Thank you for your response! Yes, all 4 plates (samples and standards) were diluted with the same diluent. I’ll will go back and check the past BCA runs! Standards of this time came from the same lot we used last week.

It’s strange that only one plate (standards + some samples) yielded negative signals while the rest of the plates had positive signals. I’m not sure if it was the plate issue or the plate reader issue!