Je-Hyan Lee et al. have published a study on cystatin C concentrations in the first 30 days of life in 127 pre-term and 119 term neonates in this edition of Pediatric Nephrology, thereby closing a knowledge gap of detailed cystatin C concentrations beyond 72 h of life by day of life and by post-conceptional age. While the study objective has merit and a large number of measurements were included, there are some methodological limitations that bring the validity of the data into question as pure reference intervals for children up to 1 month of age, mostly because of the inclusion of patients that potentially could have an impaired glomerular filtration rate (GFR), for instance due to exposure to nephrotoxic drugs. We discuss the strengths and weaknesses of the study and outline an approach to definitely close this knowledge gap. We call for a worldwide collaboration to use Box-Cox transformations similar to the methodology used with growth charts to calculate age-independent z-scores and percentiles of neonatal and infant markers of GFR. This could also lead to better definitions of acute kidney injury in infants if GFR markers cross the percentiles based on post-conceptional or chronological age.