# Pharmaceutical Calcultion

Discussion in 'Pharmacy' started by Jaguis, Mar 30, 2009.

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1. ### JaguisNew Member

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What is the final concentration of a sodium chloride solution obtained by mixing 100ml of 0.9% w/v with 200 ml of 0.45%w/v and 300ml of 0.2% w/v.

Any help pls?
2. ### xiphoid2010SDN Two Year Member

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good lord, this is really simple, just set up an equation.

100ml x 0.9% + 200ml x 0.45% + 300ml x 0.2% = 600ml x (X%)

Solve for X%.

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What kind of equation is that?
(100x.9)+(200x.45)+(300x.2)=600ml???????????????????
4. ### QuiksilverSecundum Artem PharmD

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Figure out the mass of each drug contained in each individual solution, masses are neither created nor destroyed. Add all those masses together, you now have a total amount of drug in the final mixture. Add all the volumes together. although on a technicality depending on the solvent or base liquid it may not be exactly 600 mls but since we are dealing with only water as a base liquid its 600 mls.

one thing to note, its good to think about the amount of drug in a volume as opposed to a concentration because lets just say you take 10 mls out of the 0.9% solution into the syringe to mix it with another. The concentration is the same in the syringe, but the amount of the drug is different, and the amount of drug in the syringe will change the concentration in the new solution
5. ### type b pharmDNever stressed

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good call

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0.4%
7. ### xiphoid2010SDN Two Year Member

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what's wrong? I always set up equations exactly as how the question logically flows. In this case, you got 3 solutions of different concentrations, which will give you 600 ml of what concentration? And you are solving for that "what". Basic understanding of mass and volume is assumed.

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Please send me a picture so if I see you behind the counter I can run the other way.

The way Quiksilverr described the process is the correct way to perform the calculation.

100ml x 0.9% + 200ml x 0.45% + 300ml x 0.2% = 600ml x (X%)

.9+.9+.6=600 x (x%) makes no sense and does not compute......

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Are you sure?

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yeah.. I am.
11. ### ItsOverZyvoxRetiredBanned

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That is correct.
12. ### xiphoid2010SDN Two Year Member

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looks like an old timer has gotten a little too old. My equation is correct.

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Show me the math.

So how many gms of NaCl are there in 600 ml?
14. ### fenixtnlfanP2 Wildcat

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Both ways come out exactly the same. Yeah, I'm so geeky for actually having done that but I didn't understand why xiphoid's equation wouldn't work.
15. ### powertooldSDN Two Year Member

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This is how I was taught to solve these problems: just add up the individual components and come out with a total mixture. It's probably the most logical way of doing it.
16. ### FuzzychickensP-2

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.004 x 600 = 2.4 grams

xiphoid's approach isn't excessive, excessive is how I used to approach questions where you have various concentrations and have to solve for quanities of each to produce a fixed volume of certain concentration.

I used to approach these using systems of equations before a teacher pointed out using alligation to save me the time.
17. ### xiphoid2010SDN Two Year Member

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Which part of my simple equation don't you understand? It's really just simple logic equation of "total is the sum of all of its individual components", as confirmed by all the others in this forum.
18. ### xiphoid2010SDN Two Year Member

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yeah, thanks. My equation is a simple summation of all the components to solve for a single variable, and I thought it's very self explanatory.

Does anyone understand why Old Timer is not understanding my equation? Is it that hard to understand?

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Sure, when you make a calculation like this, it is safer to show your work and break down each component of the problem. If you were my student and you had to make a compound I would not let you do what you dud because it is faster or easier. The way Quicksilver described solving it is the only safe way. I know I'm being a pain in the a**, but I have seen to many errors over the years using the method you have described. It's so easy to end up off by a factor of 10 or a factor of 100. I had a student just this past week, do what you did and they used your formula and they were off by a factor of 10....

That's why I said show me the math..... Only It's Over gave the answer, just without any math so I asked if he was sure.
20. ### QuiksilverSecundum Artem PharmD

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Its not all that simple to understand. Yeah, its a one step problem but to understand what is conceptually going on, it is easier with my method. When you have a first time learner, the only way they are going to learn it is to conceptualize it. just my 2 cents
21. ### xiphoid2010SDN Two Year Member

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Hmmm, maybe it's an asian thing. Math has always been like a natural language to me and very easy to understand and work with. Then again, maybe it's the difference in how little and much emphasis US and asian education system put on math.
22. ### xiphoid2010SDN Two Year Member

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Different people work the thought process differently. Students should use whatever thought process that works best and accurate for them, as long as the result is correct. I'll be more prone to error and not as efficient if I had to change my thought process into ones that's some one else's. My process worked best for me since I came out among the top of my class in both Pharm Calc and PK, and arrived at the answers faster than most.
23. ### QuiksilverSecundum Artem PharmD

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but lack the skills to communicate that across....

Touche my friend.
24. ### xiphoid2010SDN Two Year Member

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Math is the universal language. Learn it, speak it.

But agreed, asian education system focus way more on math and sciences. I was raised on that formula, at the expense of being more "well rounded". While I'm sure I will suck as a politician or a salesman, but let's just say no love lost there.
Last edited: Mar 31, 2009
25. ### ItsOverZyvoxRetiredBanned

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Oh yeah? What's the extrapolated Cmax for Tobramycin 7mg/kg dose?

quick.
26. ### ItsOverZyvoxRetiredBanned

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3 minutes elapsed.

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6 minutes..
28. ### xiphoid2010SDN Two Year Member

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no tobra dosing was taught. What's Tobra's Vd, infusion time, infusion method, number of doses given, how is it cleared? LOL
29. ### xiphoid2010SDN Two Year Member

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what you don't know those parameters off the top of your head? Shame on you zyvox.

3 minutes.
30. ### ItsOverZyvoxRetiredBanned

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I'm a little disappointed..

you tell me what the VD is I'll tell you the infusion time is 60 minutes.
You tell me how it's cleared and I'll tell you that only 1 dose is given.

Infusion method???????? You tell me.
31. ### ItsOverZyvoxRetiredBanned

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I was out cracking and eating walnuts. I'm off to the range. I expect answers when I get back.
32. ### xiphoid2010SDN Two Year Member

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Sorry, I'm not doing the looking up for you since you are not paying me to work for you. You want answers, you need to give me all the relevant parameters. And there are a lot of parameters. You are start off by looking up if it's one compartment model or 2. Mean while I gota go to my Business Management elective. See you when I come back, have fun researching.
33. ### ItsOverZyvoxRetiredBanned

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I already know the answer with the parameters given. Just wanted to see how fast you can calculate it because you bragged about your pk and math prowess.
34. ### QuiksilverSecundum Artem PharmD

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Don't you either need a peak or trough or better yet patient parameters in order to solve this gem. I mean all i know is the long way to the answer.

also don't you need to know dosing interval and infusion time?

PS I'm not so bad at PK myself, and im not asian
Last edited: Mar 31, 2009

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no.
36. ### QuiksilverSecundum Artem PharmD

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are you going to leave me hanging? do you assume values of some of the variables?
37. ### ItsOverZyvoxRetiredBanned

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I asked for extrapolated Cmax. What's the formula for Cmax?
38. ### codep1nkNew Member

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Cmax = too high

Last edited: May 27, 2012
39. ### QuiksilverSecundum Artem PharmD

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its multiple compartment system. However you trap your calculations within a certain time window and you don't worry about number of compartments because you are only dealing with a single compartment.
40. ### ItsOverZyvoxRetiredBanned

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that is true.
41. ### ItsOverZyvoxRetiredBanned

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What's too high? What's the optimum bacteriocidal concentration of aminoglycoside against pseudomonas?
42. ### codep1nkNew Member

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edited post =]
not too high for CF kid's daily dosing

pseudomonas where?
43. ### ItsOverZyvoxRetiredBanned

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Location of pseudomonas would be irrelevant as far as determining the optimum bacteriocidal concentration. Location of infection on the other hand is important in determining the dose to attain certain peak.
44. ### codep1nkNew Member

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Hah, that's what i meant...psa INFECTION where?
45. ### rphello*

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You (royal) are really overdoing this. I'm grateful I had a applied kinetics rotation. We could start a patient on vancomycin with just weight and SCr. I'll be darned if those levels didn't come back correct to the tenth decimal place on a few occasions. Of course we'd refine the dose after viewing the levels, but population parameters were remarkably effective.
46. ### ItsOverZyvoxRetiredBanned

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Dang, what good is all the PK education that's so convoluted with concepts and formulas that you guys can't see a clear cut day to day practical application of how to dose ?

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Thank you.

48. ### QuiksilverSecundum Artem PharmD

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Cpeak = [Dose/CL*T(infusion time)*(1-e^-kT)*e^-kt2 (1/2 hr in this case)]/1-e^-kt (dosing interval)

I am missing a clearance.

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who's royal?
50. ### codep1nkNew Member

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u can use a simpler estimate...
cmax = dose/vd

still dont have vd...but if we had a weight...you could estimate that as well with the population parameter...but no weight either.
b/c for traditional dosing, generally don't dose 7mg/kg of an aminoglyc unless its amikacin...
Last edited: May 27, 2012