ELISA Troubleshooting Guide
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A poor ELISA standard curve is most commonly caused by improper standard dilution or reconstitution, standard degradation, pipetting errors, or incomplete washing. Pipetting errors in particular can result in incorrect reagent concentrations across wells, leading to inconsistent OD values and a poorly fitted curve. To improve standard curve quality, verify dilution steps and reconstitution procedure, use calibrated pipettes, store standards according to protocol, and try log-log or 5-parameter logistic curve fitting if the curve does not fit the expected scale.
Possible Causes What You Can Do Improper standard dilutionUse appropriate diluent as blank. Make sure that the dilution is performed as according to protocol Standard improperly reconstituted Briefly spin standard vial before opening. Make sure that there is no undissolved material after reconstituting Standard degraded Store standards as according to protocol Curve doesn’t fit the scale Try plotting log-log or 5 parameter logistic curve fitPipetting error Calibrate pipettes to make sure that the correct volume is dispensed ncomplete washing Increase washing cycles -
High variation among ELISA replicates is most commonly caused by inconsistent washing, uneven sample mixing, pipetting inaccuracies, bubbles in wells, or edge effects. Proper plate handling, accurate pipetting, and equilibrating reagents and plates to room temperature before the assay can help improve ELISA reproducibility and reduce well-to-well variability.
Possible Causes What You Can Do Improper washingMake sure that the washing is done as according to protocol Poor mixing of samples Mix samples gently and evenly Dirty plate Make sure that the bottom of plate is cleanReagents too oldMake sure that the reagents are not expired. Use freshly prepared reagents Pipetting error Calibrate pipettes to make sure that the correct volume is dispensed Edge effects Make sure that the plate and reagents are equilibrated to room temperature before starting assay -
High background in ELISA is commonly caused by excessive antibody concentration, insufficient washing, nonspecific antibody binding, improper blocking conditions, or overdevelopment of substrate reactions. Optimizing antibody dilution, washing conditions, blocking buffers, and incubation parameters can help reduce background signal and improve ELISA assay specificity.
Possible Causes What You Can Do Too much antibodies was usedReduce the concentration of primary or secondary antibodies Antibodies bind nonspecifically Use blocking buffer or choose another affinity-purified antibody Too much substrate reagent usedUse substrate with higher dilutionInsufficient washingIncrease washing cycles Wrong concentration of blocking reagent Check the recommended concentration of blocking buffer Reaction not stopped Stop reactions with STOP buffer before readingPlate left too long before readingTake measurements shortly after addition of substrate and STOP buffer Insufficient Tween in wash buffer Use PBS+0.05% Tween as wash buffer Incubation temperature too highOptimize incubation temperature for each experimentPlate stacking during incubation lead to uneven temperature throughout the plateAvoid stacking plates together during incubation Pipetting error Calibrate pipettes to make sure that the correct volume is dispensedReagents not mixed properly Make sure that all reagents are mixed properly and equilibrated to room temperature before assaySalt concentration of incubation and wash buffer Increase salt concentration to reduce nonspecific interaction Substrate incubation carried out in light Perform substrate incubation in dark Dirty plate Make sure that the bottom of plate is clean -
Weak or no signal in ELISA is most commonly caused by insufficient antigen or antibody coating, or degraded substrate reagents prepared at an incorrect pH. To improve signal strength, increase the amount of coating reagent used and ensure substrate reagents are freshly prepared and at the correct pH before use.
Possible Causes What You Can Do Insufficient coating Use more antigens or antibodies for coatingSubstrate reagents have expired or prepared at a wrong pHUse fresh substrate reagents -
No signal in ELISA is commonly caused by incorrect assay setup, improper antibody selection, expired substrate reagents, or low sample concentration. To restore signal detection, verify protocol steps, use fresh reagents, check plate reader settings, and ensure that wells do not dry out during incubation.
Possible Causes What You Can Do Assay set up incorrectly Make sure that the instructions in the protocol is followed carefully Incorrect secondary antibody used Check if the correct secondary antibody is used Insufficient antibodies usedIncrease concentration of primary or secondary antibodySubstrate reagents not freshUse fresh substrate reagents Wrong settings of plate reader Check the settings (wavelength, filters, gain etc) of plate reader Insufficient incubation Follow the incubation time as indicated in the protocol bookletPlate washing too vigorousCheck the setting of plate washer. Pipette wash buffer into wells gently IWells dried out Use PBS+0.05% Tween as wash buffer Incubation temperature too highCover plate with adhesive film or incubate in humidified chamber throughout experimentEnzyme inhibitor present in buffers or reagentsInhibitors such as Sodium Azide can affect enzyme and assay performance. Ensure that there is no enzyme inhibitor in any buffers