Serum amyloid A in the assessment of early inflammatory arthritis

Objective. Acute phase serum amyloid A (A-SAA) has been reported to be more sensitive than C-reactive protein (CRP) as a marker of disease activity. I t may function in immune regulation and is linked to the development of sec ondary amyloidosis. We investigated the profile of A-SAA in early inflammat ory arthritis and compared A-SAA with CRP and erythrocyte sedimentation rat e (ESR) in relation to diagnosis and disease activity. Methods. Using a sensitive and specific ELISA, A-SAA was measured in the se rum of 140 patients with early arthritis (disease duration 2 weeks to 24 mo , mean 6 mo). CRP was determined using a standard ELISA; ESR and clinical d isease activity variables were also recorded. Results. Sixty-four patients had rheumatoid arthritis (RA), 19 psoriatic ar thritis (PsA), 28 undifferentiated arthritis (UA), and 29 other forms of ar thritis. A-SAA levels correlated with both CRP (r = 0.73, p = 0.0001) and E SR (r = 0.6, p = 0.0001). The magnitude of the A-SAA response was greater t han either the CRP or ESR, and very high A-SAA levels were observed in dise ase as early as 2 weeks. Highest A-SAA concentrations occurred in RA (media n 70.3 mg/l, maximum 1542) compared with the other groups (medians, PsA: 33 mg/l; UA: 12.3 mg/l; other arthritis: 11.2 mg/l), with values > 520 mg/l o bserved exclusively in RA. A-SAA, unlike CRP or ESR, could distinguish pati ents with a final diagnosis of RA from those who had persistent UA. In RA, A-SAA provided the strongest correlations with clinical measurements of dis ease activity. Clinical improvement was also best represented by A-SAA, whi le disease deterioration was associated with a significant increase in A-SA A values, but not CRP or ESR. Conclusion. Compared with ESR or CRP, A-SAA correlates best with markers of disease activity, and in patients with recent onset arthritis, very high l evels of SAA occur exclusively in RA. As A-SAA is sensitive to change and a ccurately reflects alterations in disease status, it is the best marker ava ilable for the assessment of inflammatory joint disease.