APLN and rheumatoid arthritis: In addition, serum protein levels of chemerin and vaspin, determined by ELISA, were found to be significantly higher by 2.49- and 1.86-fold, respectively (p < 0.001), while serum apelin and omentin levels were found to be lower by 0.58- and 0.25-fold, respectively (p < 0.001) among RA patients relative to controls.