The kidneys remove waste and water from the blood. In chronic kidney disease, kidney function worsens over time until the kidneys stop working and dialysis is needed. Dialysis uses machines to filter the blood. Caring for kidney disease involves controlling diet, controlling blood pressure, and taking medications to delay heart complications and the need for dialysis. Unfortunately, many patients do not know that they have chronic kidney disease and do not get care to delay complications. Glomerular filtration rate (GFR) is a measure of kidney function. Chronic kidney disease is defined by a GFR less than 60 mL/min per 1.73 m2. Cystatin C is a substance that increases in the blood as kidney function worsens. Some researchers believe that there is a stage between completely normal kidney function and chronic kidney disease called “preclinical kidney disease” that also puts people at risk for bad health outcomes. One way to define preclinical kidney disease is by high cystatin C levels in patients with normal GFR. If preclinical kidney disease puts people at risk for bad health outcomes, treatments for this early stage of kidney disease might improve those outcomes.