LEP and rheumatoid arthritis: However, its concentration was lower when compared with the serum leptin levels.[23] A similar situation was found in osteoarthritic synovial fluid because the severity of OA and the level of synovial fluid leptin were positively correlated.[24] Leptin levels in serum and synovial fluid of normal adults and OA/RA patients were shown in Tables 1 and 2.