Objective: To determine whether serum measures of nitric oxide production correlate with disease activity in patients with systemic lupus erythematosus (SLE).
Methods: We assayed the levels of serum nitrate/nitrite from 26 patients with SLE followed for 1-3 years and nitrotyrosine levels in sera from 28 additional patients with SLE; sera from 19 controls were tested in both assays. Lupus disease activity was determined via the physician's global assessment, the Lupus Activity Index, and the SLE Disease Activity Index (SLEDAI) at the time of serum collection for the initial set of 26 patients. Statistical correlations were determined using the Wilcoxon rank sum method and one-way ANOVA testing.
Results: Serum levels of nitrate/nitrite were significantly higher in 26 patients with SLE compared to 19 controls (SLE, mean 29.5 microM/ml, range 1-438; controls, mean 9.6 microM/ml, range 0-51; p = 0.0004). Overall, there was a significant correlation between serum nitrate/nitrite levels and SLEDAI scores (p = 0.0065). Renal variables within the SLEDAI had the highest correlation with serum nitrate/nitrite (p = 0.0028). Serum nitrotyrosine levels were also significantly higher in patients with SLE versus controls (p = 0.007) and in active SLE versus those with inactive SLE (p = 0.008).
Conclusion: Serum nitrate/nitrite levels correlated with SLE disease activity, especially nephritis, in the majority of patients studied. Serum nitrotyrosine levels also differentiated controls from patients with lupus and patients with active from those with inactive disease. Due to the ease and low cost of these assays, serum measures of nitric oxide production appear a potentially useful adjunctive laboratory measure of disease activity in SLE and further implicate nitric oxide as an important mediator of disease in SLE.