The question
A cell-based potency assay reports relative potency by comparing each sample's dose-response curve with a reference standard. Two samples both return a relative potency number from the software. Can both be reported?
The analysis
Relative potency only means something when the test curve is the reference curve shifted along the dose axis: same shape, same slope, same response range. That is the parallelism assumption. Each curve was fitted with a four-parameter logistic model, and parallelism was judged by equivalence: the 90% confidence intervals for the slope ratio and the response-range ratio must sit inside 0.80 to 1.25, limits set before testing.
| Sample | Slope ratio (90% CI) | Response-range ratio (90% CI) | Parallelism | Relative potency (90% CI) |
|---|---|---|---|---|
| Sample A | 1.01 (0.90 to 1.14) | 1.00 (0.96 to 1.05) | Parallel | 89% (80 to 100%) |
| Sample B | 0.79 (0.70 to 0.91) | 0.64 (0.60 to 0.68) | Not parallel | Not reportable |
What to do
- Make parallelism a system suitability requirement with pre-set equivalence limits, not an optional check.
- Treat a non-parallel result as a signal about the product, and investigate it rather than repeating the assay until it passes.
- Report relative potency with its confidence interval, not as a single number.
How it was done
Synthetic dose-response data, eight doses in triplicate. Four-parameter logistic fits by nonlinear least squares; confidence intervals from 400 residual bootstrap resamples; equivalence limits of 0.80 to 1.25, following the equivalence approach described in USP chapter 1032. Download the data (CSV).