Need help with pharm calculation

[kartagener]

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I can't seem to figure this out. I feel as if I'm missing something. I've asked someone and they keep coming up with the same wrong answer as me. If anyone could explain it would be greatly appreciated. My exam is in a few days. Thank You.

30 year old female brought to the ER 30 mins after her friend could not arouse her. History of IV drug abuse. Her friend says the patient injected herself with an IV drug 6 hours ago. Labs show serum conc of drug to be .30 mg/L. The drug has first order kinetics with a half life of 2 hours and a volume of distribution of 200L in this patient. What is the quantity of the drug she injected?

A)300 mg
B)480 mg
C)650 mg
D)1000 mg
E)1250 mg

 

[Mr. Mojo Risin]

What's the right answer? On first glance I came up with answer B.

 

[Transposony]

You have to calculate the Loading dose first and then double the value every 2 hours till 6 hours to reach the dose she took I/V.
Loading dose= Plasma concentration X Volume of distribution.
LD= 0.30 X 200 = 60
2 hr ago it was 120 mg
4 hr ago it was 240 mg
6 hr ago it was 480 mg.

 

[kartagener]

Thank you to the both of you but esp Transposony for the explanation.

 

[Mr. Mojo Risin]

Using the loading dose works too, but you can also use the Vd equation if you forget the Loading Dose equation. Find the plasma concentration before 3 half-lives, then multiply by the Vd.

Rearranging the Vd equation:

Amount of drug in the body = Vd x (plasma concentration of drug)