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Objective. There have been a large number of reports on alterations in the serum level of keratan sulphate (KS), a potential marker of articular cartilage degeneration in patients with arthropathy. Such studies have commonly employed ELISA using the anti-KS monoclonal antibody 1/20/5D4 (5D4-ELISA) to determine KS levels. Recently, a highly sensitive KS ELISA (HS-ELISA) kit has been developed, allowing determination of serum KS levels even in small animals, which were formerly undetectable with 5D4-ELISA. However, the effectiveness of this kit in humans has not been demonstrated. The objective of this study was to assess the usefulness of the HS-ELISA for the analysis of human serum samples. Methods. Serum samples were collected from 28 patients with knee OA and 23 healthy volunteers. KS was determined by 5D4-ELISA and HS-ELISA, and measurements were compared with those obtained by HPLC. KS levels in serum samples with protease pretreatment were also determined by HS-ELISA. Results. KS levels determined by HS-ELISA exhibited a better correlation with those determined by HPLC, and a higher diagnostic sensitivity for OA compared with 5D4-ELISA. Protease pretreatment of serum further improved the correlation between the values obtained by HS-ELISA and HPLC, as well as the diagnostic sensitivity of HS-ELISA for OA. Conclusions. HS-ELISA proved useful for determining KS level in serum and the diagnosis of OA. Pretreatment of serum samples with a protease further improved the performance of HS-ELISA. [ABSTRACT FROM AUTHOR]