Laboratory tests - antinuclear antibody (ANA)

What it is

Antibodies against nuclear (and some cytoplasmic) antigens, detected by indirect immunofluorescence on HEp-2 cells - the reference method
Reported as titre + pattern
*A screening test with high sensitivity and poor specificity* - it tells you whether to look further, not what the diagnosis is
Solid-phase assays (ELISA, multiplex) miss some patterns - if clinical suspicion is high, request IIF specifically

Positive at 1:80 in ~10-15% of healthy adults; at 1:160 in ~5%; at 1:320 in ~3%
Rises with age and is commoner in women
*In an unselected population, most positive ANAs are false positives* - the pre-test probability determines everything

Loss of tolerance to nucleic acid-protein complexes exposed during apoptosis or NETosis
Nucleic acid-containing immune complexes -> TLR7/9 on plasmacytoid dendritic cells -> type I interferon -> further B-cell activation
Autoantibodies precede clinical autoimmune disease by years

Disease	ANA sensitivity
SLE	~93-95% (a negative ANA effectively excludes SLE)
Systemic sclerosis	~85%
Polymyositis/dermatomyositis	~61%
Juvenile idiopathic arthritis	~57%
Sjogren syndrome	48-73%
Rheumatoid arthritis	~41%

2019 EULAR/ACR SLE classification requires ANA >=1:80 as the entry criterion - without it, classification cannot proceed
*A negative ANA does not exclude every autoimmune disease* - anti-Jo-1 myositis and some vasculitides are ANA-negative

Pattern	Associated antibodies / disease
Homogeneous	Anti-dsDNA, anti-histone -> SLE, drug-induced lupus, MCTD, JIA
Speckled	Anti-ENA (Ro, La, Sm, RNP) -> SLE, Sjogren, MCTD, myositis, systemic sclerosis
Nucleolar	Anti-Scl-70, anti-fibrillarin (U3-RNP), anti-RNA pol III -> diffuse systemic sclerosis, myositis
Centromere	Anti-centromere -> limited cutaneous systemic sclerosis (CREST); PBC
Peripheral / rim	Anti-dsDNA -> SLE (the most specific pattern for SLE), systemic sclerosis
Cytoplasmic	Anti-Jo-1 and other anti-synthetases, anti-ribosomal P, anti-mitochondrial (PBC)
Dense fine speckled (DFS-70)	*Isolated DFS-70 with no other specific antibody argues AGAINST systemic autoimmune rheumatic disease* - common in healthy people

Other autoimmune disease: Hashimoto thyroiditis, Graves disease, autoimmune hepatitis, primary biliary cholangitis, coeliac disease, ITP, myasthenia gravis
Infection: EBV/mononucleosis, hepatitis C, subacute bacterial endocarditis, TB, HIV, parvovirus
Malignancy: lymphoproliferative disease, solid tumours
Drugs: hydralazine, procainamide, minocycline, isoniazid, TNF inhibitors, interferon
Age, pregnancy, healthy relatives of patients with autoimmune disease
Chronic liver disease, ILD, silicosis

Titre matters: 1:40-1:80 is weak and common; >=1:320 with a specific pattern is far more meaningful
The pattern directs the reflex test, not the diagnosis
*The ANA titre does NOT track disease activity - never repeat it to monitor*

Order only with a clinical syndrome that would be explained by a systemic autoimmune rheumatic disease
- Inflammatory arthritis, photosensitive rash, serositis, unexplained cytopenias, Raynaud with abnormal nailfold capillaries, unexplained renal disease with active sediment, myositis, ILD in a young patient
**Do not order for: fatigue, non-inflammatory arthralgia, fibromyalgia, isolated positive family history**
- A positive result in these settings causes anxiety, referrals and unnecessary follow-up without changing outcomes

1. Assess pre-test probability first - a positive ANA in a patient with no clinical features usually means nothing
2. Follow the pattern to the specific antibody
- Homogeneous/rim -> anti-dsDNA, C3/C4
- Speckled -> ENA panel (Ro, La, Sm, RNP, Scl-70, Jo-1)
- Nucleolar or centromere -> scleroderma-specific antibodies + nailfold capillaroscopy
3. Add: FBE, creatinine, urinalysis with microscopy, CK, complement, and antiphospholipid antibodies where indicated
4. Reassess clinically rather than repeating serology
*Never repeat the ANA to follow the disease* - it is a diagnostic test, not a monitoring test
Anti-dsDNA and C3/C4 ARE useful for monitoring in SLE; ANA is not

SLE (>95%), systemic sclerosis (~85%), Sjogren syndrome, MCTD, myositis, RA, JIA, psoriatic arthritis
Autoimmune hepatitis and primary biliary cholangitis (often with anti-mitochondrial antibodies)
Autoimmune thyroid disease
Drug-induced lupus - anti-histone, homogeneous pattern
Chronic infection - hepatitis C, endocarditis, TB, HIV, EBV
Malignancy, especially lymphoproliferative disease
Ageing and healthy first-degree relatives
Raynaud phenomenon - ANA + abnormal nailfold capillaroscopy is the combination that predicts progression to a defined CTD

An isolated positive ANA in an asymptomatic person: ~1% or less per year progress to a defined autoimmune disease
Autoantibodies precede clinical disease by years - documented up to 9 years before SLE onset
Raynaud + positive ANA + abnormal capillaroscopy: high risk of progression to systemic sclerosis - this combination justifies follow-up
- Isolated ANA without Raynaud or capillary abnormality generally does not
Isolated DFS-70 positivity remains benign over long follow-up
The titre does not rise or fall usefully with disease activity and should not be tracked

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