How Psoriasis Is Diagnosed

Physical exam and medical history remain the primary tools

Psoriasis is an autoimmune disorder that happens because the immune system attacks skin cells as if they were harmful. That causes telltale patches of thick, red, scaly skin that can be spotted during a physical exam and are often are enough to give a psoriasis diagnosis away.

When the diagnosis is uncertain, a provider may also perform a skin biopsy to collect a sample of tissue that can be examined under a microscope.

This article covers everything your doctor will look for in an exam and what they may ask about your medical history. You'll also learn how a biopsy is done and what else you might have if it turns out you're not diagnosed with psoriasis after all.

psoriasis diagnosis
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Physical Examination

There are many health conditions that can cause itchy, scaly skin and rashes. In most cases, healthcare practitioners—especially dermatologists, who specialize in skin disorders—can tell if your symptoms are due to psoriasis or something else based on sight alone.

To get a closer look, they may view your skin through a dermatoscope. This simple handheld tool has a light and a magnifying glass. It allows your doctor to "zoom in" on your skin.

Review of Medical History

They will also spend some time asking you some questions.

There are several types of psoriasis. Some can cause symptoms that affect more than your skin, so your practitioner may ask if anything else is bothering you too. Some of these issues include:

Expect to also be asked about your medical history. Aside from other conditions you have, you will be asked about whether or not you've noticed any symptom patterns.

In preparation for your visit, it can help to write down when you have flares, how long they last, what symptoms you have, and if anything seems to make them better or worse. Take note of any treatments or illnesses that took place around the time of your flares as well.

In addition, they will likely ask you if you have any other risk factors for psoriasis, such as:

  • A family history of psoriasis or other skin conditions
  • A recent strep throat infection
  • A recent vaccine
  • A condition that weakens your immune system, like HIV
  • Other autoimmune disorders, such as celiac disease, Crohn's disease, or thyroid disease

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

Recap

Diagnosis for psoriasis starts with a physical exam. Your doctor will look at your skin and check you for other symptoms like joint pain and eye inflammation. They will also ask you if you have any other factors that could put you at risk for psoriasis.

Labs and Tests

Rashes can develop for many different reasons. So if your rash doesn't look like a typical psoriasis rash, your dermatologist may proceed with a skin biopsy.

In particular, they will likely get the tissue sample via a punch biopsy.

This procedure is done in the doctor's office with a local anesthetic to numb the skin. A tube-shaped device that snaps down when pressed (similar to a paper hold puncher) remove a tiny piece of skin.

The tissue sample is stained with a blue dye called hematoxylin-eosin. This helps the skin cells stand out when viewed under the microscope. With psoriasis, the skin cells appear acanthotic (dense and compact).

No other tests can confirm psoriasis, but your doctor may order some if they think there are other possibilities for your symptoms.

It tends to take around a week to get the results of a skin biopsy. In the meantime, your doctor may treat you for pain and discomfort.

Types of Psoriasis

Part of diagnosis is determining the type of psoriasis that you have. This gives your doctor a baseline from which to monitor your condition for any changes. It can also help whether you have a type that requires more specialized treatment.

Each has its own unique features, which can vary from person to person. In addition, some types are distinct in that they tend to affect a certain part of the body.

Around 80% to 90% of people with psoriasis have plaque psoriasis, which causes patches of red, dry skin covered with silvery-white scales. These skin plaques tend to be found on the elbows, knees, scalp, and back.

Other possibilities include:

  • Nail psoriasis: This type often occurs with skin psoriasis but can also occur on its own. It causes dents to form in the nail (nail pitting) and nail color changes. The nail plate can also lift from the nail bed and crumble.
  • Guttate psoriasis: The telltale sign of this type is a pale, pink, rash shaped like a teardrop. It tends to develop after a bacterial or viral infection, like strep throat or chickenpox. It is most common in children.
  • Pustular psoriasis: This type causes patches of red skin with pus-filled blisters, often on the soles and/or palms. A more serious form, called Von Zumbusch psoriasis, can affect the entire body and may need hospitalization.
  • Inverse psoriasis: This type is less common but more likely to occur in overweight people. It causes a rash to form in skin folds, such as the armpits and groin, under the breasts, and between the buttocks. The rash may appear moist rather than dry and scaly.
  • Scalp psoriasis: This causes patches of thickened skin on the scalp. The patches are often quite itchy and can lead to hair loss.

It's possible to have more than one type of psoriasis. For example, you may have plaque psoriasis for several years before a second type of psoriasis develops. Or, your plaque psoriasis could turn into another, potentially more severe type of psoriasis later on.

PASI Classification

If your doctor finds that you have psoriasis, they may classify your condition's severity using the Psoriasis Area and Severity Index (PASI). The index grades your condition based on how severe your symptoms are.

Symptoms on your head, arms, trunk, and legs are all assessed visually. No special tools or tests are needed.

Each symptom on each of these body parts is given a value. The values are then added up to get 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 determining the PASI score at diagnosis and repeating the PASI every few months, your dermatologist can track your response to treatment.

Recap

Your dermatologist may do a punch biopsy to verify the cause of your skin symptoms. If it is psoriasis, they may use the Psoriasis Area and Severity Index (PASI) to grade your condition's severity. The PASI may be repeated every few months.

Differential Diagnoses

Many skin diseases cause symptoms that look like psoriasis. There is always a chance you could be diagnosed with a different condition at first, such as a drug allergy or a fungal infection, only to later be diagnosed with psoriasis.

On the other hand, it's also possible for psoriasis to be suspected when it's actually another condition that's causing your symptoms.

Some of these are serious, so your doctor may want to do a few tests to rule them out before confirming what you have. This process is known as a differential diagnosis (DDx).

Conditions that can mimic psoriasis include:

Summary

Most dermatologists can tell if you have psoriasis just by looking at your skin up close. They may take a sample of tissue and examine it under a microscope to make sure your rash isn't caused by something else and confirm a psoriasis diagnosis.

Many medical conditions cause skin symptoms that can look and feel like psoriasis. Some of them can be serious, which is why it's important to have your doctor check out any changes in your skin.

A Word From Verywell

A psoriasis flare can be triggered by many things, from an infection to the weather. As the condition is a chronic one, this can be stressful.

Unfortunately, stress is also a common psoriasis trigger. It's easy to see how all of this can lead to one flare after the next.

If stress is making your psoriasis worse, try learning techniques to manage it, like yoga or meditation. You might also consider seeking help from a licensed therapist.

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, a skin biopsy. A small piece of tissue is taken from the affected area and viewed under a microscope. The biopsy can be done at the doctor's office.

  • 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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6 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. National Psoriasis Foundation. What type of psoriasis do I have?.

  2. National Psoriasis Foundation. About psoriasis.

  3. American Academy of Dermatology Association. Pustular psoriasis: Treatment options.

  4. American Academy of Dermatology Association. Types of psoriasis: Can you have more than one?.

  5. Fink C, Uhlmann L, Klose C, Haenssle HA. Automated, computer-guided PASI measurements by digital image analysis versus conventional physicians' PASI calculations: study protocol for a comparative, single-centre, observational studyBMJ Open. 2018 May;8(5):1-4. doi:10.1136/bmjopen-2017-018461

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

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