Fig. 1

A A threshold, nonlinear association between the average blood glucose levels (mmol/L) and 6-month GOS was observed in a generalized additive model (GAM). The solid red line represents the smooth curve fit between variables. Blue bands represent the 95% confidence interval from the fit; 8.88 mmol/L (blood glucose level) were its inflection points (K). The 95% confidence interval of intervals of the infection points were 7.43 and 9.74. B A similar curve was found between the average blood glucose levels and 5-year GOS, in which 8.17 mmol/L was identified as the inflection point (K) with a 95% CI: 7.43–9.48. The same analysis revealed that there was also a bell relationship between average blood glucose levels and favorable outcome group (GOS ≥ 4) at 6 months C or 5 years D. The covariates were all adjusted for Glasgow coma score and Acute Physiology and Chronic Health Evaluation II score before surgery.