Detection of serum midkine levels in cancer patients using rabbit anti-human midkine monoclonal antibodies

Midkine (MK) is a heparin-binding growth factor and was found to be highly expressed in many types of human carcinomas. MK may become a novel tumor marker. In this study, we used the rabbit specific antibodies against human MK to establish a double antibody sandwich enzyme-linked immunosorbent assay (ELISA) of MK system and applied it to detect serum MK levels in different types of cancer patients. The standard curve, precision and recovery rate were tested, respectively, and serum MK concentration of 102 cancers patients and 102 normal individuals were detected using this method. The detection range of this method was 0.2 to 10 ng/ml (R 2 = 0.97). The average intra and intro-assay coefficients of variation (CVs) were 3.6 and 7.9%, respectively. The average recovery rate was 89.9% when some standard antigens were added into the serum. The medians (25th and 75th percentiles) of serum MK levels were 1.35 ng/ml (0.96 and 1.64) in cancer patients and 0.30 ng/ml (0.23 and 0.38) in the controls; the MK levels of the patients were significantly higher than those of the controls (P<0.05). Moreover, 87.2% of the patients showed more than 0.6 ng/ml levels of MK. Serum MK could serve as a general tumor marker with a good potential for clinical application. Key words: Midkine, rabbit monoclonal antibody, sandwich ELISA, tumor marker.