ANA Screen + 11 ANA Components

Antinuclear antibodies are immunoglobulins directed against nuclear antigens. They are produced when immune tolerance to self-antigens breaks down, a hallmark of systemic autoimmune rheumatic disease. Indirect immunofluorescence on HEp-2 cells remains the gold standard screening method, detecting antibodies to a broad range of nuclear and cytoplasmic antigens. Pattern and titre provide initial clues to the underlying specificity, though specific extractable nuclear antigen testing is required for definitive characterisation.

ANA screening is the gateway test for systemic lupus erythematosus, Sjögren syndrome, systemic sclerosis, mixed connective tissue disease, and drug-induced lupus. A positive ANA prompts reflex testing for specific autoantibodies that define disease subsets and guide prognosis. Sensitivity for SLE exceeds 95 per cent, but specificity is lower because low-titre ANA occurs in healthy individuals, infection, malignancy, and other inflammatory states.

The fluorescence pattern observed on HEp-2 cells reflects the subcellular distribution of target antigens. Homogeneous staining suggests anti-dsDNA or anti-histone antibodies; speckled patterns point to antibodies against extractable nuclear antigens; nucleolar patterns are seen with anti-RNA polymerase or anti-fibrillarin; and centromere patterns indicate anti-centromere antibodies.

Pattern guides the choice of reflex tests and narrows the differential diagnosis, but overlap and mixed patterns are common, so specific antibody identification is essential for clinical decision-making.

Antibodies to double-stranded DNA are highly specific for systemic lupus erythematosus and are thought to contribute directly to tissue injury through immune-complex deposition in kidney and other organs. Titres often correlate with disease activity, particularly lupus nephritis.

Anti-dsDNA is a diagnostic criterion for SLE and a marker of renal involvement. Rising titres may herald flare, making serial measurement useful in disease monitoring. Specificity approaches 95 per cent, but sensitivity is only 50–70 per cent, so a negative result does not exclude lupus.

Smith antigen is a core component of small nuclear ribonucleoprotein particles involved in mRNA splicing. Anti-Sm antibodies are directed against these proteins and are found almost exclusively in systemic lupus erythematosus, though their pathogenic role remains uncertain.

Anti-Sm is one of the most specific antibodies for SLE (>95 per cent specificity) and is an ACR/EULAR classification criterion. However, it is present in only 10–30 per cent of patients with lupus, limiting its sensitivity. Unlike anti-dsDNA, anti-Sm titres do not correlate well with disease activity.

U1 ribonucleoprotein is another small nuclear RNP particle. Anti-RNP antibodies recognise the 70-kDa, A, and C polypeptides of U1-RNP and are characteristic of mixed connective tissue disease when present in high titre without other specific antibodies.

High-titre anti-RNP in isolation defines mixed connective tissue disease, which presents with overlapping features of lupus, scleroderma, and myositis. Anti-RNP is also found in about 30 per cent of SLE patients, often alongside anti-Sm, and may be associated with Raynaud phenomenon and lung involvement.

SSA/Ro antigens are RNA-protein complexes (Ro52 and Ro60) involved in quality control of misfolded non-coding RNAs. Antibodies to SSA/Ro cross the placenta and are implicated in neonatal lupus and congenital heart block.

Anti-SSA/Ro is found in 40–50 per cent of patients with Sjögren syndrome and 30–40 per cent with SLE, and is strongly associated with photosensitivity, subacute cutaneous lupus, and neonatal lupus. Pregnant individuals positive for anti-SSA/Ro require foetal echocardiographic surveillance because of the risk of congenital heart block.

SSB/La antigen is a 48-kDa phosphoprotein that binds nascent RNA polymerase III transcripts. Anti-SSB/La is almost always accompanied by anti-SSA/Ro, so isolated anti-SSB/La is rare.

Anti-SSB/La is found in 10–15 per cent of SLE patients and 40–50 per cent of those with Sjögren syndrome. Its presence alongside anti-SSA/Ro is associated with longer disease duration and more prominent sicca symptoms in Sjögren syndrome.

Topoisomerase I (Scl-70) is a nuclear enzyme that relieves torsional strain during DNA replication. Anti-Scl-70 antibodies are highly specific for systemic sclerosis and correlate with diffuse cutaneous disease and interstitial lung involvement.

Found in 20–40 per cent of patients with systemic sclerosis, anti-Scl-70 is a marker of diffuse cutaneous disease, progressive interstitial lung disease, and overall poorer prognosis. It is mutually exclusive with anti-centromere antibodies, which predict limited cutaneous disease.

Centromere proteins (CENP-A, -B, -C) are kinetochore components essential for chromosome segregation. Anti-centromere antibodies produce a discrete speckled pattern on metaphase cells in immunofluorescence.

Anti-centromere is the hallmark of limited cutaneous systemic sclerosis (CREST syndrome) and primary biliary cholangitis. It predicts a lower risk of severe internal organ involvement compared with anti-Scl-70, though pulmonary arterial hypertension remains a concern in long-standing disease.

Histidyl-tRNA synthetase is a cytoplasmic enzyme that charges tRNA with histidine. Anti-Jo-1 is the most common of the anti-synthetase antibodies, a family of autoantibodies associated with inflammatory myopathy.

Anti-Jo-1 defines anti-synthetase syndrome, characterised by myositis, interstitial lung disease, mechanic's hands, Raynaud phenomenon, and inflammatory arthritis. It is found in 20–30 per cent of patients with polymyositis or dermatomyositis and is a strong predictor of ILD, which may dominate the clinical picture.

Ribosomal P proteins (P0, P1, P2) are structural components of the 60S ribosomal subunit. Anti-ribosomal P antibodies are specific for systemic lupus erythematosus and have been linked to neuropsychiatric manifestations, though this association is debated.

Found in 10–20 per cent of SLE patients, anti-ribosomal P is more common in Asian and Hispanic populations. It has been associated with lupus hepatitis, nephritis, and neuropsychiatric lupus, particularly psychosis and severe depression, though not all studies confirm the neuropsychiatric link.

Chromatin (nucleosome) consists of DNA wrapped around a histone octamer. Anti-chromatin antibodies recognise the histone-DNA complex and are closely related to anti-histone and anti-dsDNA antibodies, though they constitute a distinct specificity.

Anti-chromatin antibodies are found in 50–90 per cent of patients with SLE and may be positive when anti-dsDNA is negative, adding diagnostic sensitivity. They are also seen in drug-induced lupus, where anti-histone antibodies are the classic marker.

Histones are the principal protein components of chromatin. Anti-histone antibodies target H1, H2A, H2B, H3, and H4, and are induced by drugs that cause systemic lupus-like syndromes, including procainamide, hydralazine, quinidine, and anti-TNF biologics.

Anti-histone antibodies are present in more than 95 per cent of cases of drug-induced lupus and in 50–70 per cent of idiopathic SLE. Their presence in a patient on a known culprit drug strongly supports drug-induced lupus, which typically resolves after drug withdrawal.