ELISA 4PL curve fitter — Fits the 4-parameter logistic (4PL) sigmoid y = d + (a-d)/(1+(x/c)^b) to standard OD data via non-linear regression (in-house Levenberg-Marquardt). a=upper plateau, d=lower plateau, c=EC50, b=Hill slope. Back-calculates unknown sample concentrations from OD.

(1) Fit ELISA standard curve (more accurate than linear) — GraphPad Prism 4PL alternative (2) Auto-calculate LLOQ/ULOQ (lower/upper limit of quantification) (3) Back-calculate unknown sample concentrations from OD (with CV%) (4) Cytokine, hormone, antibody quantification — sigmoidal dose-response in general

① Enter standard data (concentration, OD pairs). Typically 6–8 points (e.g., 0, 7.8, 31.25, 125, 500, 2000 pg/mL). ② Click "Fit 4PL" → LM optimization for a, b, c, d ③ Auto-output: • 4PL parameters + R² (goodness of fit) • LLOQ ≈ lowest quantifiable conc., ULOQ ≈ conc. at 95% of upper plateau • Curve plot (semi-log x) ④ Enter unknown sample OD → back-calculate concentration Outliers (residual >3σ) flagged automatically. ≥4 data points required.

Example) IL-6 ELISA standard curve Standards: 0, 7.8, 31.25, 125, 500, 2000 pg/mL OD values: 0.05, 0.12, 0.35, 0.80, 1.50, 2.40 → 4PL fit: a=2.60, b=1.05, c=180, d=0.04, R² = 0.998 → LLOQ ≈ 8 pg/mL, ULOQ ≈ 1800 pg/mL Unknown sample OD = 0.65 → conc = 78 pg/mL