Blood tests ordered for skin rash investigation
| Test | What it checks for |
|---|---|
| ANA (antinuclear antibodies) | Lupus (SLE) — butterfly facial rash with systemic features |
| Anti-dsDNA antibodies | More specific for lupus than ANA |
| ANCA | Vasculitis (granulomatosis with polyangiitis, microscopic polyangiitis) |
| Total IgE / specific IgE | Allergic conditions — urticaria (hives), atopic eczema |
| CBC | Eosinophilia (raised eosinophils) in drug reactions and parasitic infections |
| CRP / ESR | Systemic inflammation — vasculitis, infection, autoimmune |
| LFT | Liver disease causing pruritus and jaundice with skin changes |
| Blood cultures | Meningococcal septicaemia — non-blanching petechial rash |
| Viral serology | Measles, rubella, EBV (glandular fever), parvovirus, dengue |
| Coagulation (PT/INR) | Purpura from clotting disorder |
Common rash types and their causes
Urticaria (hives)
Raised, itchy, red welts that migrate around the body. Can be acute (often allergic — food, drug, insect sting) or chronic (>6 weeks — often autoimmune or idiopathic). Total IgE and specific IgE may identify allergic triggers. Acute severe urticaria with throat swelling (angiooedema) is anaphylaxis — emergency.
Eczema / atopic dermatitis
Chronic itchy, dry, red skin in flexural areas (elbows, knees, neck). Blood tests: raised total IgE, possible eosinophilia. Specific IgE may identify food or environmental triggers. Managed with emollients, topical steroids, immunomodulators and avoiding triggers.
Lupus (SLE) butterfly rash
A butterfly-shaped red rash across the nose and cheeks, worsened by sun exposure. Associated with fatigue, joint pain, kidney disease and haematological abnormalities. ANA positive in ~95%; anti-dsDNA more specific. Treat with hydroxychloroquine; immunosuppressants for severe disease.
Drug rash
Medications cause a wide variety of skin reactions — morbilliform (measles-like), urticarial, fixed drug eruption, Stevens-Johnson syndrome (severe mucous membrane involvement — emergency). CBC may show eosinophilia. The most common offending drugs: antibiotics (penicillins, sulfonamides, cephalosporins), anticonvulsants, NSAIDs, allopurinol.
Meningococcal rash — emergency
Non-blanching petechiae or purpura (purple spots that do not fade when pressed with a glass) in a sick patient with fever and stiff neck is a meningococcal septicaemia until proven otherwise. Blood cultures, immediate IV benzylpenicillin and emergency hospital admission.
Questions to ask your doctor
- Is there a systemic cause for my rash?
- Should I have ANA and anti-dsDNA for lupus?
- Could this be a drug reaction?
- Do I need a skin biopsy?
- Is there an allergic trigger to identify with IgE testing?