A reflection from a nephrology pharmacist on estimation of kidney function and what that means to clinical practice. Why are we still talking about this- why we need to not focus on a single number? A consideration of the mechanics of kidney function estimates, what is GFR and how is it measured versus how it is estimated. Highlighting the inaccuracies in estimation of GFR and the requirement to consider other factors when estimating kidney function and the impact this has on drug dosing. Some examples of going down the rabbit hole when considering dose adjustments based on kidney function and future directions for kidney function estimates.