EC50, IC50, and Kd are common measures in pharmacology and assay reports, but they describe different quantities. Therefore, interpreting them correctly starts with the assay design and experimental conditions. Interpreting them correctly requires knowing the assay design and experimental conditions.
EC50
More specifically, EC50 is the concentration that produces 50% of a measured maximal effect in a specified assay. It depends on the system, endpoint, receptor expression, and conditions; it is not a universal property independent of the experiment.
IC50
By contrast, IC50 is the concentration that inhibits a defined response by 50% under the assay conditions. It can vary with substrate or ligand concentration, incubation time, and assay setup. Avoid comparing values from different protocols as if they were interchangeable.
Kd
For binding, Kd is an equilibrium dissociation constant used to describe binding affinity in a defined binding system. A lower Kd generally indicates tighter binding under the stated model, but binding affinity alone does not determine functional activity.
Before comparing numbers
- Check the assay type and measured endpoint
- Review units, conditions, and fitting model
- Distinguish binding from functional response
- Compare only compatible experiments
These values help readers when researchers report enough method detail to explain what they measured.
Frequently asked questions
What is the difference between EC50, IC50, and Kd?
EC50 and IC50 are assay-dependent concentrations associated with a measured effect or inhibition. Kd describes binding affinity under a defined model and conditions; they are not interchangeable.
Can values from different studies be compared directly?
Usually not without considering assay methods, targets, conditions, and analysis. A lower number alone does not prove a stronger real-world effect.
What should I check in a reported value?
Review units, assay description, controls, uncertainty, and the model used to calculate it.
