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junee
06-25-2008, 08:58 PM
Six hours after 500 mg of a drug is administered by IV injection, a patient's plasma concentration is 10 ug/ml. The half life of the drug is 4 hours, MEC ( minimum effective concentration) is 2 ug/ml, how many hours after the first dose should a second dose be administered?

Ans is 15.4

Isabel
06-25-2008, 10:29 PM
Hi, Junee.
I think this is one of the blue print questions..
What I did is, since we can't use calculator, I know by theory that the next dosis should be administred in less than 5 t1/2, 4*5= 20. So in less than 20 hours. Acccording to the alternatives, 15 was the closest.
Anyone with a better idea?
Isa

vivyzeta
06-25-2008, 11:08 PM
I agree. I did in the same way.
Vivi

sara_t
06-26-2008, 06:45 AM
I believe you should keep the minimum effetive dose in mind for anwering this quesiton: the concentration after 6 hours was 10 and by 2 half lives it will decrease to 2.5 so the answer is

6+4+4=14 but definitely less that 18 becasue by passing another half life the concentration will fall below the minimum effective dose and the figure should be closer to 14 not to 18 since 2.5 is more closer to 2 than 1.25.
Sorry for not using the units!

junee
06-26-2008, 04:33 PM
Thanks you very much.