Diascopy is the medical diagnostic test for blanchability within the skin in order to determine if a rash or lession is vascular in nature (meaning that it inflammatory) or non-vascular (nevus) in nature or haemorrhagic (such as the petechial or purpura rash assocaited with meningococcal disease).
In order to perform diascopy, the paramedic or medical clinician should apply pressure to the skin with a finger or glass. If the rash/lession blanches it is determined to be vascular. If it does not blanch, then it is most likely to be non-vascular or haemorrhagic.
As a paramedic, we regularly perform diascopy when we check a rash for blanchability as part of our clinical assessment of a person with sepsis like symptoms in order to rule out meningococcal disease.