Question 1

How is IC50 calculated from a dose-response curve?

Accepted Answer

Normalise responses to the positive and negative controls, fit a 4PL model to log10 concentration, and read the inflection point C50 — the concentration giving 50% of the fitted response span. Report it with a 95% confidence interval; Labreador computes both at labreador.com/dose-response.

Question 2

What is the difference between IC50 and EC50?

Accepted Answer

They are the same mathematical parameter of the fitted curve; IC50 is used when the response decreases (inhibition) and EC50 when it increases (activation or efficacy). The distinction is pharmacological direction, not a different formula.

Question 3

What Z' factor is acceptable?

Accepted Answer

Z' above 0.5 is an excellent screening assay, 0.0-0.5 is marginal, and below 0 means the control distributions overlap and hits cannot be separated from noise.

Question 4

My IC50 is outside the tested concentration range — is it valid?

Accepted Answer

No. An IC50 outside the tested range, together with a very wide confidence interval or an unconstrained asymptote, means the curve was extrapolated. Extend the dilution series so the plateaus are captured before reporting a value.

Dose-Response & IC50/EC50 Analysis — Labreador

Import Plate Data