Purpose: The aim of this study was to define the nature of metabolic acidosis in patients with septic shock on admission to intensive care unit (ICU) using Stewart method. We also aimed to compare the ability of standard base excess (SBE), anion gap (AG), and corrected AG for albumin and lactate (AGcorr) to accurately predict the presence of unmeasured anions (UA).

Patients and methods: Thirty consecutive patients with septic shock were prospectively included on ICU admission. Stewart equations modified by Figge were used to calculate the strong ion difference and the strong ion gap (SIG).

Results: Most patients had multiple underlying mechanisms explaining the metabolic acidosis. Unmeasured anions and hyperchloremia were present in 70% of the patients. Increased UA were present in 23% of patients with normal values of SBE and [HCO3-]. In these patients, plasma [Cl-] was significantly lower compared with patients with low SBE and increased UA (103 [102-106.6] vs 108 [106-111] mmol/L; P=.01, respectively). Corrected AG for albumin and lactate had the best correlation with SIG (r²=0.94; P<.0001) with good agreement (bias, 0, and precision, 1.22) and highest area under the receiver operating characteristic curve (0.995; 95% confidence interval, 0.87-1) to discriminate SIG acidosis.

Conclusions: Patients with septic shock exhibit a complex metabolic acidosis at ICU admission. High UA may be present with normal values of SBE and [HCO3-] as a result of associated "relative" hypochloremic alkalosis. Corrected AG for albumin and lactate offers the most accurate bedside alternative to Stewart calculation of UA.