The doctor may take a biopsy (small sample of skin) or scraping and have it examined under a microscope to either rule out other disorders and/or determine what type of psoriasis it is. For the biopsy, the patient will receive a local anesthetic. This type of biopsy is usually carried out at the doctor’s office.
The following disorders that look like psoriasis may need to be ruled out:
Seborrheic dermatitis - a medical name for what is often called seborrhea. Seborrhea is a chronic (long-term) inflammatory skin disease characterized by the build-up of scales of greasy skin. There may also be yellow crusted patches, which are usually itchy. Seborrhea typically affects the scalp, but it may also affect the face, ears, eyelids, eyebrows, umbilicus, upper chest, back, as well as genitalia. Seborrhea of the scalp is often referred to as dandruff. Although not proven, many say it is caused by an allergy to wheat.
Lichen planus - a skin disease with small pink or purple spots on the arms, legs, lower back and genitalia; they are typically itchy. There may be hair loss if the scalp is affected.
Body ringworm (tinea corporis) - a fungal infection that affects the top layer of the skin Symptoms include:
Rash - with a ring-like appearance. The skin may be red and inflamed around the outside of the ring, but look fine in the middle of it.
More rings - they may multiply and grow.
Merging rings - when there are enough of them they will merge together.
Rings are raised - when you touch them they may feel slightly raised.
Itchiness - especially under the rash.
Pityriasis rosea - a common mild rash that may affect people of any age, but mainly individuals between 10 and 35 years. It may last from a few weeks to several months. The condition usually begins with a herald patch - a large spot (patch) on the chest, abdomen or back. It then spreads. It may itch, especially when it is hot. The rash usually extends from the middle of the body. The rash looks like dropping pine-tree branches.
If the doctor diagnoses psoriasis the patient will probably be referred to a dermatologist (a skin specialist doctor). If psoriatic arthritis is suspected, the patient will probably be referred to a rheumatologist (arthritis specialist doctor). Sometimes blood tests may also be ordered to rule out other forms of arthritis. X-rays of affected joints may also be taken.
