Synopsis
Use ANCA, complement and extended autoantibodies to refine a clearly defined systemic phenotype, while prioritising tissue, urine and organ assessment and avoiding false reassurance or harm from indiscriminate panels.
- ANCA testing is appropriate when small-vessel vasculitis is clinically plausible; it is not a general screen for fatigue, arthralgia or an isolated mildly raised CRP.
- Interpret antigen-specific PR3-ANCA and MPO-ANCA with phenotype, urinalysis, renal function, chest findings, ENT disease, neuropathy and histology where feasible.
- A positive ANCA can occur with infection, inflammatory bowel disease, autoimmune liver disease, medicines and other conditions, while ANCA-negative vasculitis remains possible.
- Low C3 and C4 suggest complement consumption but occur in infection, immune-complex disease and other disorders; the pattern and clinical setting determine meaning.
- Anti-dsDNA, ENA, antiphospholipid antibodies, myositis and systemic-sclerosis antibodies should follow compatible organ and examination findings rather than a blanket panel.
- Urinalysis, urine protein quantification, creatinine trend and microscopy can reveal dangerous renal disease before the complete serology profile returns.
- Antiphospholipid syndrome requires a qualifying clinical event and persistent laboratory positivity on tests at least 12 weeks apart; one result is not sufficient.
- Biopsy can distinguish vasculitis, immune-complex disease, infection and mimics and should be coordinated early without delaying treatment in a life-threatening presentation.
Key red flags
Haemoptysis, falling haemoglobin, diffuse lung infiltrates, haematuria, proteinuria and rising creatinine indicate a medical emergency requiring parallel vasculitis and infection assessment.
Investigation priorities
Support classification of suspected ANCA-associated vasculitis within a compatible organ phenotype.
Management branches
Pulmonary haemorrhage, glomerulonephritis, digital ischaemia, neuropathy, myocarditis or another rapidly progressive organ lesion.
- Stabilise physiology, obtain blood, urine, imaging and infection samples, and contact rheumatology plus the organ specialty immediately.
- Send PR3/MPO, complement and phenotype-directed serology while arranging biopsy; do not wait for the full panel when delay risks irreversible injury.
- Agree immunosuppression and antimicrobial coverage with specialists using illness severity, infection probability and tissue findings, then monitor organ response closely.