Guide to Autoimmune Blood Tests

Autoimmune blood tests and what they detect

ANA: the starting point

Complement levels in autoimmune disease

Complement proteins (C3 and C4) are part of the immune system. In lupus, immune complexes consume complement, causing low C3 and C4, especially during flares. Low complement with high anti-dsDNA is a reliable indicator of lupus disease activity and kidney involvement. Complement is normal or raised in RA and most other autoimmune conditions.

Questions to ask your rheumatologist

• Which specific antibodies are positive in my case?
• Do my antibody patterns point to one diagnosis?
• Is my complement low: indicating active lupus?
• Do I need further organ-specific investigations (kidney biopsy, lung function)?

Frequently Asked Questions

References

Sources cited on this page. PubMed links open the original abstract.

1. Tan EM, Feltkamp TE, Smolen JS, et al. Range of antinuclear antibodies in "healthy" individuals. Arthritis Rheum. 1997;40(9):1601–1611. PMID 9324014 · doi:10.1002/art.1780400909

The ANA cascade – how autoimmune testing is done stepwise

Antinuclear antibody (ANA) testing is the entry point for investigating suspected autoimmune disease. However, a positive ANA alone does not diagnose autoimmune disease – it is a screening test that triggers a cascade of more specific tests:

1. ANA by immunofluorescence: Detects antibodies that bind to nuclear material. Reported as a titre (e.g., 1:80, 1:160, 1:320) and pattern (homogeneous, speckled, nucleolar, centromere). Positive at 1:160 or above is clinically significant; 1:80 is found in up to 15% of healthy adults and 30% of healthy older adults – the threshold matters enormously.
2. If ANA positive, specific extractable nuclear antigen (ENA) antibodies are measured:
   • Anti-dsDNA: Highly specific for systemic lupus erythematosus (SLE); levels correlate with disease activity and can predict renal flares.
   • Anti-Sm: Also highly specific for SLE; less sensitive but when positive is near-diagnostic.
   • Anti-Ro (SS-A) and anti-La (SS-B): Sjögren's syndrome; also found in neonatal lupus (anti-Ro crosses the placenta and can cause neonatal heart block).
   • Anti-Scl-70 (anti-topoisomerase I): Systemic sclerosis (diffuse pattern).
   • Anti-centromere: Limited systemic sclerosis (CREST syndrome).
   • Anti-Jo-1: Antisynthetase syndrome (inflammatory myopathy + interstitial lung disease).

The choice of which ENAs to test depends on the clinical presentation – ordering the full ENA panel in every ANA-positive patient leads to false-positive results and unnecessary investigation.

Complement proteins – active disease markers in lupus

Complement C3 and C4 are consumed when the immune system activates the complement cascade – as happens in active SLE, especially lupus nephritis. Low C3/C4 alongside raised anti-dsDNA indicates active lupus flare with immune complex deposition. Serial complement measurement is used to monitor treatment response in lupus nephritis – rising C3/C4 correlates with renal improvement. Complement is normal in the autoimmune conditions that are not complement-consuming (Sjögren's, most inflammatory myopathy).

ANCA testing – vasculitis investigation

Anti-neutrophil cytoplasmic antibodies (ANCA) are separate from ANA and are used to investigate systemic vasculitis:

• PR3-ANCA (c-ANCA pattern): Associated with granulomatosis with polyangiitis (GPA, formerly Wegener's) – presenting with upper respiratory (nosebleeds, saddle-nose deformity), pulmonary (haemoptysis, cavitating nodules), and renal (rapidly progressive glomerulonephritis) involvement.
• MPO-ANCA (p-ANCA pattern): Associated with microscopic polyangiitis and eosinophilic GPA (Churg-Strauss) – asthma, sinusitis, eosinophilia, mononeuritis multiplex, and renal disease.

ANCA-associated vasculitis is a medical emergency – untreated, it causes irreversible kidney failure within weeks to months. Early diagnosis and treatment with cyclophosphamide or rituclimab plus steroids is life-saving.