There are two general types of hematologic lesions: vascular and non-vascular. Vascular lesions exist as pathologic formations of blood vessels, such as telangectasias and hemangiomas. Non-vascular hematologic lesions are the result of extravasated blood collected below the surface of the skin or trapped between layers of tissue, such as a hematoma.
The principle of diascopy relies on this difference. Diascopy entails using a thin plate of transparent glass or plastic to depress a hematologic lesion. If the lesion blanches and no longer appears with the same bloodlike color, the lesion is deemed vascular, as the glass or plastic has effectively blocked the circulatory flow of or about the lesion, causing a momentary ischemia of the lesion. If the lesion is non-vascular, pressing it with the plate will merely press on the collection of blood, but the area will not blanch.