Author's response to reviews Title: Estimation of Glomerular Filtration Rate from Serum Creatinine and Cystatin C in Octogenarians and Nonagenarians Authors:

We did our best to answer the remaining concerns of the reviewer. In particular, we understand the concerns with the standardization of cystatin C measurements. At the time of the study planning and execution there were no clear international recommendations about standardization of cystatin C measurements. The recommendation about this standardization has been made just very recently and it was not possible for us to incorporate it in the present study. We tried to deal with it using a formula proposed by Inker el al (AJKD 2011), but as pointed out by the reviewer this was not appropriate since it was calibrated using another assay (Siemens) instead of the one we used (Dako). As we could not find another way to overcome this limitation, we decided to maintain the cystatin C values without standardization and acknowledge it in the Discussion as a study limitation.