Renal, hepatic, thyroid and iron may be evaluated in patients with generalised body itching or 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, HIV and syphilis.

Additional tests in vasculitis include anti-neutrophil cytoplasmic antibodies (ANCA), antiphospholipids, thrombophilia screen.

Hormonal tests are occasionally arranged in females with acne, hirsutism and androgenic 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.

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

Skin 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.

Specialists undertake skin patch tests for patients with suspected contact allergic dermatitis they detect delayed hypersensitivity reactions. Patients should have chronic or relapsing dermatitis.