How Psoriasis Is Diagnosed

Physical exam and medical history remain the primary tools

Psoriasis is an autoimmune disorder primarily affecting the skin. The diagnosis typically involves a physical examination to identify the signs and symptoms of the disease. A review of your medical history would also be conducted to assess your risk factors for your psoriasis as well as any other explanations for your condition. Less commonly, a tissue sample may be obtained to examine under a microscopic.

Unlike some autoimmune disorders, there are no blood tests or imaging studies that can aid in the diagnosis of psoriasis.

psoriasis diagnosis
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If you notice skin changes that you believe are caused by psoriasis, it is important to have them checked by a doctor or, better yet, a dermatologist. While you may be correct in your assumption, psoriasis can mimic many common and uncommon skin disorders, some of them serious.

With that being said, it is important to know the signs and symptoms of the different types of psoriasis so that you can take appropriate action. The most common forms include:

  • Plaque psoriasis: Around 80% of people with psoriasis have plaque psoriasis, which causes patches of red, dry skin covered with silvery-white scales. These skin plaques are most commonly found on the elbows, knees, scalp, and back.
  • Nail psoriasis: Nail psoriasis typically occurs in tandem with skin psoriasis but can also occur on its own. It is characterized by the pitting, lifting, and crumbling of the nail plate, as well as white or yellowish-red discoloration.
  • Guttate psoriasis: The appearance of a pale-pink, teardrop-shaped rash could very well be a sign of guttate psoriasis. This type of psoriasis usually follows a bacterial or viral infection (such as strep throat or chickenpox) and is more common in children than adults.
  • Pustular psoriasis: Per its name, pustular psoriasis is characterized by areas of reddened skin covered in pus-filled blisters. The focal form of the disease is confined to the soles and/or palms. A more serious form, called Von Zumbusch psoriasis, can affect the entire body and require hospitalization.
  • Inverse psoriasis: Inverse psoriasis is an uncommon form of the condition in which the rash is mainly confined to skin folds (such as in the armpits and groin, under the breasts, and between the buttocks). More common in overweight people, inverse psoriasis lesions may appear moist rather than dry and scaly.

Physical Examination

In the majority of cases, your doctor will be able to diagnose psoriasis by appearance alone. The examination can be done with the naked eye or with a lighted magnifying tool called a dermatoscope. Reports of flares, in which symptoms appear spontaneously and resolve just as quickly, are also helpful in making the diagnosis.

Depending on the type of psoriasis you have, there may also be non-dermatological symptoms such as blepharitis (eyelid inflammation), uveitis (inflammation of the pigmented part of the eye), and joint pain (associated with psoriatic arthritis).

A review of your medical history is just as important. It can often reveal factors that place you at an increased risk of the disease. Examples include:

Because many other skin diseases look like psoriasis, the medical review may also reveal alternative explanations for your symptoms, including drug allergies, fungal infections, or cancer.

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

Labs and Tests

Occasionally, the appearance of a rash may be unusual and require closer examination. In such instances, a histological (microscopic) evaluation of a tissue sample may be needed. This would involve a skin biopsy of some sort.

Dermatologists will usually perform a minimally invasive punch biopsy to obtain a tissue sample. This can be done in the doctor's office with a local anesthetic to numb the skin. The procedure involves a tube-shaped device that snaps down when pressed to remove a tiny portion of skin.

The tissue sample is then stained with a blue dye called hematoxylin-eosin to help differentiate the skin cells under the microscope. With psoriasis, the skin cells appear acanthotic (dense and compacted), unlike eczema and other dermatological conditions.

It usually takes around a week to get the results of a skin biopsy, during which time presumptive treatment may be started to help relieve pain and discomfort.

PASI Classification

Once psoriasis has been diagnosed, your doctor may want to classify the severity of your condition so that your response to treatment can be monitored. The scale most commonly used is called the Psoriasis Area and Severity Index (PASI). Outside of a research setting, this may be used in people with severe or intractable (treatment-resistant) psoriasis.

The index classifies your condition based on the extent or severity of symptoms on the head, arms, trunk, and legs. Each symptom on each part of the body is assigned a value which is then added to the others for a final PASI score. The higher the value, the more severe your condition.

The PASI reference values are as follows:

  • Erythema (redness), rated on a scale of 0 to 4
  • Induration (thickness), rated on a scale of 0 to 4
  • Desquamation (scaling), rated on a scale of 0 to 4
  • Percentage of skin involved, rated from 0% to 100%

By getting a baseline value before treatment and repeating the PASI every few months, your doctor can track your response. The index requires no special tools and can be performed by a dermatologist trained to use it.

Differential Diagnoses

As distinctive as psoriasis usually is, it can sometimes be mistaken for other conditions. Because psoriasis is diagnosed primarily on appearance, a dermatologist may want to perform additional tests to exclude other possible causes. Doing so can help avoid misdiagnosis and, even more importantly, ensure that potentially dangerous diseases are not missed.

Many of these conditions have lab and imaging tests to render a definitive diagnosis or, at the very least, point the doctor in the right direction. There are several conditions commonly included in the differential diagnosis process for psoriasis:

You can help your doctor avoid misdiagnosis by keeping a record of your symptoms (including the date, description, and duration of the acute flare), as well as any other symptoms, treatments, or illnesses that occurred around or during the event.

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Article Sources
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  2. National Psoriasis Foundation. What type of psoriasis do I have?. Updated March 8, 2020.

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