Background: Serum amyloid P component (SAP) and acute phase proteins like C-reactive protein contribute to the clearance of apoptotic cells. This response is diminished in systemic lupus erythematosus (SLE).
Objectives: To analyse SAP concentrations in SLE in relation to disease activity, and investigate whether SAP reacts like an acute phase protein.
Methods: SAP was measured in 40 patients with SLE during active and inactive disease and compared with healthy controls and patients with rheumatoid arthritis and Wegener's granulomatosis. Normal SAP values were determined in 120 healthy controls by ELISA. C reactive protein and serum amyloid A (SAA) were measured in all subjects and their levels related to SAP. SAP was also measured serially in 11 patients with breast cancer treated with recombinant human interleukin-6, and in 16 patients with sepsis.
Results: In SLE, SAP was unaltered compared with healthy controls and was not influenced by disease activity, in contrast to C reactive protein and SAA, which increased during active disease. SAP increased in Wegener's granulomatosis but not in rheumatoid arthritis. The rise in C reactive protein and SAA was most pronounced in Wegener's granulomatosis with active disease. SAP did not change significantly during an acute phase response. No correlation was found between SAP and C reactive protein or SAA, but there was a correlation between SAA and C reactive protein (r = 0.4989, p = 0.0492).
Conclusions: Patients with SLE have normal circulating SAP levels. In contrast to C reactive protein or SAA, SAP does not act as an acute phase protein.