Specific rashes can require medical attention; and some Tiktokers, like @drabby6, are pointing out the benefits of using the blanching test with a glass to know when a rash could be serious. According to Dr. Dawn Davis, professor of dermatology and pediatrics at the Mayo Clinic, “Blanching of the skin is when the skin loses color from blood being displaced away from where pressure is applied” (via Today). Typically, a healthcare professional presses a glass slide onto the rash to see if it changes (turns white) or stays the same (per Healthline).
You don’t need a doctor to perform a blanch test, however. Grab a glass out of your cupboard or dishwasher to do this test at home. Meningitis Now suggests pressing the side of the glass firmly against the skin where the rash is. Watch to see how it reacts. Blanching rashes will discolor, like turning white, when pressure is applied to the area. Non-blanching skin will maintain its color. Dr. Adam Friedman, professor and chair of dermatology, told Today, “Just be careful when applying pressure because you don’t want to break the glass on the skin.”
The reason for the test is the vessels underneath the skin. When a rash is blanchable, it means that the color leaves the blood vessels when pressed. Those that do not blanche have blood that has leaked outside of the vessels.