diascopy diascopy diascopy is a test for blanchability performed by applying pressure with a finger or glass slide and observing color changes. diascopy of a red dot basal cell carcinoma on the left mid back of a 74-year-old female. the red dot basal cell carcinoma on the left mid back is circled; the tumor blanches when a glass microscope slide is pressed against it. from a case series by philip r cohen, 2017. it is used to determine whether a lesion is vascular (inflammatory or congenital), nonvascular (nevus), or hemorrhagic (petechia or purpura). hemorrhagic lesions and nonvascular lesions do not blanch ("negative diascopy"); inflammatory lesions do ("positive diascopy"). diascopy is sometimes used to identify sarcoid skin lesions, which, when tested, turn an apple jelly color.