What laboratory tests are recommended for diagnosing systemic sclerosis?

Laboratory Testing for Systemic Sclerosis

All patients with suspected systemic sclerosis should undergo antinuclear antibody (ANA) testing by indirect immunofluorescence as the initial screening test, followed by specific autoantibody profiling to identify disease-specific antibodies that predict organ involvement and prognosis. 1

Initial Screening Tests

Antinuclear Antibodies (ANA)

• ANA by indirect immunofluorescence (IIFA) on HEp-2 cells is the gold-standard screening test, detecting antibodies in 90-95% of SSc patients 2, 3
• The pattern observed (homogeneous, speckled, nucleolar, centromere) provides initial clues to specific autoantibody types 1
• ANA-negative SSc exists in approximately 5-10% of cases and represents a distinct subtype with less vasculopathy but more severe gastrointestinal involvement 3

Complete Blood Count and Basic Chemistry

• CBC, glucose, electrolytes, kidney function (creatinine, BUN), and liver enzymes (AST, ALT, alkaline phosphatase) should be obtained in all patients 1
• Alkaline phosphatase elevation may indicate primary biliary cholangitis, which occurs in 8% of limited cutaneous SSc patients, particularly those with anti-centromere antibodies 1

Disease-Specific Autoantibody Panel

Essential SSc-Specific Antibodies

The following autoantibodies should be tested as they define distinct clinical phenotypes and predict organ involvement:

• Anti-topoisomerase I (anti-Scl-70): Strongly associated with diffuse cutaneous SSc, interstitial lung disease (ILD), and digital ulcers; predicts poor prognosis 1, 4, 2
• Anti-centromere antibodies (ACA): Associated with limited cutaneous SSc, isolated pulmonary hypertension, and favorable prognosis 4, 2
• Anti-RNA polymerase III (anti-RNAP III): Associated with diffuse cutaneous disease, rapid skin progression, scleroderma renal crisis, and increased malignancy risk 1, 4, 2

Additional SSc-Associated Antibodies

These should be included in comprehensive autoantibody profiling:

• Anti-Th/To antibodies: Associated with limited cutaneous SSc, pulmonary fibrosis, and pulmonary hypertension 4, 2
• Anti-U3RNP (anti-fibrillarin): Associated with diffuse cutaneous SSc, pulmonary hypertension, gastrointestinal involvement, and less favorable prognosis 1, 4
• Anti-PM/Scl antibodies: Associated with SSc-myositis overlap syndrome, ILD, and calcinosis 4, 2
• Anti-U1RNP antibodies: Suggests mixed connective tissue disease or SSc overlap syndrome 1

Overlap Syndrome and Connective Tissue Disease Screening

Extractable nuclear antigen (ENA) panel should be performed to identify overlap syndromes and mixed connective tissue disease 1:

• Anti-RNP, anti-SSA/Ro, anti-SSB/La, anti-Smith antibodies for SLE overlap 1
• Anti-Jo-1 and other myositis-specific antibodies if muscle involvement is suspected 1
• Complement levels (C3, C4) and anti-dsDNA for SLE overlap 1

Organ-Specific Screening Tests

Pulmonary Assessment

• Pulmonary function tests (PFTs) including forced vital capacity (FVC) and diffusing capacity for carbon monoxide (DLCO) should be performed at baseline and regularly, especially in early diffuse cutaneous SSc and anti-Scl-70 positive patients 1, 5
• High-resolution CT of the chest to screen for ILD, particularly in high-risk patients (anti-Scl-70 positive, abnormal PFTs, crackles on examination) 1, 6

Cardiac Screening

• N-terminal pro-B-type natriuretic peptide (NT-proBNP) if arrhythmias or heart failure are present 1
• ECG and echocardiography if cardiac involvement is suspected 1

Renal Monitoring

• Urinalysis with microscopy for proteinuria and hematuria 1
• Blood pressure monitoring at every visit to detect early scleroderma renal crisis 6

Additional Laboratory Tests

• Thyroid function tests (TSH, free T4) if autoimmune thyroid disease is suspected 1
• Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP): Elevation may predict risk of lung fibrosis, especially in early diffuse cutaneous SSc 1
• Vitamin D and calcium levels if osteoporosis screening is indicated 1

Important Caveats and Pitfalls

• Do not rely solely on multiplex immunoassays (ELISA, line blots) for ANA screening, as they may miss certain antibodies; indirect immunofluorescence remains the gold standard 1, 3
• If clinical suspicion for SSc is high but ANA is negative, proceed with specific autoantibody testing anyway, as some SSc-specific antibodies (anti-Jo-1, anti-Ro/SSA) can occur in ANA-negative patients 1
• Immunoprecipitation is considered the gold-standard technique for autoantibody identification due to its high sensitivity and specificity, though other assays are widely used in routine practice 3
• Serial autoantibody testing is generally not useful, as antibody profiles typically remain stable over time; however, clinical phenotype may evolve 4
• Anti-RNAP III positivity warrants age-appropriate malignancy screening within 3 years of SSc diagnosis due to increased cancer risk 1

Monitoring Laboratory Tests

• Repeat PFTs every 3-6 months in early disease or if ILD is present to detect progression 1, 6
• Annual urinalysis for proteinuria starting at age 10 years or earlier if renal involvement is suspected 1
• Monitor complete blood count and liver enzymes if on immunosuppressive therapy 1