Why did my Assay / ELISA / Binding fail?
Elevated signal in blank / negative-control wells. Weak or absent readout even in positive-expected wells. Signal plateaus across a wide concentration range instead of a sigmoidal response. Poor reproducibility across technical or biological replicate wells. Standard curve R² / shape unfit for quantification
Playbooks
- Assay / ELISA / Binding: high backgroundElevated signal in blank / negative-control wells.
- Assay / ELISA / Binding: low signalWeak or absent readout even in positive-expected wells.
- Assay / ELISA / Binding: saturated curveSignal plateaus across a wide concentration range instead of a sigmoidal response.
- Assay / ELISA / Binding: high cv between replicatesPoor reproducibility across technical or biological replicate wells.
- Assay / ELISA / Binding: poor standard curveStandard curve R² / shape unfit for quantification (non-linear, noisy, or inverted).
- Assay / ELISA / Binding: edge effectsPerimeter wells systematically higher or lower than interior replicates.
- Assay / ELISA / Binding: no signalNo readable OD/fluorescence above blank, including expected positive wells.
- Assay / ELISA / Binding: hook effectVery high analyte reads paradoxically low until diluted (prozone/hook).
Calculators
FAQ — Assay / ELISA / Binding
- High background
- Elevated signal in blank / negative-control wells. Add one extra wash cycle (3× → 4–5×) with 0.05% Tween-20 — The single highest-yield fix for plate-wide background. Extend blocking to 1 h at room temp or overnight at 4°C with fresh 1–5% BSA/milk — Expired or weak blocking buffer is the second most common cause. Titrate detection antibody 2-fold down from current dilution — Reduces nonspecific binding while preserving true signal. Shorten substrate development time or read earlier — Caps background accumulation before the assay saturates.
- Low signal
- Weak or absent readout even in positive-expected wells. Increase primary/capture antibody concentration or use a fresh aliquot — Confirms whether the reagent itself has lost activity. Re-coat with fresh carbonate/bicarbonate buffer, pH 9.6, overnight at 4°C — Coating efficiency is the most common silent failure in ELISA. Let substrate develop 5–10 more minutes before stopping — Under-development is easy to miss and easy to fix. Re-run samples at a less dilute concentration — Pulls the signal back inside the standard curve's dynamic range.
- Saturated curve
- Signal plateaus across a wide concentration range instead of a sigmoidal response. Serially dilute samples 1:5 to 1:50 and re-read — Brings concentrated samples back onto the linear/sigmoidal portion of the curve. Read the plate earlier, before substrate fully develops — Prevents standards themselves from plateauing at OD ceiling. Titrate detection conjugate down 2-fold — Widens the assay's dynamic range before saturation.
- High CV between replicates
- Poor reproducibility across technical or biological replicate wells. Re-pipette using reverse pipetting technique to avoid bubbles — Bubbles are the single largest source of well-to-well CV. Fill unused perimeter wells with buffer ("moat" plate layout) — Reduces evaporation-driven edge effects on outer wells. Vortex or invert master mix immediately before every dispense — Prevents settling of denser components over the plating time.
- Poor standard curve
- Standard curve R² / shape unfit for quantification (non-linear, noisy, or inverted). Remake standards with fresh serial dilutions and new tips each step — Pipetting error is the most common poor-curve cause. Reconstitute a new standard vial per kit instructions; avoid extra freeze-thaw — Degraded stock cannot be rescued by dilution technique alone. Verify reader wavelength and blank against kit substrate instructions — Wrong λ compresses or warps the apparent curve.
- Edge effects
- Perimeter wells systematically higher or lower than interior replicates. Seal plates and use a humidity tray / moat of buffer in unused wells — Evaporation is the dominant edge-effect driver. Verify washer manifold alignment; re-run with manual edge-aware washes — Separates instrument spray patterns from true biology. Incubate single plates on a rack (not stacked) at verified temperature — Reduces thermal gradients across perimeter wells.
- No signal
- No readable OD/fluorescence above blank, including expected positive wells. Apply fresh substrate to a known-positive well and watch for development — Fastest split between substrate vs upstream assay failure. Confirm reader mode/wavelength matches the kit — Wrong readout mode presents as total zeros. Re-coat and re-run with a printed step checklist for antibody order — Skipped coating/antibody steps are common total-loss causes.
- Hook effect
- Very high analyte reads paradoxically low until diluted (prozone/hook). Re-assay samples at 1:10 and 1:100; report the dilution that falls on-curve — Dilution test is the definitive hook check. Increase detection antibody concentration one step if dilutions still hook — Some kits need richer detection for high-load matrices. Extend the standard curve upward for future runs of this sample type — Makes hooking visible before results are reported.