Healthy kidneys keep a large reserve, so they continue working well even after a big share of their capacity is lost. That is why chronic kidney disease so often causes no symptoms until it is advanced. Simple tests can reveal it much earlier, and treatment at that stage can slow it considerably. This is why kidney function tests for early detection are central to long-term health. The kidneys filter waste, balance fluids and minerals, help control blood pressure and support red blood cell production, so losing them affects the whole body.