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.

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.

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

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.

Atopic dermatitis is typically diagnosed clinically, meaning it is diagnosed based on signs and symptoms alone, without special testing. Several different forms of criteria developed for research have also been validated to aid in diagnosis. Of these, the UK Diagnostic Criteria, based on the work of Hanifin and Rajka, has been the most widely validated.

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.

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.

The pathophysiology may involve a mixture of type I and type IV-like hypersensitivity reactions.

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.

Intertrigo can be diagnosed clinically by a medical professional after taking a thorough history and performing a detailed physical examination. Many other skin conditions can mimic intertrigo's appearance including erythrasmascabies, pyoderma, atopic dermatitis, candidiasis, and seborrheic dermatitis, and fungal infections of the superficial skin caused by "Tinea versicolor" or "Tinea corporis".

LSC is typically diagnosed by careful observation and history taking. It is easily recognized (see "signs and symptoms", and "gallery"). Biopsies are not necessary, and cannot be used to distinguish LSC from psoriasis.

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.

To avoid misdiagnosis as nail psoriasis, lichen planus, contact dermatitis, nail bed tumors such as melanoma, trauma, or yellow nail syndrome, laboratory confirmation may be necessary. The three main approaches are potassium hydroxide smear, culture and histology. This involves microscopic examination and culture of nail scrapings or clippings. Recent results indicate the most sensitive diagnostic approaches are direct smear combined with histological examination, and nail plate biopsy using periodic acid-Schiff stain. To reliably identify nondermatophyte molds, several samples may be necessary.

There are four classic types of onychomycosis:

- Distal subungual onychomycosis is the most common form of "tinea unguium" and is usually caused by "Trichophyton rubrum", which invades the nail bed and the underside of the nail plate.

- White superficial onychomycosis (WSO) is caused by fungal invasion of the superficial layers of the nail plate to form "white islands" on the plate. It accounts for around 10 percent of onychomycosis cases. In some cases, WSO is a misdiagnosis of "keratin granulations" which are not a fungus, but a reaction to nail polish that can cause the nails to have a chalky white appearance. A laboratory test should be performed to confirm.

- Proximal subungual onychomycosis is fungal penetration of the newly formed nail plate through the proximal nail fold. It is the least common form of "tinea unguium" in healthy people, but is found more commonly when the patient is immunocompromised.

- Candidal onychomycosis is "Candida" species invasion of the fingernails, usually occurring in persons who frequently immerse their hands in water. This normally requires the prior damage of the nail by infection or trauma.

Characteristics may vary according to the subtype of pustular psoriasis. For example, it can be localized, commonly to the hands and feet (localized pustular psoriasis), or generalized with widespread patches occurring randomly on any part of the body (generalized pustular psoriasis). However, all forms of pustular psoriasis share in common the presence of red and tender blotchy skin covered with pustules.

Pustular psoriasis can be localized, commonly to the hands and feet (palmoplantar pustulosis), or generalized with widespread patches occurring randomly on any part of the body. Acrodermatitis continua is a form of localized psoriasis limited to the fingers and toes that may spread to the hands and feet. Pustulosis palmaris et plantaris is another form of localized pustular psoriasis similar to acrodermatitis continua with pustules erupting from red, tender, scaly skin found on the palms of the hands and the soles of the feet.

Generalized pustular psoriasis (GPP) is also known as (von Zumbusch) acute generalized pustular psoriasis in acute cases, and as impetigo herpetiformis during pregnancy. GPP is a rare and severe form of psoriasis that may require hospitalization. This form of psoriasis is characterized by an acute onset of numerous pustules on top of tender red skin. This skin eruption is often accompanied by a fever, muscle aches, nausea, and an elevated white blood cell count. Annular pustular psoriasis (APP), a rare form of GPP, is the most common type seen during childhood. APP tends to occur in women more frequently than in men, and is usually less severe than other forms of generalized pustular psoriasis such as impetigo herpetiformis. This form of psoriasis is characterized by ring-shaped plaques with pustules around the edges and yellow crusting. APP most often affects the torso, neck, arms, and legs.

Dandruff scale is a cluster of corneocytes, which have retained a large degree of cohesion with one another and detach as such from the surface of the stratum corneum. A corneocyte is a protein complex that is made of tiny threads of keratin in an organised matrix. The size and abundance of scales are heterogeneous from one site to another and over time. Parakeratotic cells often make up part of dandruff. Their numbers are related to the severity of the clinical manifestations, which may also be influenced by seborrhea.

Intertrigo is treated by addressing associated infections, by removing moisture from the site, and by using substances at the site to help maintain skin integrity. If the individual is overweight, losing weight may also help. Relapses of intertrigo are common.

Keeping the area of the intertrigo dry and exposed to the air can help prevent recurrences, as can removing moisture from the area using absorbent fabrics or body powders, including plain cornstarch and judiciously used antiperspirants.

Greases, oils, and barrier ointments, may help by protecting skin from moisture and from friction. Antifungal powders, most commonly clotrimazole 1%, may also be used in conjunction with a barrier ointment. Diaper rash ointment can also help.

Fungal infections associated with intertrigo may be treated with prescription antifungals applied directly to the skin (in most cases) or systemic antifungals, including fluconazole, nystatin, and griseofulvin.

Intertrigo is also a known symptom of vitamin B6 deficiency.

Dermographism can be treated by substances (i.e. an antihistamine) which prevent histamine from causing the reaction. These may need to be given as a combination of H antagonists, or possibly with an H-receptor antagonist such as cimetidine.

OTC Vitamin C, 1000 mg daily, increases histamine degradation and removal.

Not taking hot baths or showers may help if it is generalized (all over) and possibly for localized cases (in a specific area). If taking hot showers helps, it may be a condition called shower eczema. If it affects mainly the head, it may be psoriasis. In rare cases, allergy tests may uncover substances the patient is allergic to.

While cromoglycate, which prevents histamine from being released from mast cells, is used topically in rhinitis and asthma, it is not effective orally for treating chronic urticaria.

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.

Mild forms of IBS should be diagnosable from appearance and patient history alone. Severe cases of IBS are hard to distinguish from mild EHK.

A skin biopsy shows a characteristic damaged layer in the upper spinous level of the skin. Again it may be difficult to distinguish from EHK.

The gene causing IBS is known and so a definite diagnosis can be given by genetic testing.

Generally, lichen nitidus is asymptomatic and self-limited; therefore, no treatment is required. However, if persistent pruritus is present, or the appearance “...interferes with daily activities or outlook...” topical glucocorticoids may be tried. If the disease process is symptomatic, generalized and extensive, oral glucocorticoids may be indicated. Other reported treatments include PUVA, UVA/UVB phototherapy, astemizole, acitretin, and etretinate.

When appears with sun/humidity; air conditioning (cool dry air) reduces swelling and discomfort.

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