Background Serum amyloid A (SAA) protein has been suggested to be a more sensitive marker of disease activity than CRP in rheumatoid arthritis (RA). Few studies are carried out in patients with ankylosing spondylitis (AS).
Objectives To analyse if SAA correlates with other markers of inflammatory activity such as CRP, ESR and serum levels of IL-6 in both RA and AS. To evaluate if SAA is the most sensitive marker of clinical disease activity in RA.
Methods Using a sensitive ELISA test (N-latex SA, Dadde Behring), SAA was measured in the serum of 51 patients with RA and 44 patients with AS. CRP, ESR and serum IL-6 (ELISA) were measured simultaneously. Clinical parameters of disease activity (number of painful and swollen joints) and Thompson’s articular index were analysed also in RA patients.
Results The magnitude of the SSA response was greater than the CRP (x ± SD: SAA = 32.7 ± 56.7 mg/l vs CRP = 18.8 ± 26.3 mg/l p < 0.001). Patients with RA showed higher SAA levels than AS (x ± SD: 44.8 ± 64 mg/l vs 13.6 ± 18.9 mg/l p < 0.001). In both RA and AS, SAA correlates significantly with ESR (r = 0.51; p = 0.0001 in RA and r = 0.61; p = 0.0001 in AS), CRP (r = 0.70; p = 0.0001 in RA and r = 0.66; p = 0.0001 in AS) and IL-6 (r = 0.58 in RA; p = 0.0001, and r = 0.48 in AS; p = 0.001). In patients with RA, SAA but not CRP, ESR or IL-6, showed a positive, although weak correlation with the number of painful joints (r = 0.31; p = 0.03). Thompson articular index, but not the swollen joint count, correlates with the four parameters analysed; this correlation was better for CRP (r = 0.51; p = 0.0001) than for SAA (r = 0.39; p = 0.007) ESR (r = 0.35; p = 0.02) or IL-6 (r = 0.37; p = 0.01).
Conclusion In both RA and AS, SAA correlates well with other laboratory markers (mainly with CRP) of inflammation. However SAA is not a better marker of clinical disease activity than CRP in RA.
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