The ELISA plate reader detects no colorimetric signal or signal intensity is significantly below expected values after substrate development. Wells appear clear or only faintly colored even after standard incubation times of 10-30 minutes.
Common Causes
1Key reagent omitted during protocol execution or added in incorrect order
2Substrate prepared incorrectly or incompatible with enzyme conjugate (e.g. pNPP required for alkaline phosphatase, OPD or TMB for peroxidase); fresh H2O2 not added when required
3Washing steps too stringent, removing bound antibody-enzyme conjugates
4Incubation times inadequate for antibody binding or substrate development (typically 10-30 minutes required)
5Enzyme conjugate or substrate has lost activity due to improper storage or expiration
6Sodium azide present in buffers, inhibiting peroxidase reactions
Solutions
1Verify all reagents added in correct sequence following manufacturer protocol; use checklist during execution
2Confirm substrate matches enzyme conjugate type; prepare fresh substrate solution with fresh H2O2 immediately before use
3Use automated plate washer if available; reduce or eliminate detergent concentration in wash buffer
4Extend incubation times appropriately for system; bring substrates to room temperature (20-25°C) before use
5Test conjugate and substrate activity with positive controls; replace expired or improperly stored reagents
6Prepare buffers without sodium azide for peroxidase-based assays; verify plate reader wavelength and filter settings match substrate absorbance