When the [creatinine] is changing, the kidney function can still be tracked with a quantitative technique called kinetic glomerular filtration rate (GFR). The equation yields useful information on the severity of acute kidney injury, the clinical course of kidney and dialysis clearances, and the timing of kidney recovery. It has been validated in at least 3 independent studies, where it performed sufficiently well in intensive care unit and kidney transplant settings, and in head-to-head comparisons with biomarkers. Because it is based on a mathematical model, the kinetic GFR faces limitations depending on the accuracy of its assumptions. As the assumptions more accurately reflect the complexities of biology, some of these limitations can be overcome in a more sophisticated model. Kinetic GFR is an easy-to-use, low-cost tool that should be more widely incorporated into medical practice.