If a person has at least three of the following symptoms, they are diagnosed as having the condition:
- Being older than 16 when the symptoms appear
- Having taken medications before the symptoms appeared
- A slightly elevated, purple-red rash over one or more areas of skin. The rash doesn't whiten when pressed.
- Flat and raised lesions of different sizes over one or more areas of skin
- Changes in the cells of the small blood vessels that can be seen through a microscope