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How Is Prurigo Nodularis Diagnosed?

Medically reviewed by Kevin Berman, M.D., Ph.D. — Written by Emily Wagner, M.S.
Posted on September 21, 2022
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Prurigo nodularis (PN), also known as nodular prurigo, is a chronic inflammatory skin disease that causes pruritus (intensely itchy skin) and the formation of nodules (bumps) and lesions. PN can look similar to other inflammatory skin conditions, making it difficult to diagnose.

Dermatologists use a variety of tests to rule out other conditions and make a PN diagnosis. Receiving a correct diagnosis helps ensure you’ll get the treatment you need.

Challenges of Diagnosing Prurigo Nodularis

When it comes to diagnosing PN, doctors and dermatologists have to consider many factors. According to the American Academy of Dermatology, symptoms of PN include:

  • Itchy nodules, papules, or lesions on the skin that last for at least six weeks
  • Thickening, lichenification (skin has become thickened and leathery), and scarring of the skin
  • Burning or crawling sensation

Many of these symptoms overlap with other inflammatory skin diseases, making differential diagnosis difficult at times. (A differential diagnosis looks at the possible disorders that could be causing your symptoms.) A misdiagnosis of another disease can lead to doctors choosing an ineffective treatment plan. In some cases, PN is only diagnosed after a treatment plan doesn’t work and more tests are done.

Tests for Diagnosing Prurigo Nodularis

To accurately diagnose PN, dermatologists perform a few tests that look at the skin and the cells found within it. If PN is suspected, your health care provider may run blood tests to look at the levels of different immune cells and screen for diseases or infections that might be contributing to your PN.

It is very common for PN to occur alongside other health conditions, which may also be diagnosed with blood tests. If you have underlying health conditions that are contributing to PN, your doctor will go through your treatment options. Addressing these problems can help clear PN in some cases, improving your overall quality of life.

Physical Exam

When diagnosing PN, your primary care physician or dermatologist will likely perform a physical exam. They may also ask how symptoms affect your daily activities like going to school or work, and if they affect your sleep quality, mental health, and social interactions. They will look for signs of PN, including:

  • The presence of nodules, lesions, or papules on the skin and if they are symmetrical (the same on both sides of the body)
  • The location of the nodules and lesions — common sites for PN include the arms, legs, upper and lower back, and abdomen
  • The extent of the itchiness on a scale from mild to very severe

Dermoscopy

Dermatologists use a test called dermoscopy to look closely at the skin. This method has been traditionally used to diagnose types of skin cancer such as melanoma. However, doctors now know that it is also useful for diagnosing other skin conditions such as PN. Dermoscopy is noninvasive, meaning that it doesn’t break the skin. This lowers the risk of infection and PN flares.

Dermoscopy uses a high-powered magnifying lens that is attached to a light (called a dermatoscope) to look closely at the skin. The dermatologist places the lens onto a nodule or skin lesion and inspects the structure of the epidermis (outermost layer of skin) and the dermis (second-deepest skin layer), along with any inflammation. They may also take pictures with the dermatoscope to look at later.

The dermatologist will examine the different characteristics of the skin and note any changes in:

  • Pigmentation (how light or dark the skin is)
  • How much keratin (a protein found naturally in the skin) is present, which can indicate PN
  • The symmetry of the nodules or lesions
  • The blood vessels in the skin (vascularization)

A small study using dermoscopy to diagnose PN found that people with PN may have a distinct pattern in their lesions. The researchers noted that many participants had a “white starburst pattern” around a dark-colored crust with excess keratin in the skin.

Skin Biopsy

Another test used to diagnose PN is a skin biopsy. This is an invasive test in which your dermatologist removes a piece of skin so that they can look at it under a microscope. In suspected cases of PN, a skin biopsy takes a piece of thickened skin from a nodule or lesion to look for changes under the microscope.

There are three types of skin biopsies: excisional biopsy, shave biopsy, and punch biopsy. Studies mainly use the punch biopsy to collect samples from the outer and inner layers of skin.

In people with PN, lesions tend to have more immune cells present, such as macrophages, mast cells, and neutrophils. These cells cause skin inflammation, which leads to itchiness, redness, and swelling.

PN lesions and nodules may also have more keratin. People with PN tend to have excess keratin (known as hyperkeratinization) because constant itching and scratching causes it to build up over time.

The levels of different immune cells and keratin found from a skin biopsy will help your dermatologist make a PN diagnosis. Sometimes, the biopsy is diagnostic (something related to the identification of a problem), and other times it just supports the PN diagnosis.

Blood Tests

Let your doctor know if you have any underlying health or medical conditions. Some conditions are associated with an increased risk of PN. Blood tests look at levels of immune cells, along with your overall organ health to determine if another condition or infection is playing a role in your PN. Examples of tests your doctor may order include:

  • Complete blood cell count with differential to check for infection (measures the number of red blood cells, white blood cells, and platelets in the blood)
  • Kidney function tests
  • Thyroid function tests
  • Liver function tests
  • HbA1C blood test to check for diabetes risk
  • Screening for hepatitis B/C and HIV, which increase the risk of developing PN

Blood test results will help your doctor determine if you have another health condition or infection that may be contributing to your PN and itching.

Differential Diagnosis and Other Skin Conditions

PN often looks similar to other inflammatory skin conditions such as:

  • Atopic dermatitis (eczema) with lichen simplex chronicus
  • Bullous pemphigoid or other blistering skin diseases
  • Psoriasis
  • Actinic prurigo, another chronically itchy skin condition

With this, dermatologists may have a difficult time diagnosing PN correctly. Skin biopsies are especially useful for separating PN from other skin disorders. After a biopsy, the skin is then sent to a pathology lab where it is stained with different chemicals to look for certain proteins and immune cells.

PN tends to have more keratin present than other conditions, along with more inflammatory cells and fibrosis (buildup of thickened or scarred skin). Dermatologists may also use skin scrapings on lesions or nodules to look for any signs of infection or if they suspect scabies or fungus.

Diagnosing PN correctly helps ensure you receive the treatment you need to help manage your symptoms and break the itch-scratch cycle. Your doctor may choose to run additional tests to identify and treat other health conditions that may be contributing to PN, which can help treat your PN. Learn more about conditions related to PN.

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Posted on September 21, 2022
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Kevin Berman, M.D., Ph.D. is a dermatologist at the Atlanta Center for Dermatologic Disease, Atlanta, GA. Review provided by VeriMed Healthcare Network. Learn more about him here.
Emily Wagner, M.S. holds a Master of Science in biomedical sciences with a focus in pharmacology. She is passionate about immunology, cancer biology, and molecular biology. Learn more about her here.

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