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Old 06-25-2012
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Default Pharmacokinetics questions

An antibiotic follows first order kinetics and has a volume of distribution of 70L and a clearance of 0.5L/min. How much should be administered every six hours to obtain a therapeutic steady state plasma concentration of 4mg/L?

A. 2mg
B. 35mg
C. 280mg
D. 720mg
E. 12,600mg

please expain your answers
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Old 06-25-2012
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Hi, well the answer is D.

They are practically asking you about a maintenance dose, so we can disregard volume of distribution.
The equation is MD = Css X CL X t

Css = plasma steady state is 4mg/L
Clearance they gave you 0.5L/min
if you multiply you get 2 mg/min
You can already see that this is not the answer as the answers were given as mg and not in mg/min so you need to get rid of this min
t = time interval the drug was given
its given every 6 hours so you have to make from it minutes because when you will multiply minutes in 2mg/min the minutes will cancel out
so 6 hours = 60minX 6 = 360 minutes

2mg/min X 360 minutes = 720mg
I would say it's a pretty easy question when you know the equation and make note that you should look also on the valuves so mg/L mg/min etc they should also be taken care of , so not only the numbers

I would only add the whole Equation (Css X CL X t)/F should be divided by F which is the bioavailability usually the give the drug IV so it's 1 but maybe in some tricky questions it will be given orally (i doubt it will happen in the USMLE as i understand we cant use calculators...).

Hope it was understandable.
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Default my answer :)

Quote:
Originally Posted by numbndumb View Post
An antibiotic follows first order kinetics and has a volume of distribution of 70L and a clearance of 0.5L/min. How much should be administered every six hours to obtain a therapeutic steady state plasma concentration of 4mg/L?

A. 2mg
B. 35mg
C. 280mg
D. 720mg
E. 12,600mg

please expain your answers
I kinda disagree... they are asking about the dose needed to obtain the steady state not to sustain it. so I would go for loading dose, which would be:

LD = Vd x Cp = 70L x 4 mg/L = 280 mg
The equation should include bioavailability but bc they didn't mention it I assume it's iv drug or otherwise 1.

@modesty - could you please explain why do you think they are asking about the maintenance dose?
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Because of this sentence
"How much should be administered every six hours"
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Quote:
Originally Posted by modesty View Post
Because of this sentence
"How much should be administered every six hours"
hmm.. but when we give the patient loading doses we also give them every X hours
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Yes they are asking about maintainance dose. I got that but i didnt know we had to multiply it by the time too. I thought the answer was 2mg

How do we know when to multiply it by tiem and when not to do so?
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Quote:
Originally Posted by numbndumb View Post
Yes they are asking about maintainance dose.




where is this question from?
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I remember seeing this qn in UWorld. And, I was stumped!
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Quote:
Originally Posted by numbndumb View Post
Yes they are asking about maintainance dose. I got that but i didnt know we had to multiply it by the time too. I thought the answer was 2mg

How do we know when to multiply it by tiem and when not to do so?
They asked you how much should be given every 6 hours, thats how you know you need to consider the "t" in the equation.
also as i mentioned before its not 2mg it's 2mg/min - you can see in their answers that none of them is 2mg/min so you know that you need to multiply by minutes.
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thanku so much modesty...i was totally lost on this one....these equations n stuff drive me crazy
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Quote:
Originally Posted by modesty View Post
Hi, well the answer is D.

They are practically asking you about a maintenance dose, so we can disregard volume of distribution.
The equation is MD = Css X CL X t

Css = plasma steady state is 4mg/L
Clearance they gave you 0.5L/min
if you multiply you get 2 mg/min
You can already see that this is not the answer as the answers were given as mg and not in mg/min so you need to get rid of this min
t = time interval the drug was given
its given every 6 hours so you have to make from it minutes because when you will multiply minutes in 2mg/min the minutes will cancel out
so 6 hours = 60minX 6 = 360 minutes

2mg/min X 360 minutes = 720mg
I would say it's a pretty easy question when you know the equation and make note that you should look also on the valuves so mg/L mg/min etc they should also be taken care of , so not only the numbers

I would only add the whole Equation (Css X CL X t)/F should be divided by F which is the bioavailability usually the give the drug IV so it's 1 but maybe in some tricky questions it will be given orally (i doubt it will happen in the USMLE as i understand we cant use calculators...).

Hope it was understandable.
Thanks dude....and just one other thing there will be a calculator on the real exam just like uworld
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Quote:
Originally Posted by DefeatTheBeast View Post
Thanks dude....and just one other thing there will be a calculator on the real exam just like uworld
Wow, i thought there is no calculator as there isnt one in the NBME and i thought it will look the same as NBME test, also i saw in one video of kaplan the guy said they give easy calculations as you cant use calculator, i was totally sure there isnt one in the real exam. I also thought that i will never have to deal with the kaplan Qbank calculations about a person who drinks x osmomoles and then loses y osmoles in the urine and you need to calculate total osmolarity and stuff like that.
****!! haha
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ans is 280
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Quote:
Originally Posted by modesty View Post
Wow, i thought there is no calculator as there isnt one in the NBME and i thought it will look the same as NBME test, also i saw in one video of kaplan the guy said they give easy calculations as you cant use calculator, i was totally sure there isnt one in the real exam. I also thought that i will never have to deal with the kaplan Qbank calculations about a person who drinks x osmomoles and then loses y osmoles in the urine and you need to calculate total osmolarity and stuff like that.
****!! haha
Yeah its kinda weird how they don't have it on the nbme but have it on the real thing...you can google it too, lots of people have said there is a calculator on it
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Quote:
Originally Posted by Casandra View Post
hmm.. but when we give the patient loading doses we also give them every X hours
We give them loading dose only once then we give them the maintenance dose - Just look at the first order kinetics graph you give them 200% so after one half life they go down to 100% and you keep giving them the maintenance dose and keep them on 100% .
As you can see giving them loading dose every time will overdose them e.g - you gave 200% then waited 1 half life you are at 100% give another 200% hes in 300% then .
Also i think a question that wants us to calculate loading dose will state it in it's stem.
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