In order to adjust the dose of renally excreted drugs in response to reduced renal function, it is necessary to make a quantitative estimate of the glomerular filtration rate (GFR) of the patient. Traditionally this has been done with the use of the Cockcroft and Gault equation or a measured creatinine clearance. More recently the MDRD (Modification of Diet in Renal Disease) formula has become available, providing an estimate of GFR readily available on routine pathology reports. The presence of these different methods of assessing renal function has created some confusion for healthcare workers as to the best approach. In this paper the two methods are compared, together with a newer formula CKD-EPI (named after the Chronic Kidney Disease Epidemiology Collaborative), and a proposal is made for future practice.