[R] Estimate Slope from Boltzmann Model (package: DRC)

[Bob Arrigo]

Dear R Community,

I am using the package DRC ( to fit a boltzman model to my data.  I
can fit the model and extract the lower limit, upper limit, and ED50
(aka V50), but I cannot figure out how to get the slope of the curve
at ED50.  Is there a simple way to do this?  I've searched the mailing
list and looked through the package documentation, but could not find
anything.  I am new to r, and especially the DRC package, so any help
would be appreciated.

This gets me 3/4 of what I need:

DATA:
Dose,Response
0.85,0.0085
0.90,0.0096
0.95,0.0155
1.00,0.2725
1.05,0.3634
1.10,0.3508
1.15,0.5002
1.20,0.8826
1.25,0.7023
1.30,1.7934
1.35,0.7389

CODE:
library( drc )
mydata <- read.csv( file="test.csv", header=T )
drc.preTBS <- drm( response ~ dose, data = mydata, fct= LL.4( names =
c( "Slope", "Lower Limit", "Upper Limit", "ED50") ) )

For the data above, I would expect a slope at ED50 to be about 0.0714
(according to GraphPad).  How can I determine the slope at ED50 for a
Boltzmann fit using DRC or related packages?

Thanks much!
-Bob (Veterans Administration)