Analysis of a drug (FS) in a urine of patient using an internal standard (IS) was carried out using HPLC. The following data were obtained in anaiysis: Peak area of (IS) Conc. of (FS) Conc of (IS) ug/ml Peak area of (FS) ug/ml 0.25 0.5 4800 5000 0.50 0.5 9900 5000 0.75 0.5 15500 5000 1.00 0.5 21000 5000 25500 5000 1.25 0.5