Abstract Background Rheumatoid arthritis (RA) is a chronic, inflammatory joint condition characterized by overproduction of pro-inflammatory cytokines. We aimed to assess TNF-α levels in both serum and synovial fluid in effusive knees in RA patients and find out if synovial fluid levels correlate with ultrasound (US)-detected local knee inflammatory and/or destructive changes in these patients. Results This study included 40 patients (20 with RA, 10 with systemic lupus erythematosus (SLE), and 10 with osteoarthritis (OA)) who had knee effusion (unilateral or bilateral) upon clinical examination. The mean age of RA patients was 48.4 years; most of them were females (80%), with a median (min–max) duration of knee effusion of 2 (1.5–3) months. Serum TNF-α was significantly higher in RA vs. non-RA and in OA cases (p = 0.052, 0.022, respectively), while in the synovial fluid, the difference was not statistically significant (3.73 ± 0.72 vs. 3.48 ± 0.58 U/ml, p = 0.252). Serum TNF-α at a cut point of > 3.24 U/ml can significantly discriminate RA from OA with 65% sensitivity and 90% specificity (AUC = 0.725, P = 0.018). There was no statistically significant correlation between synovial TNF-α and US parameters of the knee, either in RA or non-RA patients. Conclusions RA, OA, and SLE effusive joints share the presence of local articular joint inflammation, while systemic inflammation is more discriminative for RA patients regarding the level of TNF-α. The lack of correlation of TNF-α with ultrasonographic findings reflects the multifactorial complexity of these autoimmune diseases.
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