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#1
06-25-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 529 Threads: 57 Thanked 284 Times in 152 Posts Reputation: 294
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

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#2
06-25-2012
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 194 Threads: 21 Thanked 147 Times in 85 Posts Reputation: 157

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.
 The above post was thanked by: anomali (06-25-2012), DefeatTheBeast (06-25-2012), Doc4Step1 (12-28-2012), numbndumb (06-25-2012), raghuvanshi (04-15-2013), zainsyed (05-01-2014)
#3
06-25-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

Quote:
 Originally Posted by numbndumb 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?

#4
06-25-2012
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 194 Threads: 21 Thanked 147 Times in 85 Posts Reputation: 157

Because of this sentence
"How much should be administered every six hours"
#5
06-25-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580
.

Quote:
 Originally Posted by modesty 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
#6
06-25-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 529 Threads: 57 Thanked 284 Times in 152 Posts Reputation: 294

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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#7
06-25-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

Quote:

where is this question from?
#8
06-25-2012
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I remember seeing this qn in UWorld. And, I was stumped!
#9
06-25-2012
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 194 Threads: 21 Thanked 147 Times in 85 Posts Reputation: 157

Quote:
 Originally Posted by numbndumb 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.
#10
06-25-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 791 Threads: 76 Thanked 673 Times in 317 Posts Reputation: 691

thanku so much modesty...i was totally lost on this one....these equations n stuff drive me crazy
 The above post was thanked by: modesty (06-25-2012)
#11
06-25-2012
 USMLE Forums Guru Steps History: Step 1 Only Posts: 452 Threads: 11 Thanked 354 Times in 197 Posts Reputation: 364

Quote:
 Originally Posted by modesty 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
 The above post was thanked by: modesty (06-25-2012)
#12
06-25-2012
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 194 Threads: 21 Thanked 147 Times in 85 Posts Reputation: 157

Quote:
 Originally Posted by DefeatTheBeast 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
#13
06-25-2012
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ans is 280
#14
06-25-2012
 USMLE Forums Guru Steps History: Step 1 Only Posts: 452 Threads: 11 Thanked 354 Times in 197 Posts Reputation: 364

Quote:
 Originally Posted by modesty 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
#15
06-25-2012
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 194 Threads: 21 Thanked 147 Times in 85 Posts Reputation: 157

Quote:
 Originally Posted by Casandra 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.
 The above post was thanked by: zainsyed (05-01-2014)

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