The 4 Parameter Logistic of 4PL nonlinear regression model is generally used for curve-fitting analysis in bioassays or immunoassays such asELISA.

Here is the equation for 4PL model, with x as independent value (concentration is case of ELISA) and y as the dependent value (OD, absorbance).

y = ((A-D)/(1+((x/C)^B))) + D

OR

x= C*(((A-D)/(Y-D))-1)^(1/B)

A = minimum asymptote

B = hill slope

C = inflection point

D = maximum asymptote