Blanching and Non-blanching rashes
Rashes can be divided into 'blanching' and 'non-blanching'. The group called 'blanching' disappears when you press it. The group called 'non-blanching' doesn't disappear when you press it. A 'petechial' rash is a non-blanching rash that is very small, like pin pricks.
The glass test
The way to tell if a rash is blanching or non-blanching is to place a clear drinking glass over the rash and press down. If you can still see the rash through the glass, it hasn't disappeared and is, therefore, a non-blanching rash. If you can't see the rash anymore, it has disappeared and is therefore, a blanching rash.
For more information on the importance of non-blanching rashes, see 'Rashes' in the symptoms section.
A blanching rash
Press a glass against a blanching rash and it disappears, you can no longer see it
A non-blanching rash
Press a glass against a non-blanching rash and it doesn't disappear, you can still see it