BACKGROUND: Determination of blood concentration is essential for the pharmacokinetic/pharmacodynamic (PK/PD) study of intravenous anesthetic agents. Knibbe's method is a rapid, simple, and reliable method to quantify propofol in low-volume samples. The aim of this study was to qualify Knibbe's method by calculating the performance error in propofol target-controlled infusion (TCI). METHODS: With ethics commitee approval and informed consent, thirty ASA 1 or 2 adult patients undergoing orthopedic surgery participated in this study. All patients were premedicated with atropine 0.5 mg IM and received general anesthesia with propofol TCI (Master TCI) supplemented by 67% N2O and 33% O2. Anesthesia was induced by propofol TCI with a target concentration of 6 microgram/ml and maintained around 3-5 microgram/ml according to bispectral index (40-50). In the middle of surgery, the target concentration was increased to 6 microgram/ml and maintained until the effect concentration matched the target concentration. Three min after equilibration, 3 ml of blood was taken from the radial artery to analyse blood concentration using HPLC with the fluorescence detection method described by Knibbe et al. Using this method, the performance error for the 1 6 microgram/ml of predicted propofol concentration in whole blood was calculated according to the following formula: measured concentration-predicted concentration/predicted concentration x 100. RESULTS: The performance errors at each concentration were -12.86, -13.61, -2.95,7.94,9.22, and 13.85% for 1-6 microgram/ml of predicted propofol concentration, respectively. CONCLUSIONS: Knibbe's method for determination of plasma propofol concentration showed itself to be accurate accuracy in that there was a relatively low performance error in the concentration range of 1 6 microgram/ml, which is considered the usual concentration range for clinical practice.