Selectivity

Fit your standard curve, then paste OD readings from at least 10 individual matrix sources spiked at LLOQ and at a high QC concentration, and get FDA Bioanalytical Guidance (2018) Table 1 selectivity verdicts for ligand-binding assays — each source within ±25% of nominal at LLOQ and ±20% of nominal at the high QC level.

1. Standard curve

Paste calibrator concentration + OD pairs, one per line (tab/comma/space separated). Extra columns are treated as OD replicates and averaged for the fit, e.g. 1.25, 0.44, 0.43, 0.46. The concentration column doubles as each calibrator’s nominal value for the back-calculation check below.

Weighting

2. Selectivity data

Enter the LLOQ and high-QC nominal concentrations, then paste each individual matrix source’s OD when spiked at LLOQ and at the high QC level. FDA Table 1 wants ≥10 individual sources — fewer is allowed here for testing, but the report will flag it.

LLOQ nominal concentration High QC nominal concentration

Source	OD at LLOQ spike	OD at High QC spike

3. Selectivity results

FDA Table 1 (Selectivity, LBA column) requires each individual matrix source to back-calculate within ±25% of nominal at the LLOQ spike and ±20% of nominal at the high QC spike.

Source	LLOQ back-calc	LLOQ % nominal	LLOQ Pass	High QC back-calc	High QC % nominal	High QC Pass	Overall

Selectivity

1. Standard curve

Per-calibrator back-calculation (±20% nominal, ±25% at LLOQ/ULOQ)

2. Selectivity data

3. Selectivity results