If you prays and there is some impurity on your garment, even if it is a little, you should repeat your prayer, unless it is a small amount of blood or pus, that one would not think of as anything significant. Most of the scholars think that a small amount of blood or pus is pardoned, because of the report narrated from ‘Aa’ishah (may Allah be pleased with her) who said: “One of us would have a garment that she wore whilst menstruating or when she became junub, then she would see a drop of blood on it, so she would put some saliva on her fingernail and scratch it. According to another version: One of us would have only one garment that she would wear when menstruating, and if any of her blood got on it, she would wet it with her saliva then scratch it with her fingernail. Narrated by Abu Dawood. This indicates that it is pardoned, because saliva is not a means of purification and by doing so the impurity would be transferred to the nail. The report indicates that this was something they did frequently. Something like this could not have been hidden from the Prophet (blessings and peace of Allah be upon him) and could not have happened except on his instructions.
