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.
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 skinSilvery white scales (plaques)Cracked, dry, and bleeding skinItching 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.
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.
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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 familiesAny recent infections or immunizations that might explain your symptomsYour 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.
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:
Inverse psoriasis is a 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 manifests with tiny red rashes, mainly on the trunk.
Your healthcare provider will also consider other skin conditions that closely resemble psoriasis, including:
Eczema Keratoderma blennorrhagicum (reactive arthritis) Lupus Lichen simplex chronicus Pityriasis rosea Squamous cell skin cancer Seborrheic dermatitis Tinea corporis Lichen planus Pityriasis lichenoides chronica
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.
