Different from those non-purpuric rash, the purpuric rash usually DO NOT blanch with pressure.
One of the ways to perform blanching is by the glass test. The picture shown is taken from this article - click here for the article. Click on the image to go to its image location.
Depending on their size, purpuric lesions are traditionally classified either:
a. petechiae (pinpoint hemorrhages less than 2 mm in greatest diameter)
b. purpura (2 mm to 1 cm - note: some put 5 mm as the cut off point)
c. ecchymoses (more than 1 cm)
(Baselga E, Drolet BA, Esterly NB. Purpura in infants and children. J Am Acad Dermatol 1997;37(5 pt 1):673-705)Again, refer to this excellent article for an approach to purpura in general. Click the URL link below to go to the article:
Kraft DM, McKee D, Scott C. Henoch-Schonlein purpura: a review. Am Fam Physician 1998; 58 (2):405-8, 11. URL:http://www.aafp.org/afp/980800ap/kraft.html