COMP is an extracellular matrix resident expressed by chondrocytes that also express other cartilage ECM members such as type IX  and type II collagen. Tendon fibroblasts do produce some COMP, but their primary ECM product is Type I collagen. Chondrocytes are historically unstable in cell culture dedifferentiating to a fibroblast phenotype; their COMP expression declining in kind. I have no experience with the ELISA kit, but western blotting of 24hr culture medium from a 25sq.cm flask in the late 90s showed a rather weak band. You may need to concentrate your medium prior to ELISA. Good luck.