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#1
03-28-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 111 Times in 74 Posts Reputation: 121
Calculating Drug Clearance

If dose is about 3000 ug/day, plasma concentration (Cp) is 15 ug/L then what will be the clearance?

a) 200 L/day
b) 45000 L/DAY
c) 0.005 L/day
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Last edited by neha_subh; 03-28-2013 at 06:21 AM.

#2
03-28-2013
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 283 Threads: 35 Thanked 205 Times in 115 Posts Reputation: 215

CL = Maintenace dose / Cp

= 3000 ug/day / 15 ug/L

= 200 day/L

Now flip everything:

1/200 /day / L

= 0.005 L/day

CL = 0.005 L/day

#3
03-28-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 111 Times in 74 Posts Reputation: 121

If dose is about 3000 ug/day (Cp) is 15 ug/L then what will be the clearance?

a) 200 L/day

CL= maintaince dose / Cp

CL= 3000 / 15

CL = 200 L/ day

kaplan asesment test question.

formulas for clearance of substance:
CL = dose / Cp
CL= free fraction x GFR
CL=rate of drug eliminatation / Cp
CL = Ke x vd
mainatince dose = CL X Css x dosing interval / f
Ko =CL X Css
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#4
03-29-2013
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Quote:
 Originally Posted by neha_subh If dose is about 3000 ug/day (Cp) is 15 ug/L then what will be the clearance? corect answer: a) 200 L/day CL= maintaince dose / Cp CL= 3000 / 15 CL = 200 L/ day kaplan asesment test question. formulas for clearance of substance: CL = dose / Cp CL= free fraction x GFR CL=rate of drug eliminatation / Cp CL = Ke x vd mainatince dose = CL X Css x dosing interval / f Ko =CL X Css

So this means that day/L is same thing as L/day and ug cancels out?
#5
03-29-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 111 Times in 74 Posts Reputation: 121

Quote:
 Originally Posted by Doctor Ali So this means that day/L is same thing as L/day and ug cancels out?
ya acording to explanation ug caceled out. thanks
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#6
03-29-2013
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Thanks for the Q

What are Ko and ke?
#7
03-29-2013
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ok, i need some help here. i really do need to learn this stuff, because i ignored it the first time around, and i never got it.

i have the formula for Clearance:

CL = rate of elimination of drug / plasma drug concentration

so according to:

CL = maintence dose/ Cp

Does that mean Maintence dose = Rate of elimination of drug?

what am i missing here?
#8
03-29-2013
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i guess i don't understand this equation:

CL = dose / Cp
#9
03-29-2013
 USMLE Forums Guru Steps History: Not yet Posts: 315 Threads: 115 Thanked 155 Times in 84 Posts Reputation: 165

is it saying that the dose of 3000 per day is also the rate of elimination per day, since that is the amount given per day?

i'm really confused here, and i'm actually reviewing pharm right now!
#10
03-29-2013
 USMLE Forums Scout Steps History: Not yet Posts: 40 Threads: 22 Thanked 3 Times in 3 Posts Reputation: 13
More questions

Does anyone have a list of calculation questions that we can work on? These type of questions are few and far between in the question banks which makes it difficult to practice on and I def need the practice! Please help. Thanks
#11
03-29-2013
 USMLE Forums Addict Steps History: Not yet Posts: 135 Threads: 3 Thanked 73 Times in 48 Posts Reputation: 83

Quote:
 CL = Maintenace dose / Cp = 3000 ug/day / 15 ug/L = 200 day/L ..... So this means that day/L is same thing as L/day and ug cancels out?
(3000 ug/day) / (15 ug/L) = (3000ug/day) X (L/15ug) = 200 L/day

If you have a fraction over a fraction, and you're confused how to solve it. Simply invert the denominator and now mutiply it with the numerator.
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You can't confuse the confused!
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 The above post was thanked by: MedicalExaminer (03-29-2013)
#12
03-29-2013
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 463 Times in 298 Posts Reputation: 473

Quote:
 Originally Posted by slowpoke (3000 ug/day) / (15 ug/L) = (3000ug/day) X (L/15ug) = 200 L/day If you have a fraction over a fraction, and you're confused how to solve it. Simply invert the denominator and now mutiply it with the numerator.
seems u have never missed ur math class

i find ur explanations very useful
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#13
03-29-2013
 USMLE Forums Guru Steps History: Not yet Posts: 315 Threads: 115 Thanked 155 Times in 84 Posts Reputation: 165

hey medicalexamer, could you please explain this concept for me? i understand the units, makes sense, but i don't get the equation. thanks!
#14
03-29-2013
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 463 Times in 298 Posts Reputation: 473

Quote:
 Originally Posted by ReggieMiller hey medicalexamer, could you please explain this concept for me? i understand the units, makes sense, but i don't get the equation. thanks!

The question stem was complex

U can get cl from different equetions

Cl=rate of elimination of drug/Cp-this equation is not useful in this particular case cause we dont know elimination rate

cl can be calculated by VdxKe as well but we dont know the Ke (elimination constant)

Third equation is maintenance dose=Cpxcl/f (as bioavailability is not given we assume its 100%=1)
So maintenance dose=Cp x Cl
as there is no other way of solving the equation, we assume above unit-dose reffers to maintenance dose so Cl would equal maintenance dose devided by plasma drug concentration
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#15
03-29-2013
 USMLE Forums Guru Steps History: Not yet Posts: 315 Threads: 115 Thanked 155 Times in 84 Posts Reputation: 165

thanks!

so would you have to assume that biovailability is 100%? or was this info given in the question stem as an IV drug?

from the info given, i wouldn't have known where to start!

#16
03-29-2013
 USMLE Forums Guru Steps History: Not yet Posts: 315 Threads: 115 Thanked 155 Times in 84 Posts Reputation: 165

ok, i think i'm getting a hang of this. tomorrow i'll do a quick run through. thanks.

the question says dose, but its actually a DOSAGE RATE (maintenence dose) that is given, which at steady state is equal to the elimination rate. i'm sure the question stem had to have mentioned 100% biovailablity, so that the denominator is 1.

so now it makes much more sense. thanks!

if possible, would someone mind posting the complete question for this? i just want to make sure i'm on the same wavelength here. it was just frustating because i had just finished doing pharmacokinestics earlier today, and was stumped with the question.

as far as my explanation above goes, is that the correct reasoning/concept?
#17
03-29-2013
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 463 Times in 298 Posts Reputation: 473

Quote:
 Originally Posted by ReggieMiller ok, i think i'm getting a hang of this. tomorrow i'll do a quick run through. thanks. the question says dose, but its actually a DOSAGE RATE (maintenence dose) that is given, which at steady state is equal to the elimination rate. i'm sure the question stem had to have mentioned 100% biovailablity, so that the denominator is 1. so now it makes much more sense. thanks! if possible, would someone mind posting the complete question for this? i just want to make sure i'm on the same wavelength here. it was just frustating because i had just finished doing pharmacokinestics earlier today, and was stumped with the question. as far as my explanation above goes, is that the correct reasoning/concept?
Yup
There has been some new questions on this topic posted
Practice on them
__________________
Everything is possible for him who believes (MARK 9:23)
245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad
 The above post was thanked by: ReggieMiller (03-29-2013)
#18
03-30-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 111 Times in 74 Posts Reputation: 121

Quote:
 Originally Posted by ReggieMiller ok, i think i'm getting a hang of this. tomorrow i'll do a quick run through. thanks. the question says dose, but its actually a DOSAGE RATE (maintenence dose) that is given, which at steady state is equal to the elimination rate. i'm sure the question stem had to have mentioned 100% biovailablity, so that the denominator is 1. so now it makes much more sense. thanks! if possible, would someone mind posting the complete question for this? i just want to make sure i'm on the same wavelength here. it was just frustating because i had just finished doing pharmacokinestics earlier today, and was stumped with the question. as far as my explanation above goes, is that the correct reasoning/concept?
Q: A patient has been using st.john's wort,an herbal medicine for treatment of depresion for several months as part of a drug study.his dose of active ingredient hypericin is standarized to 3mg/day and plasma level of this compound is measured to be 15ug/L.asuming 100% bioavailability what is clearance of hypericin in this patient??

a)5 L/DAY
B)200 L /DAY
C) 450 L/DAY
and were other options

Answer of this question is (B) : one need to calculate daily CL of hypericin in a patient who ingests 3mg/day. 100% bioavailability therefore one can conclude that all 3mg of hypericin have reached systemic circulation.if plasma conc: is 15ug/ml the clearance can b calculated by dividing the dose by concentration.it is important to note that the units for concntration must be same for dose and Cp.
CL can be calculated as folows:
CL = dose / Cp
CL= 3000ug/day/15ug/L
CL = 200 L /day.

sorry i forgot from where i wrote this question on mine kaplan notes that was incomplete, today again i went thru question pool book and finaly i got it and posted here.
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#19
03-30-2013
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 283 Threads: 35 Thanked 205 Times in 115 Posts Reputation: 215

Quote:
 Originally Posted by neha_subh ya acording to explanation ug caceled out. thanks

Thank you so much!!!!!
#20
03-30-2013
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 283 Threads: 35 Thanked 205 Times in 115 Posts Reputation: 215

Quote:
 Originally Posted by ReggieMiller ok, i need some help here. i really do need to learn this stuff, because i ignored it the first time around, and i never got it. i have the formula for Clearance: CL = rate of elimination of drug / plasma drug concentration so according to: CL = maintence dose/ Cp Does that mean Maintence dose = Rate of elimination of drug? what am i missing here?

There's nothing to be confused about. Let's walk here through. The question says if dose is about 3000 ug/day, plasma concentration (Cp) is 15 ug/L then what will be the clearance, right?

So, first thing we do is list all the formulas.

1. t1/2 = 0.7 X Vd / Cl

where Vd is volume of distribution. Cl is clearance. 0.7 is constant. t1/2 is half life.

where Cp is plasma concentration. Cp is also Css. In other words, Loading dose = Css X Vd/F. Css and Cp are EXCHANGABLE.
F is always 1 unless it is specify on the question stem. If question stem does not mention what is F, then it is 1.

3. Maintance dose = Cp X Cl/F
Maintance dose = Css X Cl/F

Again, Cp and Css are exchangable and they each mean the same. Also, F is always 1 UNLESS the question stem specifies what F is. If not, then F is 1.

Now let's look at the problem again. If dose is about 3000 ug/day, plasma concentration (Cp) is 15 ug/L then what will be the clearance?

We are given the following:

Cp = 15 ug/L
dose = 3000 ug/day. This could be loading dose or Maintance dose. We will find out when we will look at our final clue.

The question ask what will be clearance? From here, we now know that the dose is maintance dose. Because only that equation involves clearance; and it is the clearance that we need to solve for.

Rearrange the equation so that we can solve for Cl:

Maintance dose = Cp X Cl/F

Cl = F x Maintance dose/ Cp

F = 1 because it did not specify in the stem.

Cl = 1 X 3000 ug/day / 15 ug/ L

Cl = 200 L/ day or day/ L.
#21
07-17-2013
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 592 Threads: 92 Thanked 195 Times in 127 Posts Reputation: 205

Q:if drug is known to be distributed into TBW, what dose (mg) is needed to obtain an initial plasma level of 5mg/L in patient weighing 70kg?

a)210
b)150
c)110
d)50
e)35

The answer is 210, but i cant find a way to get there, please someone explain

how do you get this answer??
#22
05-26-2014
 USMLE Forums Scout Steps History: Not yet Posts: 11 Threads: 2 Thanked 0 Times in 0 Posts Reputation: 10

total body water 60 % of body . i.e 70*60/100 = 42L

42*5 = 210

 Tags Pharmacology-, Step-1-Questions

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