Most rashes are not considered significantly serious. That said, if you have a persistent rash that worsens or coincides with other symptoms aside from itching and irritation, it is wise to seek medical attention. Some of the signs that a rash could indicate a more serious condition include:
A widespread rash on most of your body. This could indicate a systemic allergic reaction or infection that requires treatment.
Fever with the rash. This, too, could be indicative of an allergic reaction or infection such as shingles, mononucleosis, or measles.
The rapid spread of the rash. This may indicate an allergic reaction to some type of medication.
Your rash starts to blister. Blistering may occur due to systemic reaction or other conditions, such as shingles.
Your rash oozes and hurts. Oozing, pain, crusting of the skin, and skin that is warm to the touch all signal that your rash may be infected.