Let’s take a look at Exodus 12:22 and the word בסף (basaf). What it can mean? A "basin" or a "threshold"?
I think it is used in both senses in other passages (both basin and threshold). My thought would be that it probably refers to basin, because if there was any special significance to it (such as, in the case that it implied that one is to slaughter the animal at the threshold), I would think the verse after it would mention seeing the blood upon the threshold as well, though it only talks about seeing the blood on the top and sides of the door frame.