Serum samples (10L) were spiked with a known amount (10L) of each standard at four different concentrations and were treated in a manner similar to that described above. A calibration
curve was constructed by plotting the peak area ratio to the IS of each spiked sample against the added concentration, and aworking curve was constructed by plotting the peak area of the IS against the concentration. Precision was expressed as the relative standard deviation (RSD, %), and the accuracy was expressed as the relative mean error (RME, %). The equations used to calculate RSD and RME were as follows: RSD (%) = (standard deviation/mean)×100 and