IC50 / EC50 & Selectivity Index
Fit 4PL IC50/EC50 and CC50 curves and calculate selectivity index
4PL regression result
Within tested range · Converged
Review the 4PL fit, top/bottom, residuals, and independent repeats together. This calculator applies no automatic acceptance threshold; approximate CIs assume independent errors and a normal approximation.
Basis: NIH/NCATS Assay Guidance Manual 4PL guidance and the SI = CC50/EC50 definition used in published antiviral assays.
Tool Guide
Definition
IC50 / EC50 & Selectivity Index — Fits log-concentration response data with a nonlinear four-parameter logistic (Hill-slope) model to estimate a relative IC50/EC50 or CC50, approximate 95% CI, Hill slope, and R². Dual-curve mode divides cytotoxicity CC50 by activity IC50/EC50 in the same concentration unit.
Purpose
(1) Calculate IC50 for enzyme/cell assays and agonist EC50 (2) Calculate cell-viability CC50 (3) Summarize activity-to-cytotoxicity separation as SI = CC50/IC50 (or EC50) (4) Flag midpoint extrapolation beyond tested concentrations (5) Copy midpoint, CI, slope, and R² for submission tables
How to Use
① Select single-curve or Selectivity Index mode. ② Enter the concentration unit and at least five distinct concentration-response pairs. ③ Choose decreasing (IC50/CC50) or increasing (EC50) for each dataset. ④ In SI mode, use the same concentration unit for activity and cytotoxicity data. ⑤ Check whether each midpoint lies within the tested range and review top/bottom, slope, R², and raw data together. Summarize replicate wells appropriately within each run and review independent experiments separately.
Examples
Example) In the same nM unit, activity IC50 = 10 and cytotoxicity CC50 = 500 → SI = 500 / 10 = 50 A quantitative SI is easiest to interpret when both midpoints are within their tested ranges. If CC50 exceeds the highest tested concentration, do not invent a number; report `CC50 > highest concentration` and `SI > bound` as appropriate.