Editor,

We thank Dr. Chia-Ter Chao for his comments about our paper. In responding, we will take the opportunity to correct a mistake. Unfortunately, in the paper, we reported the wrong dose of cimetidine: Instead of 800 mg OD (once per day), it should have been written 800 mg BID (twice a day), a dose most recommended in the literature for achieving a complete creatinine secretion inhibition (we referenced these articles in our original paper). We thank Dr. Chao for pointing out this error and giving us the opportunity to correct it [1–3]. Regarding Dr. Chao′s comment on the accuracy of the method used for evaluating the renal reserve, we would like to emphasize that to evaluate this physiological variable we obtained the DELTA value between a peak GFR value and a basal one; thus, obtained DELTA value will be an accurate representation of the magnitude of GFR increase, independently of the accuracy of the applied method for measuring it. Regarding Dr. Chao′s concern about the accuracy of urine volume measurement, we would like to clarify that all the volunteers had a normal bladder voiding (presence of urine retention was one of the exclusion criteria evaluated before the study), and additionally, their urine residual volume was checked by bladder ultrasound after each micturition (this is mandatory when a timed clearance is evaluated). We agree with Dr. Chao that more research is needed in order to know how the aged kidney handles not only creatinine but also cimetidine.