- On 14 Feb 1997 at 17:58:08, djtippey.-at-.dbtech.net (Del & Jackie Tippey) sent the message

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Greetings;

I am working on a PK program for my department's aminoglycoside /

vancomycin PK program. I have equations that will give me estimates of

levels and Vd after "n" doses - provided it is the initiation of therapy. I

was wondering if anyone knew of equations that could help me determine

levels after "n" doses if I had a ballpark pre-therapy level. I know that

the steady-state equations probably will remain the same, but I wanted to

know in case levels are drawn before steady-state so we determine expected

levels.

Thanks in advance...

-DT- - On 18 Feb 1997 at 11:48:45, "David Nix" (nix.aaa.tonic.Pharmacy.Arizona.EDU) sent the message

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The full equation for concentrations at any time is:

C(t) = D/TI/ke/V*(exp(-k*ts)-exp(-k*t))*(1-exp(-nktau))/(1-exp(-ktau))

D=dose

TI=time of infusion

k=elimination rate constant

V=volume

ts=t-TI : restrict ts>=0

tau = dosing interval

n = # of doses given

There are several bayesian estimation programs commercially available

that will handle 1 or more non-steady-state levels. However, if a

computer and software are not available, some other methods may be

useful. Below is how I have handled similar problems:

Single level:

1. assume k based on creatinine clearance

2. calculate V from the above equation

3. If volume is reasonable (.24 to 0.4 L/kg for aminoglycoside)

then accept new parameters. Otherwise, you may adjust ke

by 20-40% and recalculate V. This method will get parameters in the

"ball park".

Two levels

1. Within same dose interval. Use levels to estimate k, then

calculate V as above.

2. trough-dose-peak - This senerio provides a better estimate

of V than ke. However, you must calculate ke by trial and error.

Guess at ke and calculate V based on one of the levels. Then check

to see if the other level is predicted correctly. After some

practice, this process does speed up considerable, but it is still

time consuming. Your speed is related to your ability to guess at k

and revise the estimate, and also how close that you are trying to

approach the measured concentrations.

The use of computers and baysian approach is recommended. - On 25 Feb 1997 at 12:36:16, djtippey.aaa.dbtech.net (Del & Jackie Tippey) sent the message

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Thanks for the rapid and informative feedback.

-DT-

Del Tippey R.Ph., M.S.S.M.

DCH Regional Medical Center

801 University Blvd. E.

Tuscaloosa,AL 35405

1-205-759-7310

djtippey.aaa.dbtech.net

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