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]]
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 <winsaam at 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 at 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/
oscar linares <winsaam <at> gmail.com> writes:> > 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) >[snip]> 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.You set options(digits=2), thus AR is only printed to 2 digits (2.4), when its true underlying value is (at least to machine precision) options(digits=20)> 1/(1-exp(-k*tau))[1] 2.357362278468023 Note that options(digits=2) affects only the display of numeric values, not their underlying values. You have also done some rounding of intermediate values, which is probably a bad idea. Reading R FAQ 7.31 might be useful as well. May I respectfully suggest that you not use a system that you don't understand to compute the dosages of highly toxic drugs ????? Ben Bolker