A Methodological Study to Learn More About Kidney Function in Healthy People and in People With Reduced Kidney Function

The main purpose of this study is to investigate how reliable a test assessing the renal functional reserve (RFR) and the maximum glomerular filtration rate (maxGFR) are. MaxGFR and RFR assessments are repeated twice at 1-month intervals. If precise, this test may improve the measure of kidney function and be useful in future studies for people with chronic kidney problems. To do this, the researchers will collect and analyze information about kidney function from up to 50 adults, including healthy people and people with stable kidney problems. Each person will come to the study center for a screening visit and then three study days, each about a month apart. On each study day, participants will have blood and urine samples taken before and after drinking a protein shake. The tests will measure things like creatinine, cystatin C, and other markers in the blood and urine to see how the kidneys are working. The study will also record basic health information, weight, blood pressure, and any medicines being taken. The study will last about 2 to 2.5 months for each participant.