Young child weighing 20 kg was given a drug in the dose 100mg/kg body weight. The plasma concentration of the drug is 2mg/ dl and the clearance is 200 ml/hr. What is the time required to reach steady state plasma concentration -

Ans. is 'b' i.e., 20 hours Volume of distribution = total dose/plasma concentration Total dose= dose/ kg x body weight = 100 x 20 = 2000 mg Volume of distribution = 2000/2 = 1000 Half life = 0.693 x Volume of distribution/ clearance = 0.693 x 1000/200 = 3.5 hours Time required to reach steady state plasma concentration is 5.5 half lives = 5.5 x 3.5 = 19.25 hours Therefore the most appropriate answer is 20 hours.