The invention is a method of identifying the presence of a disease state in a mammal which is associated with degradation of connective tissue in the mammal which contains the protein known as YKL-40. The method is a competitive immunoassay for YKL-40. It can be used, for example, to identify the presence of inflammatory or degenerative joint disease and tumor metastasis (to the extent it can be correlated to serum YKL-40 levels). Serum YKL-40 levels as detected and quantified by the inventive method are also suggestive of the prognosis for the length of survival in breast cancer patients following recurrence and/or metastasis of their cancers.