A new symptom or old one that worsens. It has to last at least 24 hours before the doctor will treat it as a flare. Then, if it continues, I'm always asked if I have an infection (usually a UTI), if not, I am sent for an MRI and if there is an enhancing lesion I'll be put on Solumedrol.

An exacerbation or what is commonly referred to a" flare" is a new symptom that remains for 24 hours or longer.

I was diagnosed with RRMS 13 months ago. I just had an appointment with my neuro this week. He said he is pleased because I have not had an exasperation. I begged to differ... I have had new symptoms and worsening symptoms since my initial lesions were no longer active. I have had multiple MRIs all showing new enhanced lesions, though only one at a time and small. What will a true relapse entail? Will I just know?!

Correct, if it last for more than 24 hours with the same symptoms, it's the same lesions. If you have different symptoms it may be a new lesion. Call the Doc.