Greater than the sum of its parts: C-reactive protein and the calculated ion gap together are superior in predicting mortality in critically ill surgical patients

Introduction

Inadequate tissue perfusion and an uncontrolled systemic inflammatory response are associated with poor outcome in critically ill surgical patients. An increased concentration of unmeasured anions, reflecting hypoperfusion, and the magnitude of the early inflammatory response both correlate strongly with mortality. Our aim was to assess the relationship between these factors, and their ability in combination to predict outcome.

Methods

In a prospective study we evaluated 108 consecutive patients admitted to a surgical high dependency unit. Regional Ethics Committee approval was obtained. Serum electrolytes, albumin, phosphate, lactate and C-reactive protein (CRP) were measured on admission and on day 1. We derived the calculated ion gap (CIG) using a simplified modification of the Stewart–Figge equations.

Results

Based on previous work, thresholds of 10 mmol/l for CIG and 100 mg/l for CRP were used to categorise patients. Of the patients with a CRP < 100 mg/l, 15.4% had an elevated CIG. Of the patients with a CRP > 100 mg/l, 36.7% had an elevated CIG (P = 0.016, chi-square test). Patients (n = 63) with a CIG < 10 mmol/l and CRP < 100 mg/l had a 1.5% mortality, whereas those (n = 11) with a CIG > 10 mmol/l and CRP > 100 mg/l had a 54.5% mortality (P < 0.0001, chi-square test) (Table 1).

Table 1 abstractP7

Conclusion

Inflammation is a potent cause of oxidative stress, which in turn results in endothelial damage and increased concentrations of unmeasured anions. The combination of CRP and the CIG, as markers of inflammation and inadequate tissue perfusion, respectively, is a powerful predictor of mortality in the critically ill surgical patient.