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#1
03-21-2012
 USMLE Forums Newbie Steps History: Not yet Posts: 1 Threads: 1 Thanked 0 Times in 0 Posts Reputation: 10
Calculating drug maintenance dose!

A patient with a bacterial infection requires intravenous antibiotic therapy. The chosen drug has a clearance (Cl) of 70 mL/min. The apparent volume of distribution (Vd) is 50 L. The plan is to administer the drug intravenously every 6 h and achieve a 4-mg/L steady-state blood level of the drug. No loading dose strategy is to be used. Which of the following maintenance doses is needed to achieve this?

Can anyone help me solve this? thanks.

#2
03-21-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,353 Threads: 94 Thanked 877 Times in 507 Posts Reputation: 890

Md=cl x Css

70 x 4 = 280Mg/min
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 The above post was thanked by: achez&pains (03-21-2012)
#3
03-25-2012
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Quote:
 Originally Posted by DocSikorski Md=cl x Css 70 x 4 = 280Mg/min
in this question clearance is 70 mili litre per minute and steady state level is 4 milligrams per Litre.... how can you multiply the two??
 The above post was thanked by: DocSikorski (03-25-2012)

#4
03-25-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,684 Threads: 213 Thanked 1,600 Times in 660 Posts Reputation: 1610

Well you can convert the ml & L :

70 ml/min * 4 mg/L * 1L/1000ml
= 0.28 mg/min
= 16.8 mg/ hour

The ml & L cancel out and the answer that way is 0.28 mg/ min or 16.8 mg/hour.

Is that the right answer? I'm not sure where you would use the clearance and the volume of distribution.
#5
06-21-2012
 USMLE Forums Scout Steps History: Not yet Posts: 11 Threads: 0 Thanked 11 Times in 6 Posts Reputation: 21

Yes 0.28mg/min is the correct answer.

MD= Css*CL
4mg/L * 70ml/min * 10^-3/ml = 0.28mg/min
#6
06-21-2012
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 394 Threads: 40 Thanked 218 Times in 99 Posts Reputation: 228

Are we not missing anything ?? Q stated Md to be given every 6 hrs.. Shouldnt we consider 6h time interval to calculate Md

Md= (Cs * Cl * t) / F

Then Md will come around 100.8 mg every 6th hour.. Correct me if m wrong..
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 The above post was thanked by: drhma (06-22-2012)
#7
06-22-2012
 USMLE Forums Addict Steps History: 1 + CS Posts: 193 Threads: 29 Thanked 134 Times in 71 Posts Reputation: 144

rookie is 100% correct, there is a new UWORLD question just like that, saw it yesterday :\\

U have to make use of the dosing interval, dont forget that guyssss in the test
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#8
06-22-2012
 USMLE Forums Scout Steps History: Not yet Posts: 11 Threads: 0 Thanked 11 Times in 6 Posts Reputation: 21

hmm.. good point. Thanks for that.
#9
06-22-2012
 USMLE Forums Scout Steps History: Not yet Posts: 11 Threads: 0 Thanked 11 Times in 6 Posts Reputation: 21

Hey guys, found this explainations somewhere... hope this helps!!

The scenario in question, this makes calculations relatively straightforward, particularly because no loading dose therapy will be used.
Steady-state blood levels occur when the rate of “drug in” equals the
rate of “drug out.” The volume of distribution, given in the question, is
irrelevant for the calculations.

The rate of drug out is given as a Cl
= 70 mL/min.

Recall that the dose (
D) = Cl × CSS

Therefore, with a little rearranging, the dose can be computed as:
Desired plasma level
= Vd × Cl, or 4 mg/L × 70 mL/min.
Convert the units so they’re consistent for both variables, and don’t
forget to calculate how many minutes there are in the dosing interval, cited as 6 hours. Now, the rest of the math:

The target blood level of 4 mg/L is the same as 4 mcg/mL, and this is a
reasonable change of volume units to make for subsequent calculations,
since clearance is given in units of mL/min
4 mcg/mL
× 70 mL/min = 280 mcg/min = 0.28 mg/min

And since the drug will be given every 6 hours:
0.28 mg/min
× 60 min/h × 6 h = 100.8 mg

(closest answer is 100 mg) every 6 hours.

 The above post was thanked by: Ahmed1992 (08-03-2016), giovanna (01-11-2014), kaym (07-09-2012)

 Tags Pharmacology-, Step-1-Questions

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