Studies of serum C-reactive protein in systemic lupus erythematosus.
OBJECTIVE: To examine the relationship of serum C-reactive protein (CRP) levels to other indicators of disease activity during the course of systemic lupus erythematosus (SLE). METHODS: In 124 patients serum CRP was measured retrospectively by ELISA and in some instances by radial immunodiffusion. Serum CRP levels were compared to laboratory, clinical, and radiographic assessments of disease activity. In many patients, serial CRP levels were measured over months or years to determine whether elevations of serum CRP reflected apparent changes in other disease activity variables. CRP was also measured in lyophilized aliquots of 24 h urine samples from SLE patients and controls with other renal disorders. Parallel determinations of interleukin 6 (IL-6) were made by ELISA in healthy controls and SLE patients. RESULTS: Of the 124 SLE patients studied, most showed elevations in serum CRP levels in the course of their disease. No inverse or direct correlation was noted between serum CRP and levels of nucleosome antigen or serum IgM or IgG anti-DNA antibody. In patients with renal involvement and proteinuria, CRP was often detected in 24-h urine samples. A strong correlation (p < 0.001) was noted between CRP and IL-6 levels in healthy subjects, but no correlation was recorded between serum CRP and IL-6 in SLE. CONCLUSION: Contrary to previous reports, most patients with SLE in our study showed elevations of serum CRP during the course of their illness, and extremely high serum CRP was recorded in some patients. CRP was also found in concentrated urine samples from patients with renal involvement and often paralleled elevated serum levels. In patients, no correlation was seen between CRP serum levels and serum IL-6, whereas a strong correlation between CRP level and IL-6 was recorded in healthy subjects.