How Plaque Psoriasis Is Diagnosed

The diagnosis of plaque psoriasis is relatively straightforward. Psoriasis is a common dermatological disorder that most health providers will recognize. Pediatricians, internal medicine doctors, and family practice physicians will have some direct experience with the condition.

Unlike some skin conditions that are difficult to classify, plaque psoriasis can usually be diagnosed by its appearance alongside a review of your medical history. If there is any doubt, your doctor may refer you to a dermatologist for a skin biopsy.

plaque psoriasis diagnosis
 © Verywell, 2018

Self-Checks

Although there are no at-home tests for psoriasis, most people will be able to recognize the symptoms of the disorder, including:

  • Red, raised patches of skin
  • Silvery white scales (plaques)
  • Cracked, dry, and bleeding skin
  • Itching and burning around the patches

Moreover, the condition is characterized by flares in which the symptoms will suddenly appear and just as suddenly resolve. Joint pain, thick and irregular nails, and blepharitis (eyelid inflammation) are also common.

With that being said, it is easy to mistake psoriasis for other skin conditions such as eczema and allergic dermatitis, especially if it is your first event. It is important, therefore, to see a doctor for a definitive diagnosis rather than trying to diagnose and treat it yourself.

Self-diagnosing a skin condition is never a good idea. Not only can it lead to inappropriate treatment, but it may also delay the diagnosis of a more serious condition like skin cancer.

Medical History

A medical history is an important part of the diagnostic process. It puts into context your individual risks for plaque psoriasis and helps identify conditions that may co-occur with the disease. When taking your medical history, your doctor will want to know about:

  • Your family history of skin disorders, particularly since psoriasis runs in families
  • Any recent infections or immunization that might explain your symptoms
  • Your history of allergies

Your doctor will also want to know about any skin cleansers, detergents, or chemicals you may have been exposed and whether you have persistent or worsening joint pain.

Plaque Psoriasis Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Woman

Physical Exam

The physical exam would mainly involve the visual and manual inspection of the skin lesions. The aim of the exam is to determine whether the physical characteristics of your condition are consistent with psoriasis. The visual examination may be performed either with the naked eye or a dermatoscope (an adjustable magnifying glass with a light source).

In addition to the skin, the doctor may want to look at the condition of your nails and to check whether you have pain or inflammation in the hands, wrists, elbows, wrists, knees, ankles, and small joints of the feet. An eye exam may also be performed to see if the eyelids, conjunctiva, or cornea are affected.

Labs and Tests

There are no blood tests that can diagnose plaque psoriasis. Medical imaging is also not a part of the diagnostic process.

Only in rare instances might a doctor perform a skin biopsy to definitively diagnose plaque psoriasis. A biopsy is often performed when the symptoms are atypical or another diagnosed skin condition fails to respond to treatment.

The biopsy would be performed under local anesthesia to numb the skin before a tiny sample is obtained using either a scalpel, razor, or skin punch. The sample would then be viewed under a microscope.

Psoriatic skin cells tend to be acanthotic (thick and compressed) unlike normal skin cells or even those associated with eczema.

PASI Classification

Once psoriasis has been definitively diagnosed, your doctor may want to classify the severity of your condition. The scale most commonly used is called the Psoriasis Area and Severity Index (PASI). It is considered the gold standard for clinical research and a valuable tool for monitoring people with severe and/or intractable (treatment-resistant) psoriasis.

PASI looks at four key values—the area of skin involved, erythema (redness), induration (thickness), and desquamation (scaling)—as they occur on the head, arms, trunk, and legs. The area of skin is rated by percentage from 0 percent all the way up to 90 to 100 percent. All other values are rated on a scale of 0 to 4, with 4 being the most severe.

Generally speaking, only moderate to severe cases are classified this way, typically when "stronger" biologic drugs such as Humira (adalimumab) or Cimzia (certolizumab pegol) are being considered. Doing so not only directs the appropriate treatment but also helps track your response to therapy.

Differential Diagnoses

As part of the diagnosis, your doctor will perform a differential diagnosis to exclude all other possible causes. This is especially important since there are no lab or imaging tests to support a plaque psoriasis diagnosis.

The differential will typically begin with a review of the other types of psoriasis. While each has similar disease pathways, they have different characteristics and may have different treatment approaches as well. Among them:

  • Inverse psoriasis is less scaly rash than plaque psoriasis and mainly affects skin folds.
  • Erythrodermic psoriasis is characterized by a widespread red rash.
  • Pustular psoriasis involves pus-filled blisters on the palms and soles
  • Guttate psoriasis is more common in children than adults and manifests with tiny red rashes, mainly on the trunk.

Your doctor will also consider other skin conditions that closely resemble psoriasis, including:

Because other skin conditions can mimic psoriasis, misdiagnosis is not uncommon. If unsure about a diagnosis or unable to find relief from a prescribed treatment, do not hesitate to ask for further investigation or to seek a second opinion.

A Word From Verywell

In addition to the primary and differential diagnoses, your doctor may check for other autoimmune disorders closely related to psoriasis. Chief among these is psoriatic arthritis (which affects up to 41 percent of people with psoriasis, according to a 2015 review from the University of Pennsylvania).

A dual diagnosis can sometimes motivate for different or more aggressive forms of therapy. Other common co-occurring disorders include vitiligo and Hashimoto's thyroiditis.

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