 In this study, the investigators prospectively recruit a cohort of adult neurosurgical ICU patients to validate the investigators preliminary pharmacokinetic parameter models for vancomycin. Furthermore, the investigators will also demonstrate the contribution of different vancomycin elimination routes, renal and CSF drainage eliminating routs, to its total clearance, and the relationship between vancomycin renal clearance and creatinine clearance. Exclusion criteria include renal failure, unstable renal function, obesity, shock status, third space fluid accumulation, burn and pregnancy. The models to validate are used for empirical vancomycin dosage calculation. Therapeutic drug monitoring TDM is conducted after vancomycin serum concentration achieves steady state, and the predicted and observed serum concentration of vancomycin are compared to evaluate the consistency. On the other hand, vancomycin excreted from urine and CSF drainage fluid would be calculated to see their independent contribution for total vancomycin clearance. Meanwhile, the investigators try to investigate the mechanism for renal elimination of vancomycin by its association to creatinine clearance values calculated from urine creatinine concentration data. 