Use a glass test tube or specimen tube to press on a rash to see if it blanches – remember, petechiae and purpura don’t blanch. You can also use a glass test tube as a mini-anoscope to blanch surrounding mucosa and assist in identifying anal fissures in infants. (Anal fissures are one of the many possible causes of GI bleeding in children – check out pemplaybook.org’s latest podcast on GI bleeding)

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(From https://www.meningitisnow.org/meningitis-explained/signs-and-symptoms/glass-test/)