How Psoriasis Is Diagnosed

Because psoriasis causes such distinctive changes in the appearance of the skin, scalp, or nails, it is usually obvious when a person develops the condition. This means that, in the majority of cases, a doctor can diagnose psoriasis by taking a medical history and doing a physical exam. Very rarely a biopsy may be necessary to pin down a psoriasis diagnosis.

psoriasis diagnosis
Illustration by Verywell

Self-Checks

If you notice skin changes that you think may be symptoms or signs of psoriasis you won't be able to officially diagnosis yourself, but you can certainly react by promptly seeing a doctor. That way you can be diagnosed promptly and start treatment as soon as possible. 

If you develop a rash or changes on your scalp or nails, keep the following in mind:

  • About half of people who have psoriasis on other parts of their body also will have patches on their scalp, so if scales develop on your head as well as on, say, your arms, legs, or torso, chances are good you have psoriasis. 
  • Similarly, people with skin psoriasis also often are affected with nail psoriasis. If your fingernails or toenails develop pits (dents), become thick, misshapen, or discolored, or if the nail separates from the nail bed, you could well be looking at psoriasis. 
  • A rash that consists of pale pink, teardrop-shaped spots could be guttate psoriasis. This type of psoriasis, which usually follows a bacterial or viral infection such as strep throat, is most common in children and people under 30.
  • A type of psoriasis called pustule psoriasis looks much like it sounds: Areas of reddened skin covered in pus-filled blisters that often are confined to the soles of the feet or palms of the hands. If you develop this type of rash all over your body along with symptoms such as fever, chills, and dehydration, get medical help immediately. You may have a form of pustular psoriasis called von Zumbusch psoriasis that can require hospitalization.

    In general, any rash, especially one that's persistent or seems to come out of nowhere, should be evaluated by a doctor. These are just some of the examples of how differently psoriasis can look depending on the type. 

    Physical Exam/Medical History

    In the majority of cases, your doctor can diagnosis psoriasis by examining the symptomatic skin. The most common presentation of a psoriasis rash is reddened areas of skin that are covered with silvery scales, or plaques, that are formed when the new skin cells accumulate more quickly than older ones are shed. Your doctor will also look for other presentations, such as teardrop-shaped lesions, and examine the scalp and nails. 

    In addition, your doctor will ask questions to help pin down the diagnosis further and also provide information that might be helpful to the development of a treatment plan. For example, he may ask if there are other symptoms besides the rash, such as itching, oozing, or pain. He may also ask if you have recently been ill. 

    Labs and Tests

    Occasionally a psoriasis rash will look enough like some other skin disease that a doctor can't be 100 percent certain of the diagnosis based on a visual exam and medical history. In order to rule out other conditions, it may be necessary to do a biopsy, according to New York University Langone Health. This means taking a small sample of affected skin and having a pathologist examine it under a microscope. Some skin conditions that might look enough like psoriasis to warrant a biopsy include eczema and cutaneous lupus. 

    Dermatologists usually perform what's called a punch biopsy in order to take a skin sample. If your doctor decides to do this procedure, the first step will be to apply a local anesthetic to the area of skin that's to be tested. Next, he'll use a device that's about the size and shape of a pencil eraser with a sharp rim to puncture the skin and remove a sample of tissue. Finally, he'll close up the wound. It usually takes just a stitch or two to do this.

    It usually takes about a week to get the results of skin biopsy. During that time the incision from the biopsy will heal and you'll be ready to undergo treatment for your psoriasis or whatever skin disease the test reveals. 

    Differential Diagnosis

    As distinctive as psoriasis typically is, it sometimes can resemble other conditions closely enough to make for a questionable diagnosis. Some of the skin diseases that may be mistaken for psoriasis include eczema (which does cause itchy, red areas of skin, but with no plaques); lichen simplex chronicus (thickening and darkening of the skin caused by excessive scratching—often due to eczema); and tinea corporis (a fungal infection of the skin). 

    When psoriasis affects only the scalp, it sometimes can be confused with other conditions, such as seborrheic dermatitis. Seborrhea on the scalp typically isn't as thick and crusty as scalp psoriasis. Flakes of seborrhea usually are yellow or white, whereas those of psoriasis are a silvery-gray. 

    Nail psoriasis easily can be mistaken for a nail fungus, though the two issues can co-occur. In fact, according to the National Psoriasis Foundation (NSF), about a third of people with nail psoriasis also have a fungal infection. Although people with skin psoriasis often develop nail psoriasis, it's possible to have psoriasis of the nails only.

    While most types of psoriasis are fairly easy to identify, because there are other skin conditions that it may resemble, it's possible for a doctor to make a mistake when diagnosing you. One way to avoid a misdiagnosis is to try to be as clear as you can when your doctor asks questions about your symptoms and health history.

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    Article Sources
    • Mayo Clinic. Psoriasis: Diagnosis. Mar 6, 2018. https://www.mayoclinic.org/diseases-conditions/psoriasis/diagnosis-treatment/drc-20355845 .
    • National Psoriasis Foundation. Hands, Feet, and Nails. www.psoriasis.org/about-psoriasis/specific-locations/hands-feet-nails.