What investigations are recommended for a patient presenting with a rash and systemic symptoms?

For a patient presenting with a rash and systemic symptoms, the recommended investigations depend on the clinical context but generally include:

• Clinical assessment for Lyme disease: If Lyme disease is suspected due to symptoms such as fever, swollen glands, migratory joint pain, neurological symptoms, or characteristic rashes (e.g., erythema migrans), explore history of tick exposure and consider serological testing if erythema migrans is absent. Diagnose and treat without laboratory testing if erythema migrans is present; otherwise, use a combination of clinical presentation and laboratory tests to guide diagnosis and treatment ¹.
• Blood tests to exclude haematological malignancies: In cases of unexplained fever with rash and systemic symptoms, consider urgent full blood count within 48 hours to exclude leukaemia or lymphoma, especially if accompanied by lymphadenopathy, splenomegaly, night sweats, pallor, bruising, or petechiae ³.
• Additional blood tests: Depending on clinical suspicion, tests such as erythrocyte sedimentation rate (ESR), plasma viscosity, calcium, serum protein electrophoresis, and serum free light chains may be indicated to investigate for myeloma or other systemic causes ³.
• Consider imported infections and Kawasaki disease in children: For children presenting with fever and rash, enquire about recent travel and consider imported infections. Look for features of Kawasaki disease (e.g., conjunctival injection, cracked lips, strawberry tongue, oedema of hands/feet, cervical lymphadenopathy) and manage accordingly ².