Hello Hamza Javaid,

It is better to keep them at 4 deg C since you intend to run ELISA within 2-3 days of getting sample supernatants.

For supernatants that will be used within a few days (24-72 hours), refrigeration at 4 deg C is a common and effective method for preventing significant degradation. 

Storing them at -20 deg C will unnecessarily cause repeated freeze and thaw cycles, and this can negatively impact the stability and activity of some toxins. 

For longer periods, yes, freezing at very low temperatures (e.g., -20 deg C or -80 deg C) is recommended to preserve the integrity of toxins.

Best,

Thanks, that's quite helpful!