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Tests and Diagnosis for Psoriasis

By Cynthia Price, M.D. ; Updated August 14, 2017

Since other inflammatory diseases of the skin (like eczema) can be confused with psoriasis, it is important that a board-certified dermatologist makes the diagnosis.

Physical Examination

The dermatologist will examine the outer arms, legs and scalp. The nails need to be examined because there may be visible pits in the nails that appear much like hammered brass when the disease is flared or active. In addition, the tongue may manifest as a geographic tongue, which has white scale in a ring-like pattern.

Typically, a thorough physical examination of the scalp, skin and nails is enough to make an accurate diagnosis. Although the size of an individual lesion may vary from pinpoint to over 20 centimeters in diameter, the outline of the lesion is usually circular, oval or polycyclic (derived from several smaller units or with many sides).

Psoriasis lesions characteristically have a very sharp border and do not fade into normal skin like other inflammatory skin rashes. In addition, psoriatic lesions are sometimes surrounded by a pale blanching ring, which is commonly referred to as a Woronoff ring. The surface of psoriasis plaques at times can be removed. When this happens, a characteristic Auspitz sign is observed, which refers to a collection of pinpoint bleeding.

Skin Biopsy

A skin biopsy may be necessary if the dermatologist is considering other similar inflammatory skin rashes, such as eczema, seborrheic dermatitis, dermatomyositis, lichen planus, pityriasis rosea or tinea corporis (ringworm).

After a local anesthesia injection with lidocaine (to numb the skin) and epinephrine (to control bleeding), a plastic device is used to remove three to four millimeters of skin. Many times a simple stitch or two is necessary, which will need to be removed in two weeks. The tissue is then examined under a microscope by a dermatopathologist to confirm the diagnosis.

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The Psoriasis Area and Severity Index

Because the percentage of body surface area is important in regards to deciding which treatment would be appropriate for each individual patient, a scale for measuring the number and thickness of psoriasis plaques was developed. The most widely used measuring scale is called the PASI score. This stands for Psoriasis Area and Severity Index. These scores can be used in both clinical and research settings.

Other Tests

Radiographs assessing for joint disease may be useful in patients also suffering from psoriatic arthritis. Blood testing for inflammatory markers, such as a CRP or ESR level, may also be helpful. A rheumatologist typically makes this diagnosis because he or she studies autoimmune diseases that affect the joints.

It will then be necessary to determine a treatment regimen, which many times involves both topical creams, lifestyle and diet changes and possibly systemic oral or injectable medication, depending upon the severity of the condition. Many times collaboration with both the dermatologist and rheumatologist is necessary for optimal patient outcome.

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