How to determine an IC50

Prism can easily fit a dose response curve to determine the IC₅₀. 

1. From the Welcome dialog, choose the XY tab, drop the list of sample data sets and choose "RIA or ELISA". 
2. Note that the X values are logarithms of concentration. Prism offers built-in equations designed to handle X values as either concentration OR log(concentration). Be sure you select the correct equation when performing nonlinear regression! If you'd like to convert concentration values to log(concentration) values - or vice versa - you can use Prism's Transform analysis to convert the X values.  Also note that this sample data set includes unknown values. These are Y values  without corresponding X values. Prism can interpolate these X values.
   
   
3. Click Analyze and then Nonlinear regression. Or click the Nonlinear Regression shortcut button just above the Analyze button.
4. On the Nonlinear regression dialog, open the "Dose-Response -- Inhibition" family of equations, and choose "log(inhibitor) vs. response -- Variable slope (four parameters)". At the bottom of the dialog, check the option to "Interpolate unknowns from standard curve".
   
   
5. Click OK and view the results.
   
   
6. View the graph.
   
   

More information:

• More tutorials on curve fitting
• Learn about the equation used to fit to the data.
• Interpreting nonlinear regression results
• How exactly is the IC50 defined?
• Relative vs. absolute IC50
• IC50 vs. EC50