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#1
06-14-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,268 Times in 881 Posts Reputation: 3278
Challenging question: How long the drug would be effective?

A 400 mg dose of a drug has therapeutic efficacy for 8 h. If the clearance of the drug is 2.52 L/h and the drug is distributed into total body water, for how long would a 800 mg dose be effective in a patient weighing 60 kg?

A) 8 h
B) 10 h
C) 16 h
D) 18 h
E) 24 h
F) 28 h

Last edited by bebix; 06-15-2011 at 08:06 AM. Reason: typo, ty pass7!
 The above post was thanked by: Al-Saoudi (06-14-2011), Claus_CU (06-14-2011), nevillenic (06-15-2011), pass7 (06-14-2011), struggle (06-15-2011)

#2
06-14-2011
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Yikes, no idea!

I would guess randomly D because for some reason I feel like it would be efficacious for a bit longer than 2x the original. Either that or B

Last edited by apx85; 06-14-2011 at 02:45 PM.
 The above post was thanked by: bebix (06-14-2011)
#3
06-14-2011
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i think d answer iz 10h.. option b!!
as far as i think d drug follows a zero order kinetics!!

Last edited by sidra; 06-14-2011 at 01:15 PM. Reason: i thought again!!!
 The above post was thanked by: bebix (06-14-2011)

#4
06-14-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,268 Times in 881 Posts Reputation: 3278

Quote:
 Originally Posted by sidra i think d answer iz 8h.. option a!! as far as i think d drug follows a zero order kinetics!!
why zero order kinetics???
#5
06-14-2011
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good question.
I think the answer is D.
 The above post was thanked by: bebix (06-14-2011)
#6
06-14-2011
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I would guess B..
 The above post was thanked by: bebix (06-14-2011)
#7
06-14-2011
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@bebix...where did u find this qs???
#8
06-14-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,268 Times in 881 Posts Reputation: 3278

Quote:
 Originally Posted by Claus_CU @bebix...where did u find this qs???
this is from Kaplan....
#9
06-14-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,268 Times in 881 Posts Reputation: 3278

Well, the correct answer is D) 18 h

First, we need to calculate the half time of this drug:

general equation: t1/2 = 0.7 / ke, where ke = Cl / VD

In this case VD = 60% of 60 Kg = Total body water = 36 L

ke = 2.52 / 36 = 0.07

then, t1/2 = 0.7 / 0.07 = 10 h

We know that "400 mg dose of a drug has therapeutic efficacy for 8 h" [which is NOT the same as t1/2]

So, if we use 800mg => 1 t1/2 later (10 hr later) => 400 mg => and this 400mg still has a therapeutic efficacy for 8 h.

for how long would a 800 mg dose be effective in this patient then?

10h + 8h = 18h
 The above post was thanked by: apx85 (06-14-2011), dr z (06-15-2011), Lujain (06-15-2011), rseruy (12-08-2012), Silversurfer (10-15-2011), struggle (06-15-2011)
#10
06-14-2011
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Gotta get lucky with a guess every now and then!

Good question! Should help me remember the relationship between t(1/2) and Vd/CL
 The above post was thanked by: bebix (06-14-2011)
#11
06-14-2011
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but it doesn't say whether it's first-order-elimination or zero-order elimination???????????

but it doesn't say whether it's first-order-elimination or zero-order elimination??????????? how about zero order elimination,?
Quote:
 Originally Posted by bebix Well, the correct answer is D) 18 h First, we need to calculate the half time of this drug: general equation: t1/2 = 0.7 / ke, where ke = Cl / VD In this case VD = 60% of 60 Kg = Total body water = 36 L ke = 2.52 / 36 = 0.07 then, t1/2 = 0.7 / 0.07 = 10 h We know that "400 mg dose of a drug has therapeutic efficacy for 8 h" [which is NOT the same as t1/2] So, if we use 800mg => 1 t1/2 later (10 hr later) => 400 mg => and this 400mg still has a therapeutic efficacy for 8 h. for how long would a 800 mg dose be effective in this patient then? 10h + 8h = 18h
 The above post was thanked by: bebix (06-15-2011)
#12
06-14-2011
 USMLE Forums Guru Steps History: 1+CK+CS+3 Posts: 406 Threads: 30 Thanked 141 Times in 109 Posts Reputation: 151

Quote:
 Originally Posted by bebix Well, the correct answer is D) 18 h First, we need to calculate the half time of this drug: general equation: t1/2 = 0.7 / ke, where ke = Cl / VD In this case VD = 60% of 60 Kg = Total body water = 36 L ke = 2.52 / 36 = 0.07 then, t1/2 = 0.7 / 0.07 = 10 h We know that "400 mg dose of a drug has therapeutic efficacy for 8 h" [which is NOT the same as t1/2] So, if we use 800mg => 1 t1/2 later (10 hr later) => 400 mg => and this 400mg still has a therapeutic efficacy for 8 h. for how long would a 800 mg dose be effective in this patient then? 10h + 8h = 18h
when calculating the Ke= 2.52(ml/min )and 36 L, since its given in 2 different units,shouldnt we match the unit first???
i mean this is wat i got from the calculation ..
t1/2= 0.7 x3600/2.52 =1000mints =16.6hrs
so 800mg =16+8hrs=24hrs ?????
wat do u think??
 The above post was thanked by: bebix (06-15-2011)
#13
06-15-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,268 Times in 881 Posts Reputation: 3278

Quote:
 Originally Posted by pass7 when calculating the Ke= 2.52(ml/min )and 36 L, since its given in 2 different units,shouldnt we match the unit first??? i mean this is wat i got from the calculation .. t1/2= 0.7 x3600/2.52 =1000mints =16.6hrs so 800mg =16+8hrs=24hrs ????? wat do u think??
@kingwang = zero order elimination drugs have no fixed half life (a constant amount of the drug is eliminated per unit of time)...so, you couldnt do the math in the first place.

@pass7 = you are absolutly right...the clearence should be 2.52L/h

Last edited by bebix; 06-15-2011 at 08:06 AM.
 The above post was thanked by: pass7 (06-16-2011)
#14
06-15-2011
 USMLE Forums Guru Steps History: Not yet Posts: 324 Threads: 70 Thanked 239 Times in 144 Posts Reputation: 249

actually by changing ml to L and min to hours we are getting wrong calculations or may be i am weak in maths,,
#15
06-15-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,268 Times in 881 Posts Reputation: 3278

Quote:
 Originally Posted by struggle actually by changing ml to L and min to hours we are getting wrong calculations or may be i am weak in maths,,
your math is just fine!...the typo was in the book...since the right answer is 18h, the clearence should be L/h....pass7 was right

 The above post was thanked by: struggle (06-15-2011)

#16
06-15-2011
 USMLE Forums Veteran Steps History: 1 + CS Posts: 216 Threads: 56 Thanked 116 Times in 56 Posts Reputation: 116

sheesh.....I just followed a quick and easy method

Vd = 60 x .6 = 36
CL = given as 2..

1T(1/2) = .7(Vd/CL) = 10

800mg after 1 half life = 400 mg (takes 10 hrs)

400mg = efficacious for 8hrs

10+8 = 18

 Tags Pharmacology-, Step-1-Questions

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