] in the diagnosis of UI acidosis.


Anion gap Base excess Stewart-Figge acid–base model Metabolic acidosis Unmeasured ions 


Author contributions

MA designed the study, performed statistical analysis and co-authored the manuscript. MP, CK and EK collected and analyzed the data and co-authored the manuscript. ST and ES participated in data analysis and co-authored the manuscript. DC co-authored and reviewed the manuscript for important intellectual content. All authors read and approved the final manuscript.

Compliance with ethical standards

Conflict of interest

The authors declared that they have no conflict of interest.


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Copyright information

© Springer Nature B.V. 2019

Authors and Affiliations

  1. 1.Department of Pulmonary Medicine“Georgios Papanikolaou” General Hospital of ThessalonikiExohiGreece

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