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What is a skin blanching test? And can it tell you if a rash is serious?

A skin blanching test involves pressing down on the skin to see if redness or discoloration disappears.
Forearm showing Capillaritis
If a rash is non-blanching, that means the discoloration does not fade or disappear when pressure is applied.andylid / Getty Images stock

When you press down on your skin for a few seconds and move your finger away, does that area of skin look lighter before it returns to its normal color? This is called blanching.

Some social media accounts are sharing this test as a hack for parents to determine if a child’s rash requires medical attention. But can this blanching test actually be used at home to determine whether a rash is serious or not? Here's what experts think.

What is a skin blanching test?

“Blanching of the skin is when the skin loses color from blood being displaced away from where pressure is applied,” Dr. Dawn Davis, professor of dermatology and pediatrics at the Mayo Clinic in Rochester, Minnesota, told TODAY. “For example, if I push on my skin … and it looks slightly pale when I release my finger ... that’s because I’ve temporarily pushed or stopped the blood flow to that area,” Davis added. 

In medicine, this is called a “diascopy,” said Davis, and it involves taking a glass slide and pushing it onto the lesion or skin that appears red to see if the discoloration disappears with pressure to determine what type of rash or lesion is present. 

You can also do this at home with a clear drinking glass, which makes it easier to see the skin underneath where the pressure is applied, Dr. Adam Friedman, professor and chair of dermatology at George Washington School of Medicine and Health Sciences told TODAY. “Just be careful when applying pressure because you don’t want to break the glass on the skin, I’ve seen that happen before,” said Friedman.

Why do some rashes "blanch" and others don't?

“The reason why an area of skin is red is because the blood vessels are dilated or widened, and there’s more blood flow to that area,” said Friedman. So a “blanchable” or “blanching” rash occurs when the redness (erythema) or discoloration disappears with pressure, but then returns because the blood is still inside the vessels so it's being moved around, he added. 

If a rash is non-blanching, that means the discoloration does not fade or disappear when pressure is applied, the experts noted. This is because there is bleeding or blood leaking from damaged vessels under the skin (also called purpura), which can be caused by a variety of conditions, Friedman explained. “If the blood is out of the vessels … you won’t get blanching (because) you can’t really move it around,” he added. 

It’s important to note that rashes or discoloration can look different on different skin tones. “Blanching of darker skin, and even recognizing there is erythema, is harder,” said Friedman, adding that what may appear reddish or purplish on lighter skin can appear as brown, black or even off-white on darker skin. “Being able to appreciate that blanching … could require more pressure,” said Friedman. 

What are “blanching” rashes?

“Oftentimes people believe that if something blanches, that means that it’s not harmful, but that’s not true, they can be harmful,” said Davis.

Extreme blanching of the skin can occur due to a lack of circulation, said Davis, which can be caused by frostbite or Raynaud’s phenomenon. “The blood vessels constrict due to either a disease or temperature changes, and that can leave the tips of the fingers or the toes white,” said Davis.

Another common blanching rash is caused by broken or dilated blood vessels (also called telangiectasia), which are typically benign and harmless, said Davis. “However, there are times when there’s not consistency between something blanching or not and being benign versus harmful,” she said, adding that sometimes telangiectasia can be a sign of a more serious internal disease. 

“In the early stages of vasculitis (inflammation of blood vessels), which can be very harmful, a lot of times the lesions will blanch because there’s not enough inflammation or destruction yet for the blood to leak,” Davis explained.

Allergic reactions and insect bites can also cause blanching rashes, Friedman explained. “If someone develops urticaria or hives, which can be due to a response to a food or medication …  that is blanchable,” said Friedman. Hives are typically not dangerous, he added, but when coupled with symptoms like wheezing or difficulty breathing, can constitute an emergency. 

“It all goes back to what the rash is coming along with,” Friedman added. 

There are plenty of blanchable rashes that, in conjunction with systemic features (like fever, joint pain, organ dysfunction, diarrhea, vomiting), could be an emergency, says Friedman. “There are multiple life-threatening drug reactions that have a blanchable rash … that you absolutely don’t want to dismiss,” said Friedman.

Are non-blanching rashes serious?

There are a variety of things that can cause a “purpuric” rash or bleeding under the skin, said Friedman, and some are more serious than others.

Bacterial meningitis can cause a non-blanching rash, said Friedman, and as well as other bacterial infections such as meningococcemia and gonococcemia. “(These) are actually a systemic infection that’s manifesting in the skin because of the dissemination of that bacteria through the blood,” said Friedman.

Purpuric rashes or vasculitis can also be caused by viruses like hepatitis C, autoimmune diseases, and even certain fungal infections in immunocompromised patients, Friedman explained. 

“In any of these cases, you wouldn’t see this (non-blanching rash) alone … patients would have other symptoms,” said Friedman, adding that these concerning scenarios are relatively rare.

Meningitis symptoms include a high fever, stiff neck, severe headache, confusion, and vomiting, according to The Mayo Clinic.

The texture of a non-blanching rash can also offer clues. “If you have what’s called palpable purpura, meaning a non-blanchable rash that’s raised that you can feel, our mantra in dermatology is it’s vasculitis until proven otherwise,” said Friedman. 

“Vasculitis means there’s inflammation attacking the vessels to the point where the vessels are being damaged or even destroyed,” said Friedman, adding that this can be caused by severe drug reactions and malignancy (cancers). If you suspect the non-blanching rash is vasculitis, Friedman suggests getting workups done by a doctor to identify the cause.

Some non-blanchable rashes are not dangerous at all, said Friedman, and one of the most common is called benign pigmented purpura. Others include solar purpura, which are broad purplish patches typically found on sun-exposed areas of elderly people, said Friedman, and bruising or broken blood vessels from other activities. “These are bothersome, but benign,” he added. 

While not all non-blanchable rashes are an emergency, “it’s never a bad thing to go get it figured out,” said Friedman, adding that individuals should watch out for any other symptoms.

“If a patient develops a non-blanching rash, they should seek health care advice more proactively,” said Davis.

Can this test be used to determine if a rash is serious?

The short answer is no — you can’t necessarily determine whether a rash is benign or serious only based on whether it blanches or not, said Davis, adding that there are multiple differentiating factors.

“Unfortunately, there’s no hard and fast rule of thumb to distinguish whether a rash is harmful or not based on any one specific characteristic,” she added. 

While the blanching test can help determine which type of rash you have, it is not a diagnostic test.

“You want to distinguish between blanching and non-blanching, but then there’s a laundry list of other things you need to flush out with respect to ... is it just a rash or is it a rash associated with something else,” said Friedman. Both experts recommend watching out for other signs and symptoms if you have a rash. 

One of these is a fever. “I think a fever and a rash should always prompt some investigation, and absolutely fever and a non-blanchable rash, that’s even more concerning,” said Friedman.

“If the child or the adult feels truly lethargic, toxic, or acutely febrile … that would require urgent or emergent care,” said Davis. 

Another important thing to look out for is how the rash is spreading. “If the rash progresses without explanation .... that could potentially be more concerning and you should seek medical evaluation sooner rather than later,” said Davis. 

“Anytime you develop a rash, whether that be an adult or a child, it’s helpful to be mindful of your overall health at that moment,” said Davis. “Because often the skin is the window to the body’s overall health,” she added.