1. Confirm the plate and controls before fitting
Confirm blanks, positive and negative controls, standards, QCs, and unknowns are mapped to the intended wells. Inspect raw absorbance, fluorescence, or luminescence before blank correction or normalization.
Review replicate and spatial behavior. Edge effects, dispensing order, incubation timing, washes, substrate development, and reader settings can create patterns that a standard curve cannot diagnose.
2. Fit the standard curve over its supported range
The exact synthetic standards below follow an increasing 4PL with lower 0.080, upper 2.300, midpoint 6.0 ng/mL, and slope 1.20. Fit response as a function of concentration using the stated equation, then plot every observation and residual.
A successful verification fit should recover those parameters approximately; ordinary ELISA data require weighting, replicate, recovery, uncertainty, and independent-QC review. Define any standard exclusion rule before routine use because removing a boundary standard can change the supported range.
response = 0.080 + (2.300 − 0.080) ÷ [1 + (6.0 ÷ concentration)^1.20]| Standard (ng/mL) | Observed response |
|---|---|
| 0.25 | 0.128 |
| 0.5 | 0.187 |
| 1 | 0.312 |
| 2 | 0.549 |
| 4 | 0.925 |
| 8 | 1.380 |
| 16 | 1.777 |
| 32 | 2.037 |
3. Inversely predict the unknown’s plate concentration
An unknown response of 1.718 lies within the worked standard responses. Inverting the fitted equation returns approximately 14.2 ng/mL: concentration = 6.0 ÷ {[(2.300 − 0.080) ÷ (1.718 − 0.080) − 1]^(1 ÷ 1.20)}.
Retain the response, 14.2 ng/mL plate concentration, curve parameters, inversion formula, and range state before applying off-plate preparation factors. Near flat asymptotes, a small response change can produce a large concentration change; an extrapolated number is not reportable merely because software calculates it.
6.0 ÷ {[(2.300 − 0.080) ÷ (1.718 − 0.080) − 1]^(1 ÷ 1.20)} ≈ 14.2 ng/mL4. Apply the dilution factor once
Suppose the fitted curve returns 14.2 ng/mL for an unknown well and the sample was prediluted 1:20. The preparation-adjusted concentration is 14.2 × 20 = 284 ng/mL.
Report the 14.2 ng/mL plate concentration, the factor of 20, and the 284 ng/mL final result. Apply replicate and range rules at the method-defined stage; do not hide an out-of-range well by averaging it with another dilution.
14.2 ng/mL × 20 = 284 ng/mL5. Decide whether the unknown is reportable
Apply control, standard, QC, replicate, dilutional-consistency, and reportable-range criteria exactly as defined. Preserve each observed value and outcome; a passing average must not conceal a failed well unless the approved aggregation rule explicitly allows it.
If another dilution is required, retain the first result and its reason for reanalysis. Do not silently replace it with the later value.
6. Trace the final concentration back to the well
Include source and plate identity, blank or normalization rules, standards and QCs, exact model and settings, parameters, exclusions, recoveries, unknown replicates, inverse predictions, dilution factors, range states, final results, and software version.
A reviewer should be able to move from 284 ng/mL through the factor of 20 and 14.2 ng/mL plate concentration to the exact response and fitted standard curve.
Limits and model alternatives
A 4PL is a common symmetric sigmoid model; some bounded ranges are linear, and representative data may support asymmetry or weighting. Those are method-development choices, not routine remedies for a difficult plate.
This educational workflow does not establish universal ELISA acceptance criteria or claim a current Provenarium ELISA analysis capability. A team must qualify or validate its configured workflow for intended use.
Frequently asked questions
Is 4PL always the right model for ELISA?
No. Select the model and range using representative assay data, residuals, recovery, precision, and the intended procedure. Some bounded ranges are linear; some curves show supported asymmetry.
Can I remove a failed standard point?
Only under a predefined, justified rule. Preserve the value and reason, assess the effect on curve and range, and report the exclusion.
Should unknowns be averaged before applying dilution factors?
Follow the defined method and calculate from unrounded values. Preserve each replicate, plate-level interpolation, factor, and final aggregation rule.