IC50 using microsoft excel - (Aug/10/2010 )

hi,

what is the right way of determining IC50 using microsoft excel? I went across several articles that used excel to calculate ic50, and there is one term 'linear interpolation'.I'm not really sure what it is. I just want to know, whether, for all this while i calculate the correct way using excel. What I did was, I transform the concentration values into log, then I construct scatter graph using excel with y-axis=% inhibition and x-axis= log conc. Finally I add trendline and got linear equation y=mx + c. Frm this equation, I substitute y with 50, and get x=log conc. I antilog the value and get the true x value. Is this linear interpolation?? I attach one of the graph and the way I calculate for reference.

Thank you in advance

Sorry I dont use excel to calculate IC50, I use SigmaPlot. However you can check your calculated values online here:

http://www.changbioscience.com/stat/ec50.html

Beware of using a linear trendline, if your inhibition profile is not linear this will give you messed up values. I think you need more high and low concentration points on your graph, so you encompass no inhibition all the way through to full inhibition, 3 points for each is what I aim for to get a good curve for robust IC50 calculation.