The term chronic kidney disease (CKD) is used worldwide to mean any form of kidney disease that goes on for more than a few months.
The word 'chronic' doesn’t necessarily mean 'serious' and the word 'disease' includes any abnormality of kidney structure or function, whether or not it is likely to cause a person to feel unwell or to develop complications.
Chronic kidney disease
There are various stages of CKD, ranging from mild loss of kidney function to complete kidney failure, but not all CKD conditions progress to the most serious stage. Most people fall into the mild to moderate categories where hospital-based care is not necessary.
If you have CKD your doctor should regularly test your urine and blood to find out if there is any kidney damage, how serious the condition is, whether it is getting worse and what treatments you might need to manage your symptoms.
How is kidney disease measured and divided into stages?
Kidney disease is measured and categorised using a blood test which gauges the eGFR (estimated glomerular filtration rate) and a urine test which shows the ACR (urine albumin:creatinine ratio).
Glomerular filtration rate: GFR and eGFR
The main function of the kidneys is to clear water-soluble waste products (such as creatinine) from the blood. The efficiency of clearance is measured using the GFR
- The GFR (glomerular filtration rate) is a measure of how much blood (in millilitres) is ‘cleaned’ by the kidneys in one minute. By using a blood test to measure creatinine levels we can calculate the estimated GFR, known as eGFR. We do this using a formula that takes body size into account. The average adult body size is 1.73 m2. A normal GFR is around 100 ml/min/1.73m2
- Because the average normal eGFR is 100, the eGFR can be seen as a percentage of normal kidney function. However, values as low as 60 are considered normal if there is no other evidence of kidney disease
- The eGFR is used to calculate the ‘G stage’ of CKD (as indicated in the Stages of CKD chart)
- The eGFR calculation can be inaccurate in people who have much more, or much less muscle than average people of their age, sex, and ethnic origin. For instance, it can be falsely low in body builders, and falsely high in people with anorexia nervosa.
Albuminuria: the urine albumin:creatinine ratio
A simple urine test called the urine Albumin:Creatinine ratio (ACR) is also performed to look for signs that protein is leaking into the urine (a condition called albuminuria or proteinuria). This is an important sign of kidney damage. The ACR is used to calculate the ‘A stage’ of CKD (as indicated in the Stages of CKD chart below).
There are three recognised stages of albuminuria:
- A1 – normal to mildly increased urine protein levels (lower than 3mg/mmol)
- A2 – moderately increased urine protein levels (between 3-30mg/mmol)
- A3 – severely increased urine protein levels (higher than 30mg/mmol)
Combining your ACR ratio with your eGFR can help doctors try to predict whether your kidney disease is likely to progress and whether you are more likely to develop complications such as heart and circulatory problems. Other factors are also taken into account, e.g. age. In general, the higher the ‘A’ stage, the more likely it is that you will benefit from blood pressure-lowering treatment.
Explaining the stages of kidney disease
This chart shows the stages of kidney disease and outlines how disease is likely to progress if kidney function reduces and urine protein levels increase.