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Author: Hon A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand, December 2016.
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.
Determination of allergy requires a careful history and examination, and testing should have a specific question in mind.
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 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.
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.
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 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.
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