When skin changes color, determining the cause often involves the blanching test. This test differentiates between redness caused by blood flowing within vessels and discoloration resulting from blood that has leaked into the surrounding tissue. Understanding the distinction between a blanchable lesion and a non-blanchable one, such as a bruise, is an important diagnostic tool. This distinction guides medical professionals in understanding the underlying physiological process and deciding whether a lesion is a harmless injury or a sign of a more serious condition.
What Does It Mean for Skin to Be Blanchable?
A skin lesion is considered blanchable if its redness or discoloration temporarily disappears or turns white when gentle pressure is applied. This temporary color change happens because the pressure physically pushes the blood out of the superficial capillary beds, the tiny blood vessels just beneath the skin’s surface. As soon as the pressure is released, the blood flows back into the capillaries, and the original color returns.
The underlying reason a lesion is blanchable is that its redness is caused by vasodilation, the widening of the blood vessels, or an increased amount of blood flow within the intact capillaries. Conditions like a mild sunburn, inflammation from a rash, or temporary redness from a pressure mark are classic examples of blanchable lesions. In these cases, the blood vessels are functioning normally, and the blood is contained entirely within the circulatory system.
Why Bruises Are Non-Blanching
Bruises (contusions or ecchymoses) are non-blanching because the discoloration is caused by blood that has escaped the vessels, not blood flowing within them. A bruise forms following blunt force trauma that damages small blood vessels, causing blood to leak (extravasate) into the surrounding soft tissues beneath the skin. This pooling of blood creates the characteristic blue, purple, or black mark.
When pressure is applied to a bruise, the extravasated blood is already outside the circulatory system and fixed in the tissue, meaning it cannot be pushed away or displaced. The color remains visible through the pressure, confirming the lesion is non-blanching. Over time, the body’s natural cleanup process breaks down the hemoglobin, the oxygen-carrying protein in the leaked red blood cells. This breakdown produces different colored pigments, such as biliverdin (green) and bilirubin (yellow), which is why a bruise changes color as it heals.
Differentiating Non-Blanching and Blanchable Lesions
Differentiating between blanching and non-blanching skin changes is a powerful diagnostic tool in clinical settings. While a bruise is the most common non-blanching mark resulting from trauma, other non-blanching lesions can indicate more concerning underlying medical issues. These include petechiae (tiny, pinpoint red dots) and purpura (larger red-purple patches). Both are caused by bleeding under the skin but are not necessarily due to a direct injury.
A non-blanching rash that appears suddenly, is widespread, or is accompanied by other symptoms like fever, vomiting, or confusion warrants immediate medical attention. These lesions can be signs of serious conditions such as sepsis, meningitis, or clotting disorders. To check a mark at home, a person can gently press a clear drinking glass or a finger against the area for a few seconds. If the color disappears under the pressure, it is blanchable, but if the color remains visible through the glass, it is non-blanching and should be evaluated by a healthcare provider.