pharm, steady state quesiton

[TJDoc7]

Came across a question that kind of confused me, wanted to know if anyone could clear it up for me...I understand the 1/2 life/percentages:
(1=50%, 2=75%, 3=88, 4=94), BUT came across a question that asked in such a way with what percentage would be present in body/eliminated? I don't have it to post, but just to clarify (i'm not sure now)...

1 half life= 50% remains, 50% drug elim.
2 1/2 life = 75% remain, 25% elim.
3 1/2 life = 88% remain, 12 elim.
4 1/2 life = 94% remain, 6 emil.

...or is the remain/elim the part that is switched and why I got it wrong...

Thanks.

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[NervousPreMed87]

I think you have the remain/elim part switched...

The way you have it now, as what remains, implies that the percentage in the body is increasing with time, which doesn't make sense...

 

[MilkmanAl]

Isn't it a lot easier to just take powers of .5 to see how much you have left (and then subtract from 1 if you need how much you've eliminated) rather than memorizing numbers?

 

[TJDoc7]

It is related to drug at interval dosing though...so by starting with let's say 100units...goes down to 50 (1st)...add 100...150...goes back down to 75 (2)...add 100...175...goes back down to 88 (3)...add 100...188...goes back down to 94 (4)-acceptable steady state...it's pretty much the same method you are talking about the the .5, I just happened to remember it this way/way I was taught...but my question isn't about coming up the with numbers, what gave me trouble was interpretation of them...so, again, if anyone can clear it up...in terms of the half lifes/dosing, the 50/50, 75/25, 88/12, 94/6 pertain to remaining drug/eliminated ratio after four half lifes?

 

[MilkmanAl]

The numbers in your original post are correct but switched and only apply if you've stopped giving the medication. For example, using your numbers, 75 is obviously not 75% of 200, the total dose given after 2 half-lives.

Oh wait, I think I see what you're saying. When dosing at the medication's half life, the 50, 75, 87.5, 94, etc. progression will indicate what percentage of the steady state concentration you're at. The steady state concentration will be the dose you're giving, regardless of loading dose or gradual increase. Deviate from dosing per half-life, though, and things aren't quite as ideal.

 

[TJDoc7]

The numbers in your original post are correct but switched and only apply if you've stopped giving the medication. For example, using your numbers, 75 is obviously not 75% of 200, the total dose given after 2 half-lives.

Oh wait, I think I see what you're saying. When dosing at the medication's half life, the 50, 75, 87.5, 94, etc. progression will indicate what percentage of the steady state concentration you're at. The steady state concentration will be the dose you're giving, regardless of loading dose or gradual increase. Deviate from dosing per half-life, though, and things aren't quite as ideal.

Just found the question, don't want to post it verbatim as it is off of a question bank (don't need any copyright issues right now), but overall, it's saying,

"guy with an infection is given antibiotic w/1/2 life of 3 hours. what percentage of the INITIAL dose will remain in his body 6 hours later?"

...so obviously, 2 half lives will pass (3H x 2 = 6), so then we apply this to the format which I said (and you outlined) earlier, with 1t = 50, 2t = 75, 3t = 88, 4t = 94. Now checking with the answer, and what you said, correct answer is 25% (because you take the two half lives, 75% ELIMINATED, 25% initial dose REMAINS). So, like you said, numbers from above need to be switched.

That said, but I always thought this was in regard to dosing though...meaning if you wanted to get someone to a certain point when they had enough drug "in" them, you had to do 4 half lives...but that is related to the maintence dose and not related to the half live/steady state principle at all (which according to this question, is primarily dealing w/eliminaton)?

 

[TJDoc7]

sorry, I just read it again...I think I was WAY overthinking it...it's not even a dosing/steady stage question, it looks like a straight half life question and has nothing to do w/dosing...serves me right for overthinking it.

But just to clarify, was I was saying earlier w/the steady state, I believe that DOES in fact apply to administration of a drug at given intervals to get someone to a state w/steady (rate in = rate out) of the drug...that is correct then, right (disregard the example above w/this now)...

 

[TJDoc7]

thanks for your help...was actually making it harder on myself than it was...mis-read the one question and it made me kind of lose track of the difference between the straight half life/steady state concentration questions...but to sum up, just in case I (hopefully) didn't confuse anyone...

questionds dealing w/straight half life problems (like the one above), I over-thought it, but all it is, keep "halfing" the amount (ex. 100--50--25--etc..., with eliminated>remaining).

What I was confused over, was the steady state principle, which says steady in = steady out, 4-5 1/2 lifes is acceptable for step...meaning, 1half life = 50%, 2half life = 75%, 3half life = 88%, 4half life = 94% (which I now believe I am right in saying, that percentage pertains to the steady state REMAINING in the body...need those four dosings to get to a constant state).

Thanks again for help.