What investigations are necessary for a patient presenting with a new rash and systemic symptoms suggestive of an autoimmune condition?

For a patient presenting with a new rash and systemic symptoms suggestive of an autoimmune condition, the necessary investigations include:

• Blood tests for autoimmune markers: These may include rheumatoid factor and anti-cyclic citrullinated peptide (anti-CCP) antibodies if rheumatoid arthritis is suspected, especially if synovitis is present on clinical examination ⁷.
• Full blood count (FBC): To assess for cytopenias or other hematological abnormalities that may indicate systemic autoimmune or hematological disease ².
• Inflammatory markers: Such as erythrocyte sedimentation rate (ESR) or plasma viscosity to evaluate systemic inflammation ².
• Serological testing for specific infections that can mimic or trigger autoimmune conditions: For example, Lyme disease serology if there is a history or suspicion of tick exposure and compatible symptoms ¹.
• Screening for associated autoimmune diseases: In cases like type 1 diabetes, screening for coeliac disease and thyroid disease may be appropriate ⁵,⁶.
• Skin biopsy: May be considered if the rash is atypical or diagnosis is unclear, to help differentiate autoimmune from infectious or other causes (implied from clinical practice).
• Additional viral serologies: In pregnant patients or where viral exanthems are possible, testing for rubella, parvovirus B19, and measles immunity may be indicated ³,⁴.

Referral to specialist care should not be delayed if autoimmune disease is strongly suspected, especially with persistent synovitis or systemic involvement ⁷.