Dear Oscar,
The problem has to do with rounding (because you set the global digits
value to 2). Although what you see is 2.4*270=670; when R actually
calculates it, it is using full precision. If you set
options(digits=7) # the default
you will see that AR=2.357362, not 2.4.
HTH,
Joshua
On Sun, May 2, 2010 at 4:48 PM, oscar linares <wins...@gmail.com> wrote:
> Dear Rxperts,
>
> Running the following code:
> =======================================================
> twlo=10; twhi=20; wt=154; vd=0.5; cl=0.046; tau=6; t=3; F=1;
>
> wtkg <- wt/2.2 # convert lbs to kg
>
> vd.pt <- wtkg * vd # compute weight-based vd (L)
> cl.pt <- wtkg * cl # compute CL (L/hr)
> k <- cl.pt/vd.pt # compute k (hr^-1)
>
> cave <- round((twhi - twlo)/log(twhi/twlo),2)
> doserate <- (cl.pt * cave)/F
>
> # maintenance dose (Dm)
> total_dose_tau <- round(doserate * tau,0)
>
> Cmax <- total_dose_tau/vd.pt
> Cmin <- Cmax * exp(-k * tau)
>
> AR <- 1/(1 - exp(-k * tau))
>
> Cmaxss <- Cmax * AR
> Cminss <- Cmin * AR
> Cfluctss <- (Cmaxss - Cminss)
> Ct <- (Cmaxss * exp(-k * t))
>
> dose_loading_vd <- round(Cmaxss * vd.pt,0)
> dose_loading_dm <- round(total_dose_tau * AR,0)
>
> options(digits=2)
>
> c("Ther. Win. (mg)"=twlo,"to"=twhi,"Wt (kg)"=wtkg,"Vd (L)"=vd.pt,"CL
> (L/hr)"=cl.pt,"Cave (mg)"=cave,"D/tau (mg/hr)"=doserate,"tau
> (hr)"=tau,"Total Dose (mg)"=total_dose_tau,"Cmax (mg)"=Cmax,"Cmin
> (mg)"=Cmin,"AR"=AR,"Cmaxss (mg/L)"=Cmaxss,"Cminss (mg/L)"=Cminss,"Cfluct
> (mg/L)"=Cfluctss,"C(t) (mg/L)"=Ct,"DL wVd (mg)"=dose_loading_vd,"DL wDm
> (mg)"=dose_loading_dm)
> ===========================================================================
> Gives
> Ther. Win. (mg) to Wt (kg) Vd (L) CL
> (L/hr) Cave (mg)
> 10.0 20.0 70.0 35.0
> 3.2 14.4
> D/tau (mg/hr) tau (hr) Total Dose (mg) Cmax (mg) Cmin
> (mg) AR
> 46.5 6.0 279.0 8.0
> 4.6 2.4
> Cmaxss (mg/L) Cminss (mg/L) Cfluct (mg/L) C(t) (mg/L) DL wVd
> (mg) DL wDm (mg)
> 18.8 10.8 8.0 14.3
> 658.0 658.0
>
> But, there is an error from R?
>
> AR * total_dose_tau = 658 but that is wrong
>
> Total Dose (mg) = 279 * AR (=2.4) = 670
>
> Please help. I don't want to harm anyone. A difference of 12 mg of a highly
> toxic drug can at the very least cause significant harm.
>
>
> --
> Oscar
> Oscar A. Linares, MD
> Translational Medicine Unit
> La Plaisance Bay, Bolles Harbor
> Monroe, Michigan 48161
>
> Department of Medicine,
> University of Toledo College of Medicine
> Toledo, OH 43606-3390
>
> Department of Internal Medicine,
> The Detroit Medical Center (DMC)
> Harper University Hospital
> Wayne State University School of Medicine
> Detroit, Michigan 48201
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
--
Joshua Wiley
Senior in Psychology
University of California, Riverside
http://www.joshuawiley.com/
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