The extractable nuclear antigens (ENAs), another group of nuclear antigens (nonhistone proteins) to which autoantibodies may develop, are so named because of their presence in saline solution extracts of certain nonhuman cells. The most common ENAs are RNP and Sm.
Anti-RNP is elevated in 35%–40% of patients with SLE and in patients with other connective tissue diseases, notably MCTD. MCTD is characterized by high levels of anti-RNP without autoantibodies to dsDNA or Sm. The disease resembles SLE but is not accompanied by kidney involvement.
Anti-Sm is specific for SLE but occurs in only approximately 30% of the patients. The levels of anti-Sm may be related to disease activity in SLE.
Anti-Sjögren syndrome (anti-SS)-A (Ro) has been detected in approximately 25% of patients with SLE and in 40%–45% of patients with Sjögren syndrome. Anti–SS-B (La) is found in approximately 10%–15% of patients with SLE and up to 60% of patients with Sjögren syndrome. Anti-scleroderma 70 (anti–Scl-70) is considered specific for scleroderma (systemic sclerosis). These autoantibodies are found in up to 60% of patients with scleroderma with extensive cutaneous disease and interstitial pulmonary fibrosis.
Anti–histidyl transfer RNA synthetase antibodies (anti–Jo-1) occur in approximately 20% of patients with myositis, usually in patients with interstitial pulmonary fibrosis and symmetrical polyarthritis.
The ELISA assay is a screen for several nuclear antibodies. If the ENA screen result is borderline or positive, the following tests (Table 8.10) will be set up to determine the particular SRD.
Normal Findings for ENA and Individual Autoantibody Tests
Negative: <20 U/mL by ELISA
Borderline: 20–25 U/mL
Positive: >26 U/mL
Collect a 7-mL blood serum sample in a red-topped tube. Observe standard precautions.
Label the specimen with the patient’s name, date, and test(s) ordered and place in a biohazard bag for transport to the laboratory.
Results of serum tests for autoantibodies should not be relied on extensively to establish the diagnosis of a connective tissue disease. They must always be interpreted in conjunction with clinical findings.
Testing for autoantibodies to ENAs is not useful in patients without demonstrable ANAs.