We thank Delanaye and Pottel for their response letter, which raises some important points. Our main goal was to assess the relative accuracies of eGFR equations based on creatinine, cystatin C, BTP, and B2M in hospitalized older adults. We found that the addition of cystatin C to creatinine-based equations improved accuracy, whereas the addition of BTP and/or B2M did not. This corroborates earlier literature, but it is one of only a few such studies among elderly multimorbid patients. These patients are at a disproportionately high risk of inaccurate GFR estimates and adverse outcomes related to such errors1, but they are underrepresented in the development of most modern eGFR equations.