Diagnosis of systemic lupus erythematosus.

Systemic lupus erythematosus is a multisystem inflammatory disease that is often difficult to diagnose. Before the diagnosis can be established, four of 11 clinical and laboratory criteria must be met. Antinuclear antibody titer is the primary laboratory test used to diagnose systemic lupus erythematosus. Because of the low prevalence of the disease in primary care populations, the antinuclear antibody titer has a low predictive value in patients without typical clinical symptoms. Therefore, as specified by the American College of Rheumatology, this titer should be obtained only in patients with unexplained involvement of two or more organ systems. Patients with an antinuclear antibody titer of 1:40 and characteristic multiorgan system involvement can be diagnosed with systemic lupus erythematosus without additional testing; however, patients with an antibody titer of 1:40 who fail to meet full clinical criteria should undergo additional testing, including tests for antibody to double-stranded DNA antigen and antibody to Sm nuclear antigen. While an antinuclear antibody titer of less than 1:40 usually rules out systemic lupus erythematosus, patients with persistent, characteristic multisystem involvement may be evaluated for possible antinuclear antibody-negative disease.