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

Guttate psoriasis accounts for approximately 2% of psoriasis cases.

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.

Experienced practitioners may make the diagnosis clinically. If the diagnosis is in doubt, tests may be performed to rule out similar conditions such as Lyme disease, ringworm, guttate psoriasis, nummular or discoid eczema, drug eruptions, other viral exanthems.The clinical appearance of pityriasis rosea is similar to that of secondary syphilis, and rapid plasma reagin testing should be performed if there is any clinical concern for syphilis. A biopsy of the lesions will show extravasated erythrocytes within dermal papillae and dyskeratotic cells within the dermis.

A set of validated diagnostic criteria for pityriasis rosea is as follows:

A patient is diagnosed as having pityriasis rosea if:

1. On at least one occasion or clinical encounter, he / she has all the essential clinical features and at least one of the optional clinical features, and

2. On all occasions or clinical encounters related to the rash, he / she does not have any of the exclusional clinical features.

The essential clinical features are the following:

1. Discrete circular or oval lesions,

2. Scaling on most lesions, and

3. Peripheral collarette scaling with central clearance on at least two lesions.

The optional clinical features are the following:

1. Truncal and proximal limb distribution, with less than 10% of lesions distal to mid-upper-arm and mid-thigh,

2. Orientation of most lesions along skin cleavage lines, and

3. A herald patch (not necessarily the largest) appearing at least two days before eruption of other lesions, from history of the patient or from clinical observation.

The exclusional clinical features are the following:

1. Multiple small vesicles at the centre of two or more lesions,

2. Two or more lesions on palmar or plantar skin surfaces, and

3. Clinical or serological evidence of secondary syphilis.

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.

In most patients, the condition lasts only a matter of weeks; in some cases it can last longer (up to six months). The disease resolves completely without long-term effects. Two percent of patients have recurrence.

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.

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".

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.

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.

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 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.

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.

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.

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.

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.

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.

An ultraviolet light can be used in the early phase of this disease for identification and to determine the effectiveness of treatment. Skin with vitiligo, when exposed to a blacklight, will glow blue. In contrast, healthy skin will have no reaction.

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.

In cases of extensive vitiligo the option to de-pigment the unaffected skin with topical drugs like monobenzone, mequinol, or hydroquinone may be considered to render the skin an even colour. The removal of all the skin pigment with monobenzone is permanent and vigorous. Sun-safety must be adhered to for life to avoid severe sunburn and melanomas. Depigmentation takes about a year to complete.

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.

Shampoos use a combination of special ingredients to control dandruff.