Is it permissible to pray in a garment with a few drops of blood on it, or on something from the underwear?

The basic principle is that the prayer of one who prays with blood on his garment is invalid due to its impurity. However, a small amount of it is pardoned and does not invalidate the prayer according to most scholars. The preponderant opinion is that the reference for determining a small amount is common practice (al-urf). If the blood is considered a small amount by common practice, then prayer with it is permissible; otherwise, it is not. This ruling is general for everything worn on the body.