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#1




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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#2




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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#3




my answer :)
Quote:
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




Because of this sentence
"How much should be administered every six hours" 
#5




.
hmm.. but when we give the patient loading doses we also give them every X hours

#6




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




where is this question from? 
#8




I remember seeing this qn in UWorld. And, I was stumped!

#9




Quote:
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




thanku so much modesty...i was totally lost on this one....these equations n stuff drive me crazy
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modesty (06252012) 
#11




Quote:

The above post was thanked by:  
modesty (06252012) 
#12




Quote:
****!! haha 
#13




ans is 280

#14




Quote:

#15




Quote:
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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