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#1




Difference between rate of elimination and clearance?
Which statement is accurate for the drug shown in the example below?
100 mg 2hr » 50 mg 2hr > 25 mg 2 h r » 12.5 mg A. The rate of elimination is constant B. The elimination halflife varies with the dose C. The volume of distribution varies with the dose D. The clearance varies with the dose E. The rate of elimination varies directly with the dose Sent from my iPad using Tapatalk
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#2




A
The rate of elimination is constant  A it is.

#3




The answer is E
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#4




No its not E
its A. Rate is constant, 50% is eliminated. Its First Order kinetics. I feel like Kaplan Biobchem afterchapter questions are poorly written
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#5




Kaplan says this
Quote:
Answer E. In firstorder kinetics, the elimination rate of a drug is directly proportional to its plasma concentration, which in turn is proportional to the dose. Drugs that follow firstorder elimination have a constant elimination halflife similar to the example given in the question. Likewise, clearance and volume of distribution are pharmacokinetic characteristics of a drug that do not routinely change with dose, although they may vary in terms of disease or dysfu.netion.
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#6




Quote:
I totally agree with DocSikorski. Actually I had a similar problem with that question at the end of the General Principles in Pharm. Rate=fraction > here rate is constant  it's 50%. As DocSikorski pointed out it's First Order Kinetics, with T1/2 =2 hrs. It's the amount of the eliminated drug what is proportional to plasma level, not the rate as they say in answer E). If we give 100 mg after 1 x T1/2 we'll have 50 mg. If we give 50 mg of the same drug after 1 x T1/2 we'll have 25 mg. So the rate remains constant (50%), it's the eliminated amount what changes and is proportional to plasma level/administered dose (the higher the dose, the more we loose after T1/2). If anyone can see any incorrectness in this logic pleeeease let me know! 
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#7




Quote:
Also rate of elimination = coc/time Which keep changing with every half life ? Sent from my iPad using Tapatalk HD
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#8




[QUOTE=Kabutar111;108662]I know its stupid but can you please explain the difffrence b/w clearance and rate of elimination?
Also rate of elimination = coc/time Which keep changing with every half life ? Stupid question? Wait till you see some of mines in this forum I have like tonz of them! I’ll try to explain. Please don’t be harsh on me if I don’t succeed 1) Clearance is the volume of plasma from which certain substance is removed in a given amount of time. Let’s say that Clearance for urea is 100ml/min (this is just an example not a real urea clearance value!). That means that it takes kidneys 1 min to remove all of the urea that was in 100 ml of plasma. 2) Rate of elimination shows how the amount of the substance in the plasma that we see decreasing with time. Let’s say 100 mg of the drug with T1/2=4hrs is administered (so it’s First Order Kinetics). So first we have 100 mg (amount of the drug), after 4 hrs (T1/2) we have 50 mg left. It means 50 mg was eliminated. To calculate the ratio we take that eliminated 50 mg and we divide it by the initial level (100 mg). 50mg/100mg = 50%. The elimination rate is 50% (constant here!). For drugs with ZeroOrder Elimination Rate: Let’s say that initial amount in plasma is 100 mg and a constant amount of drug eliminated per unit time (let’s say also 4 hrs) is 10 mg. We start with 100 mg, after 4hrs we have 90 mg of the drug left in plasma. So we see a decrease by 10 mg. Elimination rate is 10 mg/100 mg = 10%. After another 4 hrs we have 80 mg of the drug left. Again drop by 10 mg but the initial amount here was 90 mg so the elimination rate is 10/90=11,1%. Try to do the calculations after next 4,8 and 12 hours – you’ll see the pattern Here the elimination rate changes with the dose of the drug. 
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#9




[QUOTE=Casandra;108668]
Quote:
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#10




he is right it is First order elimination

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#11




drug clearance is not constant for a drug with a zero oder elimination..what does it mean.. can any 1 explain ??

#12




[QUOTE=Casandra;108668]
Quote:
I agree with everything except that rate of elimination does not need to be converted into percentage, It's just the amount of the substrate eliminated per unit of time as you said but not the percent. That's why the rate of elimination varies directly with the dose in firstorder kinetics and is constant in Zeroorder kinetics. So the answer to the test is E. This is the halflife what is constant in firstorder kinetics and varies with the zeroorder kinetics. 
#13




[QUOTE=DrThea;447786]
Quote:

#14




[QUOTE=eizou;448410]
Quote:
Quote:

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#15




[QUOTE=DrThea;449074][QUOTE=eizou;448410]
Quote:

#16




Just another quick question that popped to my head; So the clearance remains constant in FOE? Increasing the dose of the drug will therefore increase the ROE but not the clearance right? Am I getting this right?

#17




Quote:
You are welcome You are right. Clearance stays the same regardless of the plasma concentration of the drug eliminating with the firstorder kinetics. I think because constant proportion is eliminated per unit of time this means that constant volume of blood is cleared per unit of time. Let's say 100 mg of drug is given, after 4 hours blood concentration is 20 mg/l, because 50 mg was removed that means that 2.5l per 4 hours was cleared (50 mg per 4 hours divided by 20 mg/l). By the same way after next 4 hours 25 mg was removed and so blood concentration should halved also  10 mg/l, because 25 mg was removed in 4 hours 25/10=2.5l of blood was cleared again. Also with the zeroorder kinetics clearance isn't constant. 
#18




I honestly don't get the example you used.

#19




Answer must be E
IN zero order kinetics rate of elimination is constant, which is independent of plasma conc Where as in first order kinetics, as given in above question rate of elimination is proportional with plasma conc... 
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