Psoriasis is the most common autoimmune disease in the United States. According to the National Psoriasis Foundation, about 7.5 million Americans are affected. Yet despite its prevalence, many people still don’t know what it is — or what it looks like.

Psoriasis is a chronic autoimmune condition that speeds up the growth of skin cells and causes dry, itchy, and sometimes painful lesions or bumps on your body. It may sometimes be mistaken for eczema or dermatitis. To distinguish psoriasis from these and other skin conditions, it is helpful to know how psoriasis itself can appear. Each type of psoriasis has different features:

  • Psoriasis vulgaris (plaque psoriasis). This is the most common form of the disease, accounting for about 80 to 90 percent of cases. It causes symmetrical, well-defined, itchy, bright red, raised patches, or plaques, which are covered by silvery scales. The plaques can appear on most any part of the body other than mucous membranes, but typically will show up on elbows, knees, shins, the lower back, the belly button, and the buttocks’ crease.
  • Guttate psoriasis. This form of the psoriasis appears suddenly as small red spots that look like drops. It is often linked to a streptococcal or other bacterial infection.
  • Inverse psoriasis. This is a type of psoriasis that typically appears in the armpits, groin, under the breasts, and in skin folds around the genitals and buttocks. Inverse psoriasis is so named because it's most common in areas usually spared by the more common plaque-like psoriasis. However, it is possible to have both forms at the same time. The affected patches are usually very red but lack scales.
  • Pustular psoriasis. This form of psoriasis is distinguished by pus-filled bumps on the skin. It can be triggered by certain medications, topical agents, UV light, infections, pregnancy, and stress.
  • Erythrodermic psoriasis. This variation of psoriasis causes a bright red sheen to form on large sections of the body that looks similar to the shell of a cooked lobster. The affected areas are usually very itchy and painful.

Skin Conditions That Look Like Psoriasis

Other skin conditions might seem like psoriasis, but there are differences, from the shape of the borders of the affected areas to the color and thickness of the scales. Here are some similar-looking skin conditions that may even occur simultaneously with psoriasis:

  • Seborrheic dermatitis (seborrhea). While this can be confused with plaque-type psoriasis, the scales of psoriasis tend to be thicker and the lesions have much more clearly defined borders. Seborrhea involves only the oil-producing areas of the skin around the scalp, face, chest, and, less frequently, groin and upper back. Seborrhea lesions are poorly defined and pink with yellow-brown scales. A rash on the face could be either psoriasis or seborrheic dermatitis, and the two conditions can appear at the same time.
  • Dandruff. Seborrhea on the scalp, known as dandruff, produces fine, greasy scales and usually is distributed generally over the head.
  • Eczema. Atopic dermatitis, or eczema, is more common than psoriasis and more likely to be diagnosed by primary care physicians. It appears frequently on the back of the knee or in front of the elbow — a much more limited area than the common forms of psoriasis. What triggers the rash also helps differentiate atopic dermatitis from psoriasis. Atopic dermatitis can be brought on by outside irritants such as dust, foods, or pollen. In addition, the skin lesions of eczema can get infected with bacteria. By comparison, such irritants generally do not trigger psoriasis, and psoriasis lesions are not usually susceptible to secondary infections.

Other more serious conditions that mimic the appearance of psoriasis include mycosis fungoides, a rare form of lymphoma, and pityriasis rubra pilaris, a rare skin disorder.

Getting the Right Diagnosis

Your doctor might do a biopsy in the case of an unclear diagnosis or when the psoriasis is particularly severe. A biopsy allows the dermatologist to look at a tissue sample under a microscope. Examination of the cell architecture, blood vessels, and other structures can help distinguish psoriasis from lookalike conditions. “Several diseases have overlapping characteristics — often it’s not black and white,” says Stephen Templeton, MD, a dermapathologist and dermatologist with Finan-Templeton Dermapathology Associates in Sandy Springs, Ga.

When a person has more than one skin condition is present, a complete and precise diagnosis will allow a physician to avoid treatments that interfere with each other or interact negatively. Because some psoriasis treatments can have significant consequences, such as possible liver damage associated with the drug methotrexate, an accurate diagnosis is especially crucial, says Dr. Templeton.