There is no consensus about how to classify the severity of psoriasis. Mild psoriasis has been defined as a percentage of body surface area (BSA)≤10, a Psoriasis Area Severity Index (PASI) score ≤10, and a dermatology life quality index (DLQI) score ≤10. Moderate to severe psoriasis was defined by the same group as BSA >10 or PASI score >10 and a DLQI score >10. The DLQI is a 10 question tool used to measure the impact of several dermatologic diseases on daily functioning. The DLQI score ranges from 0 (minimal impairment) to 30 (maximal impairment) and is calculated with each answer being assigned 0–3 points with higher scores indicating greater social or occupational impairment.

The psoriasis area severity index (PASI) is the most widely used measurement tool for psoriasis. PASI assesses the severity of lesions and the area affected and combines these two factors into a single score from 0 (no disease) to 72 (maximal disease). Nevertheless, the PASI can be too unwieldy to use outside of research settings, which has led to attempts to simplify the index for clinical use.

Guttate psoriasis can typically be diagnosed by clinical examination alone. If necessary, a skin biopsy can be used to support the diagnosis.

Another classification scheme considers genetic and demographic factors. Type 1 has a positive family history, starts before the age of 40, and is associated with the human leukocyte antigen, "HLA-Cw6". Conversely, type 2 does not show a family history, presents after age 40, and is not associated with "HLA-Cw6". Type 1 accounts for about 75% of persons with psoriasis.

The classification of psoriasis as an autoimmune disease has sparked considerable debate. Researchers have proposed differing descriptions of psoriasis and psoriatic arthritis; some authors have classified them as autoimmune diseases while others have classified them as distinct from autoimmune diseases and referred to them as immune-mediated inflammatory diseases.

The classification of exfoliative dermatitis into Wilson-Brocq (chronic relapsing), Hebra or pityriasis rubra (progressive), and Savill (self-limited) types may have had historical value, but it currently lacks pathophysiologic or clinical utility.

Guttate psoriasis accounts for approximately 2% of psoriasis cases.

Treatments vary widely, and many different drugs have been documented as being successful. Some medications are successful in some patients, while unsuccessful in others. Below is a list of some medications used to treat GPP:

- Enbrel (Etanercept)

- Methotrexate

- PUVA

- Hydroxyurea

- Dapsone

- Systemic corticosteroids

- Cyclosporin A

- Adalimumab

- Etretinate

- Isotretinoin (Accutane)

- Acitretin (Neotigason)

Von Zumbusch observed a male patient, who had had classic psoriasis for several years, and who then went through recurrent episodes of bright [erythema] and [edema], which became studded with multiple pustules. Von Zumbusch observed this patient through nine hospital admissions over 10 years.

You have to treat the primary cause or the exacerbation may persisist and reincide.

Topical steroids are the primary category of medications used to treat exfoliative dermatitis (ED). A sedative antihistamine may be a useful adjunct for pruritic patients, since it helps patients to sleep at night, thus limiting nocturnal scratching and excoriations. Antimicrobial agents often are used if an infection is suspected to be precipitating or complicating exfoliative dermatitis. Other drugs specifically indicated for management of underlying cause of exfoliative dermatitis may be necessary.

Dyshidrosis is diagnosed clinically, by gathering a patient's history and making careful observations (see signs and symptoms section). Severity of symptoms can also be assessed using the dyshidrotic eczema area and severity index (DASI). The DASI has been designed for clinical trials and is not typically used in practice.

Eosinophilic folliculitis may be suspected clinically when an individual with HIV exhibits the classic symptoms. The diagnosis can be supported by the finding of eosinophilia but a skin biopsy is necessary to establish it. Skin biopsies reveal lymphocytic and eosinophilic inflammation around the hair follicles.

The causes of a rash are numerous, which may make the evaluation of a rash extremely difficult. An accurate evaluation by a provider may only be made in the context of a thorough history (What medication is the patient taking? What is the patient's occupation? Where has the patient been?) and complete physical examination.

Points to note in the examination include:

- The appearance: "e.g.", purpuric (typical of vasculitis and meningococcal disease), fine and like sandpaper (typical of scarlet fever); circular lesions with a central depression are typical of molluscum contagiosum (and in the past, small pox); plaques with silver scales are typical of psoriasis.

- The distribution: "e.g.", the rash of scarlet fever becomes confluent and forms bright red lines in the skin creases of the neck, armpits and groins (Pastia's lines); the vesicles of chicken pox seem to follow the hollows of the body (they are more prominent along the depression of the spine on the back and in the hollows of both shoulder blades); very few rashes affect the palms of the hands and soles of the feet (secondary syphilis, rickettsia or spotted fevers, guttate psoriasis, hand, foot and mouth disease, keratoderma blennorrhagicum);

- Symmetry: "e.g.", herpes zoster usually only affects one side of the body and does not cross the midline.

A patch test may be ordered, for diagnostic purposes.

The Nail Psoriasis Severity Index (NAPSI) is a numeric, reproducible, objective, simple tool for evaluation of nail psoriasis. It evaluates several signs separately, each on a 1–3 scale: pitting, Beau's lines, subungual hyperkeratosis and onycholysis. A 2005 study proposed a modified NAPSI scale for persons with psoriasis and named the title of their publication "Modification of the Nail Psoriasis Severity Index". Then, in 2007, a study found that there was a high level of inter-rater variability of the 2003 NAPSI scale and proposed another index which was, like the 2005 article, a modification of the 2003 article, and was named modified NAPSI.

A 2008 study found that Cannavo's qualitative system correlated with NAPSI (P<0.001) and is less time-consuming.

There is a risk of misdiagnosis with onychomycosis.

Eosinophilic folliculitis associated with HIV infection typically affects individuals with advanced HIV and low T helper cell counts. It affects both men and women as well as children with HIV and is found throughout the world.

EF may also affect individuals with hematologic disease such as leukemia and lymphoma. It may also affect otherwise normal infants in a self-limited form. HIV-negative individuals can also develop EF — this is more common in Japan.

Pustular psoriasis is classified into two major forms: localized and generalized pustular psoriasis. Within these two categories there are several variants:

The term pustular psoriasis is used for a heterogeneous group of diseases that share pustular skin characteristics.

A skin biopsy can be performed to test for EAC; tests should be performed to rule out other possible diseases such as: pityriasis rosea, tinea corporis, psoriasis, nummular eczema, atopic dermatitis, drug reaction, erythema migrans and other rashes.

There are many treatments available for dyshidrosis. However, few of them have been developed or tested specifically on the condition.

- Barriers to moisture and irritants, including barrier creams and gloves.

- Topical steroids - while useful, can be dangerous long-term due to the skin-thinning side-effects, which are particularly troublesome in the context of hand dyshidrosis, due to the amount of toxins and bacteria the hands typically come in contact with.

- Potassium permanganate dilute solution soaks - also popular, and used to 'dry out' the vesicles, and kill off superficial "Staphylococcus aureus", but it can also be very painful. Undiluted it may cause significant burning.

- Dapsone (diamino-diphenyl sulfone), an antibacterial, has been recommended for the treatment of dyshidrosis in some chronic cases.

- Antihistamines: Fexofenadine up to 180 mg per day.

- Alitretinoin (9-cis-retinoic acid) has been approved for prescription in the UK. It is specifically used for chronic hand and foot eczema. It is made by Basilea of Switzerland (BAL 4079).

- Systemic steroids can be taken orally to treat especially acute and severe cases of dyshidrosis.

Available studies lack sufficient power to extrapolate a standardized therapeutic regimen.

As of April 2009, an assessment of the evidence for the efficacy and safety of the treatments for nail psoriasis is in progress.

- Infliximab appears to be the most effective treatment for nail psoriasis to date.

- Results from low-dose acitretin therapy show NAPSI score reductions comparable with those studies evaluating biologic drugs for nail psoriasis and suggest that low-dose systemic acitretin should be considered in the treatment of nail psoriasis.

The diagnosis of SSSS is made clinically. This is sometimes confirmed by isolation of "S. aureus" from blood, mucous membranes, or skin biopsy; however, these are often negative. Skin biopsy may show separation of the superficial layer of the epidermis (intraepidermal separation), differentiating SSSS from TEN, wherein the separation occurs at the dermo-epidermal junction (subepidermal separation). SSSS may be difficult to distinguish from toxic epidermal necrolysis and pustular psoriasis.

Treatment differs according to what rash a patient has been diagnosed with. Common rashes can be easily remedied using steroid topical creams (such as hydrocortisone) or non-steroidal treatments. Many of the medications are available over the counter in the United States.

The problem with steroid topical creams i.e. hydrocortisone; is their inability to penetrate the skin through absorption and therefore not be effective in clearing up the affected area, thus rendering the hydrocortisone almost completely ineffective in all except the most mild of cases.

Subacute cutaneous lupus erythematosus (SCLE) is a clinically distinct subset of cases of lupus erythematosus that is most often present in white women aged 15 to 40, consisting of skin lesions that are scaly and evolve as polycyclic annular lesions or plaques similar to those of plaque psoriasis.

Characteristically the lesions appear in sun-exposed areas such as the vee of the neckline or the forearms, but not the face. It may be brought on by sun-sensitizing medications, but is usually associated with autoimmune disorders such as rheumatoid arthritis and Sjögren's syndrome.

Treatment generally involves sun avoidance and protection and topical corticosteroids. Sometimes systemic drug treatment is necessary. Besides corticosteroids other immunosuppressants such as methotrexate are also used.

Lesions of SCLE may have an annular configuration, with raised red borders and central clearing.

Subcutaneous sarcoidosis (also known as "Darier–Roussy disease" and "Darier-Roussy sarcoid") is a cutaneous condition characterized by numerous 0.5- to 0.3-cm deep-seated nodules on the trunk and extremities.

Scar sarcoid (also known as "Sarcoidosis in scars") is a cutaneous condition characterized by infiltration and elevation of tattoos and old flat scars due to sarcoidosis.

Mucosal sarcoidosis is a cutaneous condition characterized by pinhead-sized papules that may be grouped and fused together to form a flat plaque.

Erythrodermic sarcoidosis is a cutaneous condition and very rare form of sarcoidosis.

Sarcoidosis involves the skin in about 25% of patients. The most common lesions are erythema nodosum, plaques, maculopapular eruptions, subcutaneous nodules, and lupus pernio. Treatment is not required, since the lesions usually resolve spontaneously in two to four weeks. Although it may be disfiguring, cutaneous sarcoidosis rarely causes major problems.

No treatment is usually needed as they usually go away anywhere from months to years. The lesions may last from anywhere between 4 weeks to 34 years with an average duration of 11 months. If caused by an underlying disease or malignancy, then treating and removing the disease or malignancy will stop the lesions. It usually doesn't require treatment, but topical corticosteroids may be helpful in reducing redness, swelling and itchiness.

Some supported and not supported methods of having an effect on EAC include:

- Photosensitive so it can be moved/reduced with appropriate sunlight.

- Vitamin D

- Immune system - hence it will increase in size/number when the immune system is low or overloaded.

- Hormone Drugs

- Disulone

- Stress reduction

- Topical calcipotriol - a topical vitamin D derivative has been known to be beneficial

Urticarial erythema multiforme is an unusual reaction virtually always associated with antibiotic ingestions, characterized by skin lesions that consist of urticarial papules and plaques, some of which clear centrally forming annular lesions, but with no true urticarial lesions.