Detecting glycosaminoglycans (GAGs) and/or determining the level of one or more glycosaminoglycans can be useful, e.g., in identifying or monitoring various medical conditions, the status of patients having various medical conditions, and/or the response to treatment of individuals having various medical conditions. The present invention provides methods for detecting glycosaminoglycans and/or determining the level of glycosaminoglycans through the use of, e.g., mass spectrometry.