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#1
04-30-2012
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Drug Dose, Half Life, and Plasma Level

A drug acheives a plasma level of 16mg/L shortly after the administration of 1st oral dose. If the half-life and the dosing interval are both 6 hrs, what is the approximate plasma level shortly before the administration of the 5th dose?

A. 15mg/L
B. 24mg/L
C. 28mg/L
D. 30mg/L
E. 31mg/L

This is from Kaplan book.
How will we solve this question?? I'm unable to understand it!

#2
04-30-2012
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Is that E?
#3
04-30-2012
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E) 31 mg/L

16 after 6 hours 8+16 = 24 after 6 hours 12+16 = 28 after 6 hours 14+16 = 30 after 6 hours 15+16 = 31 .... that's after 4 doses (just before the administration of the 5th dose).

With these types of questions if you don't know the equation use the simple method ... it saves you time in doing the calculation.
 The above post was thanked by: curiousmind (07-16-2012), Oung (04-30-2012)

#4
04-30-2012
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i posted this question before
16->24->28->30->31
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#5
04-30-2012
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Quote:
 Originally Posted by Hope2Pass E) 31 mg/L 16 after 6 hours 8+16 = 24 after 6 hours 12+16 = 28 after 6 hours 14+16 = 30 after 6 hours 15+16 = 31 .... that's after 4 doses (just before the administration of the 5th dose). With these types of questions if you don't know the equation use the simple method ... it saves you time in doing the calculation.
which formula can we use here?
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#6
04-30-2012
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.

This will be useful to understand the concept. One you get it, you'll be able to do this type of question with way much shorter way

Ok. so what we’re dealing here is trying to reach steady state.

Given: T1/2 = dosage interval

The following number correspond to the order numbers of the dose:
1) Right after 1st dose the concentration is 16mg/L

2) After 6 hours (t1/2) – right before giving the 2nd dose the concentration goes down to 8 mg/L. as the stem said at this time we give another dose so add 16mg/L to that 8.
So right after giving the second dose we have: 16 mg/L + 8 mg/L = 24mg/L

3) After another 6 hours (t1/2) – right before giving the 3rd dose the concentration goes down by 50% (50% x 24 mg/L) – so now we have 12 mg/L. And again as the stem said we give another dose, so add 16mg/L to that 12 mg/L we just calculated = 28 mg/L

4) After another 6 hours (t1/2) – right before giving the 4th dose the concentrations goes down by 50% (50% x 28mg/L) – so now we have 14 mg/L. And again as the stem said we give another dose, so add 16mg/L to that 14 mg/L we just calculated = 30 mg/L

5) After another 6 hours (t1/2) – right before giving the 5th dose the concentrations goes down by 50% (50% x 30 mg/L) – so now we have 15 mg/L. And this is what they asked about – the concentration of the drug right before administering the 5th dose.

I hope that helps

Last edited by Casandra; 04-30-2012 at 11:08 PM.
 The above post was thanked by: dr.aysa (05-01-2012), gayathri (05-01-2012), Hope2Pass (04-30-2012), neelima208 (05-01-2012), one_destiny (05-02-2012), steps_8186 (05-01-2012), Usman2Good (07-20-2013), USMLE cell (05-01-2012)
#7
04-30-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 607 Times in 340 Posts Reputation: 617

Quote:
 Originally Posted by Hope2Pass E) 31 mg/L 16 after 6 hours 8+16 = 24 after 6 hours 12+16 = 28 after 6 hours 14+16 = 30 after 6 hours 15+16 = 31 .... that's after 4 doses (just before the administration of the 5th dose). With these types of questions if you don't know the equation use the simple method ... it saves you time in doing the calculation.
@Hope2Pass - this first 16 is already after the 1st dose so 31mg/L is what we have after administering the 5th dose not before

The answer for that question is 15mg/L.
#8
04-30-2012
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Quote:
 Originally Posted by Casandra This will be useful to understand the concept. One you get it, you'll be able to do this type of question with way much shorter way Ok. so what we’re dealing here is trying to reach steady state. Given: T1/2 = dosage interval The following number correspond to the order numbers of the dose: 1) Right after 1st dose the concentration is 16mg/L 2) After 6 hours (t1/2) – the concentration goes down to 8 mg/L. as the stem said at this time we give another dose so add 16mg/L to that 8. So right after giving the second dose we have: 16 mg/L + 8 mg/L = 24mg/L 3) After another 6 hours (t1/2) – the concentration goes down by 50% (50% x 24 mg/L) – so now we have 12 mg/L. And again as the stem said we give another dose, so add 16mg/L to that 12 mg/L we just calculated = 28 mg/L 4) After another 6 hours (t1/2) – the concentrations goes down by 50% (50% x 28mg/L) – so now we have 14 mg/L. And again as the stem said we give another dose, so add 16mg/L to that 14 mg/L we just calculated = 30 mg/L 5) After another 6 hours (t1/2) – the concentrations goes down by 50% (50% x 30 mg/L) – so now we have 15 mg/L. And this is what they asked about – the concentration of the drug right before administering the 5th dose. I hope that helps
This Is what I meant
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#9
04-30-2012
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Quote:
 Originally Posted by Casandra @Hope2Pass - this first 16 is already after the 1st dose so 31mg/L is what we have after administering the 5th dose not before The answer for that question is 15mg/L.
I did the same blunder

At least the concept I remembered
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#10
04-30-2012
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Great Explanation.
Quote:
 Originally Posted by Kabutar111 This Is what I meant
 The above post was thanked by: Kabutar111 (04-30-2012)
#11
04-30-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,684 Threads: 213 Thanked 1,652 Times in 669 Posts Reputation: 1662

Quote:
 Originally Posted by Casandra @Hope2Pass - this first 16 is already after the 1st dose so 31mg/L is what we have after administering the 5th dose not before The answer for that question is 15mg/L.

ah stupid mistake.

Thanks for clarifying that!
#12
05-01-2012
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Great explanation

Thanks a trillion Casandra !!
#13
05-01-2012
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yess.....i got it 15 mg/dl....but after solving calculation i was a little bit worried about my conception whether i got it right or wrong as other posts were referring to another answer.........but later post of casandra filled my confidence all in at once.....thanks bro
#14
05-01-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 607 Times in 340 Posts Reputation: 617
;)

Quote:
 Originally Posted by Master shifu yess.....i got it 15 mg/dl....but after solving calculation i was a little bit worried about my conception whether i got it right or wrong as other posts were referring to another answer.........but later post of casandra filled my confidence all in at once.....thanks bro
I guess you meant "sis"

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#15
05-02-2012
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Yeah.. I got it right

15mg/L it is read Just before 5th dose i.e. it's past 4 half lives

#16
05-02-2012
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you have 5 doses
dose 1 ..results in 16 mg blood level
dose 2 ..results in half of the first dose , 8 +16 ( the new dose)..it is 24
dose 3 ..24/2+16 which is 28
dose 4 ..28/2=16 which is 30
before administration of the 5th dose , the plasma level is 30/2 which is 15
#17
05-03-2012
 USMLE Forums Addict Steps History: Step 1 Only Posts: 193 Threads: 16 Thanked 103 Times in 67 Posts Reputation: 113

Quote:
 Originally Posted by Casandra I guess you meant "sis" Glad I could help guys
OOppssss.....my mistake......:sorry:

 Tags Pharmacology-, Step-1-Questions

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