Mika Laava

To generate a four-parameter logistic (4PL) curve fit using MedCalc, begin by entering your data in the required format—usually with the independent variable (e.g., concentration) in one column and the dependent variable (e.g., response) in another. Then, go to Statistics > Curve fitting > Nonlinear regression, and choose the 4PL model from the list of available functions. The 4PL model is commonly written as:

(see attached equation)

where A is the minimum asymptote, D is the maximum asymptote, C is the inflection point (EC50), and B is the slope.

Once selected, MedCalc will estimate the parameters and generate a curve fit, along with goodness-of-fit metrics like R² and standard error. Be sure to assess the residuals and confidence intervals to evaluate the model’s adequacy. This function is especially useful in bioassays, ELISA analysis, and dose-response modeling.