Porphyrin is just mucus and it is normal for them to have some discharge occasionally or after sleeping. Young rats will frequently have streams of it when adjusting to the air in a new home (it resembles a bloody nose and is quite unnerving but they are not hurt at all). Diseases that accompany heavy amounts of porphryn, however, often require medication and the most common ones, if left untreated, become chronic. I knew a rat with a problem with mycoplasma, the most common rat-killing virus, and it is able to be cured if detected immediately. If it is not treated, it can either A. not harm the host anymore but spread to other rats (it is airborne) or B. kill the rat over months or weeks.
I hope that answers your question.