We agree with the first conclusion of Iversen et al1 regarding the added value of cystatin C to estimate glomerular filtration rate (GFR) when used in combination with serum creatinine (sCr). This result is a confirmation of previous observations from much larger cohorts.2,3 It is a pity that this analysis has not taken into consideration the new European Kidney Function Consortium (EKFC) cystatin-based equation (and the combined equation).2 Also, the reference method used for GFR measurement is not ideal.