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.

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.

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.

A diagnosis of psoriasis is usually based on the appearance of the skin. Skin characteristics typical for psoriasis are scaly, erythematous plaques, papules, or patches of skin that may be painful and itch. No special blood tests or diagnostic procedures are usually required to make the diagnosis.

The differential diagnosis of psoriasis includes dermatological conditions similar in appearance such as discoid eczema, seborrhoeic eczema, pityriasis rosea (may be confused with guttate psoriasis), nail fungus (may be confused with nail psoriasis) or cutaneous T cell lymphoma (50% of individuals with this cancer are initially misdiagnosed with psoriasis). Dermatologic manifestations of systemic illnesses such as the rash of secondary syphilis may also be confused with psoriasis.

If the clinical diagnosis is uncertain, a skin biopsy or scraping may be performed to rule out other disorders and to confirm the diagnosis. Skin from a biopsy will show clubbed epidermal projections that interdigitate with dermis on microscopy. Epidermal thickening is another characteristic histologic finding of psoriasis lesions. The stratum granulosum layer of the epidermis is often missing or significantly decreased in psoriatic lesions; the skin cells from the most superficial layer of skin are also abnormal as they never fully mature. Unlike their mature counterparts, these superficial cells keep their nucleus. Inflammatory infiltrates can typically be visualized on microscopy when examining skin tissue or joint tissue affected by psoriasis. Epidermal skin tissue affected by psoriatic inflammation often has many CD8+ T cells while a predominance of CD4+ T cells makes up the inflammatory infiltrates of the dermal layer of skin and the joints.

A doctor will take a thorough medical history, and may take blood tests as well as examining liver and kidney function. Improvements have also been reported from treating malnutrition associated with zinc deficiency and other minerals. Intracellular (red blood cell) assays are more sensitive than tests for plasma levels.

In 1991, a case was reported of a man having plaque psoriasis and treating it with UV radiation at a tanning salon. After receiving a partial thickness burn from overexposure, he presented with annular pustular psoriasis, which cleared after 21 days, only to reoccur every 3 to 6 weeks for a year.

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.

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.

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.

Psoriatic erythroderma (also known as erythrodermic psoriasis) represents a generalized form of psoriasis that affects all body sites, including the face, hands, feet, nails, trunk, and extremities. First-line treatments for psoriatic erythroderma include immunosuppressive medications such as methotrexate, acitretin, or ciclosporin.

Longitudinal leukonychia is far less common and features smaller 1mm white longitudinal lines visible under the nail plate. It may be associated with Darier's disease.

Longitudinal erythronychia presents with longitudinal red bands in the nail plate that commence in the matrix and extend to the point of separation of the nail plate and nailbed, and may occur on multiple nails with inflammatory conditions such as lichen planus or Darier's disease.

The treatment of an ingrown toenail partly depends on its severity.

Mild to moderate cases are often treated conservatively with warm water and epsom salt soaks, antibacterial ointment and the use of dental floss. If conservative treatment of a minor ingrown toenail does not succeed, or if the ingrown toenail is severe, surgical treatment may be required. A "gutter splint" may be improvised by slicing a cotton-tipped wooden applicator diagonally to form a bevel and using this to insert a wisp of cotton from the applicator head under the nail to lift it from the underlying skin after a foot soak.

Onychauxis presents with thickened nails without deformity, and this simple thickening may be the result of trauma, acromegaly, Darier's disease, psoriasis, or pityriasis rubra pilaris, or, in some cases, hereditary.

Trachyonychia, sometimes called sandpapered nails, is a condition characterized by rough accentuated linear ridges (longitudinal striations) on the nails of the fingers and toes. When the condition occurs on all the twenty nails of the fingers and toes, it is known as twenty-nail dystrophy, most evident in childhood, favoring males.

Trachyonychia causes the nails to become opalescent, thin, dull, fragile, and finely longitudinally ridged, and, as a result, distally notched. It can be a manifestation of lichen planus, psoriasis, alopecia areata, immunoglobulin A deficiency, atopic dermatitis, and ichthyosis vulgaris.

"The longitudinal striations can occur as a normal part of the aging process", and not until the nails start to thin and get a sandpaper look is the condition called trachonychia. The nails are opalescent and frequently are brittle and split at the free margin. There has been evidence of the condition as a cutaneous manifestation of lichen planus. It has also been associated with other diseases such as eczema, psoriasis, alopecia areata, and atopic dermatitis. Trachonychia is often seen in vitiligo patients – suggesting that they are more susceptible to this condition.

Many conditions affect the human integumentary system—the organ system covering the entire surface of the body and composed of skin, hair, nails, and related muscle and glands. The major function of this system is as a barrier against the external environment. The skin weighs an average of four kilograms, covers an area of two square meters, and is made of three distinct layers: the epidermis, dermis, and subcutaneous tissue. The two main types of human skin are: glabrous skin, the hairless skin on the palms and soles (also referred to as the "palmoplantar" surfaces), and hair-bearing skin. Within the latter type, the hairs occur in structures called pilosebaceous units, each with hair follicle, sebaceous gland, and associated arrector pili muscle. In the embryo, the epidermis, hair, and glands form from the ectoderm, which is chemically influenced by the underlying mesoderm that forms the dermis and subcutaneous tissues.

The epidermis is the most superficial layer of skin, a squamous epithelium with several strata: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. Nourishment is provided to these layers by diffusion from the dermis, since the epidermis is without direct blood supply. The epidermis contains four cell types: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. Of these, keratinocytes are the major component, constituting roughly 95 percent of the epidermis. This stratified squamous epithelium is maintained by cell division within the stratum basale, in which differentiating cells slowly displace outwards through the stratum spinosum to the stratum corneum, where cells are continually shed from the surface. In normal skin, the rate of production equals the rate of loss; about two weeks are needed for a cell to migrate from the basal cell layer to the top of the granular cell layer, and an additional two weeks to cross the stratum corneum.

The dermis is the layer of skin between the epidermis and subcutaneous tissue, and comprises two sections, the papillary dermis and the reticular dermis. The superficial papillary dermis with the overlying rete ridges of the epidermis, between which the two layers interact through the basement membrane zone. Structural components of the dermis are collagen, elastic fibers, and ground substance. Within these components are the pilosebaceous units, arrector pili muscles, and the eccrine and apocrine glands. The dermis contains two vascular networks that run parallel to the skin surface—one superficial and one deep plexus—which are connected by vertical communicating vessels. The function of blood vessels within the dermis is fourfold: to supply nutrition, to regulate temperature, to modulate inflammation, and to participate in wound healing.

The subcutaneous tissue is a layer of fat between the dermis and underlying fascia. This tissue may be further divided into two components, the actual fatty layer, or panniculus adiposus, and a deeper vestigial layer of muscle, the panniculus carnosus. The main cellular component of this tissue is the adipocyte, or fat cell. The structure of this tissue is composed of septal (i.e. linear strands) and lobular compartments, which differ in microscopic appearance. Functionally, the subcutaneous fat insulates the body, absorbs trauma, and serves as a reserve energy source.

Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as many nonpathologic states (like, in certain circumstances, melanonychia and racquet nails). While only a small number of skin diseases account for most visits to the physician, thousands of skin conditions have been described. Classification of these conditions often presents many nosological challenges, since underlying etiologies and pathogenetics are often not known. Therefore, most current textbooks present a classification based on location (for example, conditions of the mucous membrane), morphology (chronic blistering conditions), etiology (skin conditions resulting from physical factors), and so on. Clinically, the diagnosis of any particular skin condition is made by gathering pertinent information regarding the presenting skin lesion(s), including the location (such as arms, head, legs), symptoms (pruritus, pain), duration (acute or chronic), arrangement (solitary, generalized, annular, linear), morphology (macules, papules, vesicles), and color (red, blue, brown, black, white, yellow). Diagnosis of many conditions often also requires a skin biopsy which yields histologic information that can be correlated with the clinical presentation and any laboratory data.

The diagnosis is based on the combination of the symptoms. Generally, people are diagnosed with yellow nail syndrome if they have two or three of the three classical symptoms (yellow nails, lymphedema and lung problems). The nail changes are considered essential for the diagnosis, but they can be subtle.

Pulmonary function testing can show obstruction of the airways. People with pleural effusions may show evidence of restriction in lung volumes due to the fluid. Analysis of the fluid in pleural effusions generally shows high levels of protein but low levels of cholesterol and lactate dehydrogenase, but about 30% of effusions are chylous (chylothorax) in that they have the characteristics of lymph.

A lymphogram may be performed in people with lymphedema. This can show both under developed (hypoplastic) lymphatic ducts and dilated ducts. Dye may be found in the skin months after the initial test. Scintigraphy of lymph flow (lymphoscintigraphy) shows delays in drainage of lymph (sometimes asymmetrically), although this test can also be normal.

This disease is caused by problems in the circulatory system, so when it is presented, in the beginning it is important to follow several recommendations. The person needs to keep the legs elevated as much as possible to help the return of the blood. Whenever sitting down, the person needs to keep the legs on a foot stool. At night it is advisable to sleep with a pillow under the lower legs. In the evening, t is not unusual for legs to be swollen. The volume of the lower leg can increase to up to 100ml after a long working day or up to 200ml after a long-haul flight without moving.

In the example of the 41-year-old Japanese man the lesions were much improved by washing and topical use of corticosteroids for two months, also oral antibiotics like cephalexin are used if cellulitis is present. Moist exudative inflammation and moist ulcers respond to tepid wet compresses of Burow’s solution or just saline or water for 30 to 60 minutes several times a day. But in worse cases, edema that does not disappear spontaneously within a few hours or after a walk, is described as pathological, so it needs to have a special treatment. It is very important to say that Papillamitosis, bilateral and marked edema with few symptoms is mostly caused by the systemic circulation (heart, kidneys, liver).

Papillamitosis is associated, as has been mentioned before, with symptoms and/or clinical signs such as dilated superficial veins, varicose veins and changes in the skin. Edema and its complication Papillamitosis are only partially reversible and soon becomes hard, which is mainly confirmed on palpation. All skin structures are affected and this is characterized by the term. Lymphoedema may develop in many cases accompanied by acral thickening of the skin folds, hyperkeratosis and papillomatosis.

Bisphosphonate therapy has been suggested as a first-line therapeutic option in many case reports and series.

Treatment with tumor necrosis factor alpha antagonists (TNF inhibitors) have been tried in few patients with limited success. Other drugs that are used in psoriatic arthritis, to which SAPHO syndrome is closely related, have also been used in this condition. They include NSAIDs, corticosteroids, sulfasalazine, methotrexate, ciclosporin and leflunomide.

Some patients have responded to antibiotics. The rationale for their use is that Propionibacterium acnes, a bacterium known for its role in acne, has been isolated from bone biopsies of SAPHO patients.

Treatment of manifestations: special hair care products to help manage dry and sparse hair; wigs; artificial nails; emollients to relieve palmoplantar hyperkeratosis.

Ultrasonography and magnetic resonance imaging of the hands and/or feet have been proposed as useful diagnostic investigations in RS3PE.

Some studies linked RS3PE to HLA-B27 whereas others have not.

Pachyonychia congenita may be divided into these types:

- Pachyonychia congenita type I (also known as "Jadassohn–Lewandowsky syndrome") is an autosomal dominant keratoderma that principally involves the plantar surfaces, but also with nails changes that may be evident at birth, but more commonly develop within the first few months of life.

- Pachyonychia congenita type II (also known as "Jackson–Lawler pachyonychia congenita" and "Jackson–Sertoli syndrome") is an autosomal dominant keratoderma presenting with a limited focal plantar keratoderma that may be very minor, with nails changes that may be evident at birth, but more commonly develop within the first few months of life.

Drug-induced nail changes are caused by drug administration which may result in a large number of nail abnormalities, usually with multiple nails being affected at the same time.

People with yellow nail syndrome have been found to have a moderately reduced lifespan compared to people without the condition.