Skin blanching is a medically informative phenomenon that occurs when external pressure is applied to the skin. It is the temporary whitening or paling of a red or discolored area, which fades away almost immediately when the pressure is removed. This observation indicates that the redness is due to blood still contained within the underlying vessels. The presence or absence of blanching holds significant meaning in diagnosing various skin conditions and monitoring tissue health.
The Physiological Mechanism of Blanching
The skin’s color, particularly redness, is largely determined by the blood flowing through the tiny vessels directly beneath the surface, known as capillaries. These capillaries form a dense network in the upper layers of the skin, the dermis, and are responsible for supplying oxygen and nutrients.
When pressure is exerted on the skin, it physically compresses these vessels, momentarily squeezing the blood out of the area. This displacement causes the skin to lose its red hue, resulting in the temporary white or pale appearance of blanching. Once the pressure is released, the capillaries quickly refill with blood, a process known as reperfusion, and the skin color returns to its original state within a few seconds. This rapid return of color confirms that the microcirculation—the blood flow through the smallest vessels—is intact and functional.
The Diagnostic Significance of Non-Blanching
The opposite of blanching, known as a non-blanching rash, is often a sign of a more concerning medical issue. A non-blanching mark is one where the redness or discoloration persists even when pressure is applied to the area. This difference indicates that the underlying cause of the color is not blood contained within the capillaries, but rather blood that has leaked out of the vessels and into the surrounding skin tissue. This process, called extravasation, results in small spots known as petechiae, or larger patches called purpura.
Because the blood is no longer contained within the vessels, external pressure cannot displace it, causing the discoloration to remain visible. The non-blanching nature is a warning sign because it can be associated with serious, rapidly progressing conditions, including meningococcal sepsis or certain types of vasculitis.
A simple test, often called the “glass test” or diascopy, is performed by pressing a clear glass or plastic slide firmly against the affected area of the skin. If the rash is visible through the pressed glass, it is non-blanching and warrants immediate medical evaluation. While many non-blanching rashes have less serious causes, such as minor trauma or viral illnesses, the potential for a life-threatening infection requires prompt assessment by a healthcare professional.
Specific Skin Conditions Where Blanching is Relevant
The blanching response is routinely used in clinical settings to assess the status of several common skin conditions and injuries. In the context of pressure ulcers, or bedsores, blanching is used to determine the severity of tissue damage. Redness that blanches when pressed is an early sign of temporary pressure, while persistent redness that does not blanch indicates a Stage 1 pressure injury, meaning tissue damage has already begun. This non-blanching redness suggests that the capillary beds have been damaged by sustained pressure, leading to localized bleeding and inflammation.
The blanching test also helps differentiate between various types of rashes, such as hives, medically known as urticaria. Hives are typically raised, red welts that will blanch when pressed, confirming that the redness is caused by temporary swelling and capillary dilation due to histamine release. This blanching feature helps distinguish hives from other non-blanching rashes that might suggest a more systemic disorder. Doctors also use blanching to classify vascular lesions, such as distinguishing a simple spider vein, which is blanchable, from a deeper hematoma that may be non-blanching because the blood is extravasated into the tissue.