SLE: investigations

Immunology

• 99% are ANA positive
• 20% are rheumatoid factor positive
• anti-dsDNA: highly specific (> 99%), but less sensitive (70%)
• anti-Smith: most specific (> 99%), sensitivity (30%)
• also: anti-U1 RNP, SS-A (anti-Ro) and SS-B (anti-La)

Monitoring

• ESR: during active disease the CRP is characteristically normal - a raised CRP may indicate underlying infection
• complement levels (C3, C4) are low during active disease (formation of complexes leads to consumption of complement)
• anti-dsDNA titres can be used for disease monitoring (but note not present in all patients)