I have a question from my graduate pharmacology class and have been trying, and trying to find an answer! If anyone has any help or insight it would be greatly appreciated!

Ques. A comatose patient is admitted to the Emergency Department four hours after taking an overdose of phenobarbital. The plasma level of the drug at the time of admission is 100mg/L, and the apparent volume of distribution, half life, and clearance of phenobarbital are 35L, 4 days, and 6.1L/day. Respectively. The ingested dose was approximately:

a.) 1g, b.) 3.5g, c.) 6.1g, d.) 40g, e.) 70g, f.) 110g

I was looking for a dose/plasma ratio or some type of formula and have been unsuccessful!

Not sure if it is appropriate to give a graduate student an answer, an answer I am 100% is in the book or lecture.

If the clearance is 6.1L/day, that's about 0.2542L/hr, which means about 1.01667L in those 4 hours since ingestion. So, if plasma drug level was 100mg/L on arrival, the level was at least (100mg/L)x(1.01667L)=101.667mg four hours ago. But I'm really talking out of my behind... I don't know about pharmacology. I have no idea how apparent distribution and half-life are supposed to figure into the equation.

I suggest pharmacokinetics chapter of your textbook. :)

Or, if the distributed amount is supposed to be in equilibrium with plasma level, that would mean the total of 3500mg=(100mg/L)x35L, at the time of the test. I'm really just throwing out ideas. I don't know what I'm talking about...

If you ever figure it out, will you post? I'm very curious...