How does chronic kidney disease progress?

Chronic kidney disease has five stages:

• Stage 1: Urine tests show clear signs of kidney damage. But healthy parts of the kidneys help them to keep working normally overall.
• Stage 2: As well as the signs of kidney damage, the kidneys aren't working at full strength. But their function is only slightly reduced, and there are usually no noticeable symptoms.
• Stage 3: Kidney function is moderately reduced.
• Stage 4: Kidney function is significantly reduced. Problems such as itching, anemia, acidosis or bone pain may occur.
• Stage 5: End-stage kidney disease: The kidneys are no longer able to clean the blood well enough, often resulting in severe uremia (poisoning due to a build-up of waste products in the body). Dialysis or a kidney transplant are then needed in order to do the kidneys' job or replace them.

The health implications of chronic kidney disease will also depend on how healthy someone is otherwise. For that reason, doctors also look into conditions that may make chronic kidney disease get worse faster – including heart disease, poorly regulated high blood pressure or diabetes.

This is important for individually adjusting treatment with medication – or for planning further treatment steps far enough in advance: Someone who has a high risk of kidney failure in the foreseeable future can talk about what treatment would be best with their doctor before it’s too late. Further and more frequent testing might also be needed.

Kidney function and kidney damage are closely related. The glomerular filtration rate is the most important measure of kidney function. Evidence of kidney damage is provided mainly by the amount of protein in the person's urine.

Each kidney has about one million tiny “filtering stations” in it called renal corpuscles. The glomerular filtration rate (GFR) shows how much blood is filtered per minute. In healthy people, this is between 85 and 135 milliliters per minute – based on a body surface area of 1.73 m² that is used for calculating the GFR.

It would be very difficult to measure the GFR directly. That's why it's usually estimated – for instance, using the level of creatinine in the blood. Creatinine is a breakdown product of the muscles. The most commonly used formula for estimating the GFR also includes things like the person's age and sex.

The kidneys usually almost completely prevent certain substances in the blood from entering urine. Kidney damage may become noticeable when those substances are found in urine. Examples include blood proteins like albumin. They are only found in significant amounts in urine if the kidneys are damaged. The amount of protein in urine is taken as a sign of how bad the damage is.