Aims : To standardize the histopathological assessment of synovial membrane specimens in order to contribute to the diagnostics of rheumatic and non‐rheumatic joint diseases.

Methods and results : Three features of chronic synovitis (enlargement of lining cell layer, cellular density of synovial stroma, leukocytic infiltrate) were semiquantitatively evaluated (from 0, absent to 3, strong) and each feature was graded separately. The sum provided the synovitis score, which was interpreted as follows: 0–1, no synovitis; 2–4, low‐grade synovitis; 5–9, high‐grade synovitis. Five hundred and fifty‐nine synovectomy specimens were graded by two independent observers. Clinical diagnoses were osteoarthrosis (n = 212), post‐traumatic arthritis (n = 21), rheumatoid arthritis (n = 246), psoriatic arthritis (n = 22), reactive arthritis (n = 9), as well as controls (n = 49) from autopsies of patients without joint damage. Median synovitis scores when correlated with clinical diagnoses were: controls 1.0, osteoarthritis 2.0, post‐traumatic arthritis 2.0, psoriatic arthritis 3.5, reactive arthritis 5.0 and rheumatoid arthritis 5.0. The scores differed significantly between most disease groups, especially between degenerative and rheumatic diseases. A high‐grade synovitis was strongly associated with rheumatic joint diseases (P < 0.001, sensitivity 61.7%, specificity 96.1%). The correlation between the two observers was high (r = 0.941).

Conclusion : The proposed synovitis score is based on well‐defined, reproducible histopathological criteria and may contribute to diagnosis in rheumatic and non‐rheumatic joint diseases.