- Estimated GFR ≥ 40 ml/min/1.73m2 while taking ACEI/ARB therapy. The GFR will be estimated using the 4 variable MDRD equation as published in the NKF-CKD guidelines.[9] The same NKF-CKD guidelines also promote the use of estimated GFR (GFRest) values rather than serum creatinine levels or CrCl measurements as the preferred non-invasive method of determining glomerular filtration rates.[9]