LabreadorLabreador / Bioassay analysis
Free 4PL Calculator
Fit a sigmoidal calibration curve and convert response into concentration with Labreador. Use the synthetic example below or open the full ELISA analysis for your plate data.
Free · No account · Local web calculations · Research use only
Worked example
Synthetic increasing calibration: A = 0.1 OD, D = 2.1 OD, B = 1.2, C = 16 ng/mL. Standards: 1, 2, 4, 8, 16, 32, 64, 128 ng/mL. A corrected response of 1.1 OD maps to 16 ng/mL before dilution. A perfect synthetic fit is not evidence of experimental assay validity.
Equation and units
y = D + (A - D) / (1 + (x / C)^B) x = C * ((A - D) / (y - D) - 1)^(1 / B)
Full analysis workflow
- Import your plate-reader values and assign blanks, standards and samples.
- Enter standard concentrations and select 4PL; choose weighting according to your assay variance.
- Review residuals, recovery and replicate CV%, not R² alone.
- Report only concentrations within the justified calibration range and apply the sample dilution factor.
Interpretation and limitations
A and D describe the zero-dose and high-dose asymptotes in this equation; their roles depend on curve direction. C is the half-span concentration in 4PL, not automatically a clinically meaningful IC50. In 5PL, the added asymmetry parameter changes the midpoint interpretation. Labreador does not extrapolate sigmoid sample concentrations beyond its calibration range.
Common questions
- Where can I fit a 4PL standard curve online for free?
- Labreador's ELISA module fits 4PL and 5PL curves, interpolates sample concentrations and reports calibration diagnostics without an account. Web calculator data is processed locally in the browser.
- Is 5PL always better than 4PL?
- No. 5PL adds an asymmetry parameter. Use it when justified by the response and residuals; an extra parameter or higher R² alone does not establish a better calibration.
- What should I look for in the best 4PL calculator for my assay?
- Check the model convention, weighting, residuals, back-calculated recovery, replicate variability, calibration limits and reproducible exports. Compare the same data and assumptions; no single calculator is universally best.