How Plaque Psoriasis Is Diagnosed

The diagnosis of plaque psoriasis is relatively straightforward but typically requires medical expertise to differentiate it from similar skin conditions. Plaque psoriasis, by far the most common form of psoriatic disease, is characterized by the appearance of raised scaly, white patches known as plaques. Dermatologists are usually able to diagnose psoriasis based on a review of your symptoms and medical history.

However, there are times when additional tests may be needed. This may involve a skin biopsy to look for definitive signs of the disease under the microscope. A differential diagnosis may be used to exclude other conditions that mimic plaque psoriasis, including other less common forms of psoriasis.

plaque psoriasis diagnosis
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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 healthcare provider 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.

Plaque Psoriasis Healthcare Provider Discussion Guide

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

Doctor Discussion Guide Woman

Physical Examination

The physical exam will 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. A healthcare provider will examine your skin either with the naked eye or a dermatoscope, an adjustable magnifying glass with a light source.

In addition to the skin, your healthcare provider may want to look at the condition of your nails and 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 corneas are affected.

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 during your appointment, your healthcare provider will want to know about:

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

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

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 healthcare provider perform a skin biopsy to definitively diagnose plaque psoriasis. A biopsy may be performed when the symptoms are atypical or another diagnosed skin condition fails to respond to treatment.

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

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

Determining Severity

Once psoriasis has been definitively diagnosed, your healthcare provider may want to classify the severity of your condition.

The scale most commonly used to do this 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% to 100%. 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 directs the appropriate treatment, but it also helps track your response to therapy.

Differential Diagnoses

As part of the diagnosis, your healthcare provider 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:

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

Because other skin conditions can mimic psoriasis, misdiagnosis is not uncommon. If you're 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 healthcare provider may check for other autoimmune disorders closely related to psoriasis. Chief among these is psoriatic arthritis, which affects up to 41% 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.

Frequently Asked Questions

  • Who diagnoses and treats psoriasis?

    A dermatologist typically diagnoses and treats psoriasis plaques. However, psoriasis is also treated by a rheumatologist because it is an autoimmune condition. A dermatologist treats the skin, while a rheumatologist treats the underlying autoimmune disease.

  • How is plaque psoriasis diagnosed?

    Plaque psoriasis is diagnosed based on a visual examination and a review of your symptoms and medical history.

  • Is there a lab test for plaque psoriasis?

    No. There are no blood tests that can diagnose plaque psoriasis. In rare instances, your healthcare provider might perform a skin biopsy. 

8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Lyons F, Ousley LE. Dermatology for the Advanced Practice Nurse. New York, NY: Springer Publishing Company, LLC.

  2. Golińska J, Sar-pomian M, Rudnicka L. Dermoscopic features of psoriasis of the skin, scalp and nails - a systematic review. J Eur Acad Dermatol Venereol. 2019;33(4):648-660. doi:10.1111/jdv.15344

  3. Psoriasis: Who gets it and causes. | American Academy of Dermatology.

  4. Kim WB, Jerome D, Yeung J. Diagnosis and management of psoriasis. Can Fam Physician. 2017;63(4):278-285.

  5. Armstrong AW, Parsi K, Schupp CW, Mease PJ, Duffin KC. Standardizing training for psoriasis measures: effectiveness of an online training video on Psoriasis Area and Severity Index assessment by physician and patient raters. JAMA Dermatol. 2013;149(5):577-82. doi:10.1001/jamadermatol.2013.1083

  6. Gisondi P, Del giglio M, Girolomoni G. Treatment Approaches to Moderate to Severe Psoriasis. Int J Mol Sci. 2017;18(11) doi:10.3390/ijms18112427

  7. About Psoriasis. National Psoriasis Foundation - About Psoriasis.

  8. Guttate psoriasis: MedlinePlus Medical Encyclopedia. MedlinePlus.

Additional Reading

By Ruth Jessen Hickman, MD
Ruth Jessen Hickman, MD, is a freelance medical and health writer and published book author.