Reply--Gender difference in GFR dependent on age

Sir, Rook et al. have made some comments on my work on renal function in relation to age [1]. They state that my conclusion is not warranted, since I have no longitudinal data. It is true that I have no longitudinal data, but I think these authors will agree with me that it is very hard to follow longitudinally healthy adults from age 20 to 70 years with adequate renal function tests, including continuous infusion and urine sampling. They also state that I have very few donors above 50 years of age; I believe that I pointed this out adequately in my study, in the Abstract, the Introduction, the Discussion and in the Conclusion. Moreover, I think that the period before 50 years of age is of special interest, since several authors claim that the decline in GFR does not start until 40–60 years of age [2–4]. Furthermore, that age range is the time when females still have an influence of oestrogens, which I think is the reason for the absence of decline in GFR and ERPF with age in females. Rook et al. also claim that the number of subjects studied was relatively small. Looking at the data presented by Rook et al., the age distribution of their subjects does not seem uniform. Based on the figures in their study and calculating the number of males and females in each decade up to the age of 50 years, i.e. the same age range as in mine, the numbers in their data and in mine are as follows: