Rowe C, et al. - In 20 nephrology outpatients (10 male, 10 female; median age 71, range 50-80 years) suffering moderate CKD [glomerular filtration rate (GFR) 30-59 mL/min/1.73 m²], researchers analyzed biological variability of measured and estimated GFR. Over four consecutive weeks, participants were asked to undergo weekly GFR measurement by iohexol clearance. At the same time, these subjects underwent GFR estimation using the Modification of Diet in Renal Disease [MDRD], Chronic Kidney Disease Epidemiology Collaboration [CKD-EPI]_creatinine, CKD-EPI_cystatinC and CKD-EPI_{creatinine+cystatinC} equations. Compared with measured GFR, a lower within-subject variability of estimated GFR was identified. In clinical practice, GFR changes can be determined using reference change values. Therefore, patients can be monitored over time with the estimates of GFR, at least as reliably as with measured GFR.