Contact dermatitis (CD) is a common inflammatory skin reaction that develops after the skin comes into contact with a specific substance. It can be triggered by a direct irritant, such as a strong cleaning agent, or by an allergen, like nickel or poison ivy. The hallmark is a red, itchy rash that appears in the area of exposure, sometimes accompanied by small blisters. Understanding whether this redness will temporarily disappear when pressure is applied—known as blanching—requires a look into the underlying physiological mechanism that causes skin redness.
The Mechanism of Blanching
Blanching is the temporary whitening of the skin that occurs when external pressure is applied to a reddened area. When inflammation is present, the body increases blood flow to the affected area through vasodilation, which is the widening of capillaries. This widening causes the skin to appear red (erythema).
When firm pressure is exerted onto the reddened skin, the dilated capillaries are physically compressed. This compression temporarily pushes the blood out of the vessels and away from the pressed area, causing the area to turn pale or white.
Once the pressure is released, the capillaries immediately refill with blood, and the original redness returns almost instantly. This temporary color change confirms that the redness is caused by active blood flow within intact, dilated vessels.
Contact Dermatitis and the Blanching Test
Contact dermatitis rashes generally do blanch when tested. The redness is caused by the inflammatory process, which involves the release of chemical mediators, such as histamine, that trigger localized vasodilation.
To confirm blanching, a simple test called diascopy can be performed using a clear glass or a finger. By pressing firmly against the reddened area, the observer checks if the color fades to white or a lighter shade. If the color disappears or pales significantly, it confirms the redness is due to blood held within the vessels, which is consistent with contact dermatitis.
The two main types, irritant contact dermatitis (ICD) and allergic contact dermatitis (ACD), both present as superficial inflammatory responses. ICD results from direct damage to the skin barrier, while ACD is a delayed hypersensitivity immune reaction. Since the capillaries remain intact and the redness is caused by vasodilation, the erythema associated with these conditions typically blanches.
Understanding Non-Blanching Rashes
A rash that does not blanch indicates a different, often more medically significant, physiological process. Non-blanching rashes occur when the redness is caused by blood that has leaked out of the vessels and into the surrounding skin tissue, known as purpura.
Purpura appears as spots or patches that retain their red, purple, or brown color even when pressure is applied. Since the blood is no longer contained within the vessels, external pressure cannot displace it. Purpuric lesions are classified based on size: petechiae are small, pinpoint spots less than three millimeters in diameter, and ecchymoses are larger patches, essentially bruises.
The presence of a non-blanching rash indicates an underlying issue involving the integrity of the blood vessel walls or a problem with the blood’s clotting ability. Vascular damage can be caused by conditions such as vasculitis or clotting disorders. A non-blanching rash accompanied by fever may signal a serious medical issue, such as meningococcal sepsis, and requires immediate medical attention.