TTR and chronic kidney disease: In severe stages of CKD (<29 mL/min/1.73 m2), blood-derived peptides such as alpha-1 antitrypsin, vitamin-D binding proteins, transthyretin, and serum amyloid A-1 were found to be highly abundant in the urinary proteome, while in early CKD stages, analysis showed modification of collagen fragments, which suggests changes in the turnover of the extracellular matrix (ECM), the principal structure of fibrotic tissue [10].