SLE is an autoimmune disorder that affects multiple organ systems. The cause is unknown, but the hallmark of the disease is the production of autoantibodies to the cell nucleus. The incidence of SLE varies significantly among various ethnic groups. Incidence also varies based on sex. Data suggests that the incidence rates of SLE with onset <19 years is 6 to 18.9/100,000 in white females, with higher rates in African American (20–30/100,000) and Puerto Rican females (16–36.7/100,000). The male:female ratio is 1:4.4.

Although the cause is unknown, there is a strong genetic component to SLE, with a 25% to 50% concordance rate in monozygotic twins. Approximately 10% to 16% of patients have a first- or second-degree relative with the disorder. An association with human leukocyte antigen has been found. The high female-to-male ratio, coupled with the fact that pregnancy frequently causes flares and worsens nephritis, strongly suggests that female hormones may play a role in development of the disease. Several drugs are known to induce lupuslike disease including, but not limited to, hydralazine, procainamide, isoniazid, minocycline, and D-penicillamine.

The clinical findings of SLE are a result of inflammation and blood vessel abnormalities. Bland vasculopathy, vasculitis, and immune-complex deposition have all been described. Patients frequently present with complaints of fever, diffuse hair loss, weight loss, fatigue, and arthralgias or arthritis (especially of small joints and wrists). Lymphadenopathy and hepatosplenomegaly may also be present. The differential diagnosis should include malignancy, infection, or other autoimmune disorders (i.e., rheumatoid arthritis, and Sjögren syndrome).

ANAs are seen in up to 95% of patients with SLE. ANA testing is sensitive, but not very specific for SLE. ANA also occurs in up to 5% of the normal population. Antibodies to double-stranded DNA (anti-ds DNA) and to the Sm nuclear antigen (anti-Sm) are found only in patients in SLE.

The diagnosis is based on history, physical exam, and laboratory findings. Diagnostic criteria have been established by the American Academy of Rheumatology. These criteria are sensitive (78% to 96%) and specific (89% to 96%) for SLE. See Table 114.1.