Specificity

Fit your standard curve, then compare a clean reference sample against samples spiked with potential interfering substances, to get FDA Bioanalytical Guidance (2018) Table 1 specificity verdicts for ligand-binding assays — each interferent within ±20% of the reference (±25% if tested at the LLOQ/ULOQ 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. Specificity data

Enter the OD of a clean reference sample (no interferent), then the OD of each sample spiked with a potential interfering substance at a relevant concentration. Each interferent is compared against the reference sample’s own back-calculated concentration — not a separately-entered nominal.

Clean reference sample OD This test is performed at the LLOQ or ULOQ level (use the wider ±25% band)

Interferent	OD

3. Specificity results

FDA Table 1 (Specificity, LBA column) requires each interferent’s back-calculated concentration to fall within ±20% of the clean reference sample’s back-calculated concentration (±25% if this challenge is being run at the LLOQ or ULOQ level).

Interferent	Back-calc conc.	% of reference	Tolerance	Pass

Specificity

1. Standard curve

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

2. Specificity data

3. Specificity results