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 of the disease.

A review of your medical history would also be conducted to assess your symptoms and to rule out any other explanations for your condition. Less commonly, a tissue sample may be obtained to examine skin cells from the affected area with 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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Self-Checks

If you notice skin changes that you believe are caused by psoriasis, it is important to have them checked by a doctor. Your doctor may refer you to a dermatologist, which is a doctor who is specialized in treating skin disorders.

Symptoms of the most common types of psoriasis 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 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 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 may 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.
  • Scalp psoriasis: This causes patches of thickened skin, often with an itchy sensation, on the scalp. Sometimes hair loss can occur in the affected area as well.

With many types of psoriasis, you may have flares, in which symptoms appear spontaneously and resolve just as quickly.

Physical Examination

In the majority of cases, your doctor will be able to diagnose psoriasis by its appearance alone. The examination can be done with the naked eye or with a dermatoscope, which is a lighted magnifying tool.

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

Your doctor will review your medical history with you, including factors that worsen or relieve your symptoms.

Conditions that can put you at risk of psoriasis include:

Because many other skin diseases look like psoriasis, you might be diagnosed with a different condition that's causing your symptoms, such as a drug allergy or a fungal infection.

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 not have the typical characteristics of psoriasis. If the diagnosis is not clear, your doctor may need to do a histological (microscopic) evaluation of a tissue sample. This would involve a skin biopsy.

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 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).

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 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 perform additional tests to exclude other possible causes. Psoriasis can mimic many common and uncommon skin disorders, some of them serious.

Conditions commonly included in the differential diagnosis for psoriasis include:

You can help with your diagnosis 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.

Frequently Asked Questions

  • Can psoriasis be detected by blood test?

    No, there is no specific blood test that is used to diagnose psoriasis. However, blood tests can be helpful in ruling out other conditions. Your doctor may order blood tests for this reason.

  • How does a dermatologist diagnose psoriasis?

    Psoriasis is diagnosed with a physical examination, and sometimes with a skin biopsy. A small piece of tissue is collected from the affected area and examined with a microscope. The biopsy procedure can be done at the doctor’s office, and your doctor may use a local anesthetic to numb your skin before taking a tissue sample.

  • What can be mistaken for psoriasis?

    Several skin conditions can appear similar to psoriasis. These include eczema, contact dermatitis, lupus, lichen simplex chronicus, onychomycosis, pityriasis rosea, squamous cell skin cancer, dandruff, and ringworm. 

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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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