Introduction

An experienced clinician can often diagnose a skin condition without the need for investigations. However, at times, skin swabs, scrapings and nail clippings, biopsies, and blood samples are sent to the laboratory, and allergy testing or imaging is arranged. Investigations may also be used to monitor the effects of systemic treatments.

Microbiology

• Standard skin or wound swabs have a cotton-tip on a plastic shaft and are typically placed in Amies gel transport medium for routine bacterial culture, and if appropriate, antibiotic sensitivities.
• Vaginal or male urethral swabs may be collected in a buffer for polymerase chain reaction (PCR), a molecular biology technique used to detect chlamydia, gonorrhoea, trichomoniasis and other organisms.
• Skin scrapings, nail clippings and extracted hair are treated with potassium hydroxide (KOH) and examined by direct microscopy before being plated out for fungal culture. An interim report may describe hyphae, arthrospores and mycelia typical of dermatophyte or yeast. Culture results and the specific organisms identified are reported about 4 weeks later.
• Viral transport medium is used to detect herpes simplex and herpes zoster by culture, immunofluorescence or PCR
• Skin specimens in a small amount of sterile saline may also be processed for gram stain, microscopy and culture, including low-temperature culture for atypical mycobacteria and identification of organisms such as leishmaniasis and deep fungi.

Histopathology

• Most skin biopsy samples are sent to the laboratory in 10% buffered formalin, with 24-hour fixation and processing time prior to histological examination by a histopathologist. Reports are issued in about a week. Interpretation of skin biopsies requires considerable training and experience.
• Samples for immunofluorescence should be placed in a culture medium, saline or liquid nitrogen and should be sent to the laboratory urgently.
• Frozen sections do not have fixative and can be immediately viewed by a pathologist. In dermatology, they are mainly used to determine if a basal cell carcinoma has been fully surgically removed by Mohs surgery (microscopically controlled excision). Considerable skill and training are required to interpret frozen sections.
• Cell cytology of the base of a blister can be assessed on a smear of tissue applied to a glass slide and fixed by air-drying, ie, Tzanck smear. It is quick but is not very reliable for diagnosis.

Haematology

• Neutrophilia is typical of infection but may also accompany severe inflammatory disorders including generalised pustular psoriasis, and neutrophilic dermatoses such as Sweet syndrome or pyoderma gangrenosum.
• Eosinophilia is associated with atopic eczema, scabies, bullous disease and lymphoma. It can be quite nonspecific in patients with erythroderma.
• Lymphocytosis is associated with viral infections and certain bacterial infections such as tuberculosis and syphilis.

Other blood tests

• Renal, hepatic, thyroid and iron may be evaluated in patients with generalised pruritus, vasculitis or systemic symptoms.
• Blood glucose and glycosylated haemoglobin detect and monitor diabetes, which may be of relevance in infection or skin diseases associated with metabolic syndrome.
• Antinuclear and extractable nuclear antigen antibodies and tissue autoantibodies are assessed in patients with suspected connective tissue or autoimmune disease.
• Proteins, including immunoglobulins, cryoproteins and complement, are assessed in patients with vasculitis or connective tissue disease.
• Specific serology is requested for infections, eg, hepatitis B, hepatitis C, human immunodeficiency virus (HIV), syphilis.
• Additional tests in vasculitis include anti-neutrophil cytoplasmic antibodies (ANCA), antiphospholipid and thrombophilia screen.
• Hormonal tests are occasionally arranged in females with acne, hirsutism and androgenetic alopecia if symptoms indicate these tests are warranted, or if virilism is present.
• Urinary and faecal or serum porphyrins are requested in patients that may have a cutaneous porphyria.
• Cytogenetic tests including karyotyping, fluorescent in situ hybridisation and comparative genomic hybridisation.
• Blood-based melanoma detection tests are under investigation (liquid biopsy).

Allergy tests

Determination of allergy requires a careful history and examination, and testing should have a specific question in mind.

Prick tests

Prick tests for immediate hypersensitivities are not very useful in the investigation of skin disease. These tests are sometimes undertaken in patients with suspected contact urticaria or latex allergy. They are not useful in atopic dermatitis or acute/chronic spontaneous urticaria. IgE tests have limited use in patients with type 1 hypersensitivity reactions.

Patch tests

Patch tests are undertaken by specialist clinics for patients with suspected contact allergic dermatitis; they detect delayed hypersensitivity reactions. Patients should have a chronic or relapsing dermatitis.

• A baseline series of common allergens and additional batteries of test substances are selected, depending on the site of rash or occupational exposures.
• Standardised concentrations of allergens are sourced from specialised companies.
• Test patches are applied to the upper back on Day 0, removed on Day 2, and reviewed on Day 4, and sometimes, day 7.
• Photopatch tests require a duplicate set of photoallergens to be applied, with exposure to ultraviolet radiation to one of the sets of allergens on the day of removal.
• Interpretation of equivocal results can be challenging.
• Atopy patch tests are sometimes used in patients with atopic dermatitis with possible exacerbation by type 1 allergens such as food or house dust mite.

Imaging

Radiological examinations

X-rays, CT scans, MRI scans and ultrasound tests are not routinely requested in dermatology but may be arranged for systemic symptoms or monitoring treatment. They are essential for investigating advanced melanoma. See Radiographic investigations in melanoma.

Wood light examination

Exposure to long-wavelength UVA emitted by a Wood lamp is mainly used to investigate pigmentary disorders and chronic superficial skin infections such as pityriasis versicolor, where fluorescence supports a specific cause.

Digital dermatoscopy

Digital dermatoscopy is digital imaging of skin lesions under dermatoscopy. Ideally, these are accompanied by macroscopic and location images. Dermatoscopic images can be taken using a variety of dermatoscopes, adapters and video and still cameras using the contact and non-contact, polarised and unpolarised systems.

Mole mapping is a system in which the location of melanocytic naevi is mapped to a mannequin, total body photographs are taken, and macroscopic and digital images are recorded for lesions of clinical concern. The procedure is repeated at intervals to monitor patients with a high risk of melanoma, particular if they have many naevi or naevi with an unusual appearance.