What Happens in CKD

Overview

This page explains more about the various functions of the kidneys.
Since the kidneys have so many functions, there are many possible symptoms of CKD. The symptoms you might see depend upon which functions are affected.
This page also explains why CKD is so common in cats, and why CKD cannot normally be detected until two thirds of function has already been lost.

Table of Contents

Main Functions of the Kidneys

The kidneys have five main functions:

the regulation of fluid levels in the body
the regulation, including filtering and disposal, of waste products in the body
the regulation of electrolytes (salts in the body’s cells which are necessary for survival) and minerals in the body
stimulation of red blood cell production
production of renin, which controls blood pressure

In cats with CKD, as the kidneys become more and more damaged and their ability to function declines, an imbalance can arise in any or all of these areas. So, for example, a cat whose kidneys struggle with the production of red blood cells will develop anaemia.

How the Kidneys Work

In order to understand what happens in CKD, it helps to have a rough idea of how the kidneys work.

Nephrons

The main work of the kidneys is performed by units called nephrons. The nephrons filter the blood flowing into the kidneys via their glomeruli (see below). Healthy cats have about 170,000 – 190,000 nephrons, far more than they need, which is known as the renal reserve.

Khan Academy has a clear video showing how the nephrons work.

Glomeruli and Glomerular Filtration Rate (GFR)

Each nephron contains a glomerulus, a very fine blood vessel. The glomeruli perform the first stage in the process of filtering waste products to produce urine. GFR measures their filtering ability.

If the glomeruli are damaged, blood proteins, which would normally be recycled into the bloodstream, may leak out into the urine (proteinuria).

The glomerular filtration rate (GFR) is an accurate measure of how effectively the glomeruli and nephrons are working, but unfortunately this is difficult to measure, so it is not checked very often.

Long Beach Animal Hospital clearly explains more about the way the kidneys work.

Kidneys (2025) Preminger GM MSD Manual explains kidney function (with diagrams).

There are a number of different causes of kidney disease. Management and treatment of chronic kidney disease in cats (2016) Caney S In Practice Oct 2016 pp10-13 states “CKD is the end result of a wide range of primary disorders that cause irreversible damage to nephrons, eventually leading to reduced glomerular filtration rate (GFR).”

Interstitial nephritis is the most commonly seen type of kidney problem in cats, so when a vet says a cat has CKD, s/he often means chronic interstitial nephritis.

Chronic interstitial nephritis is not a disease as such. Renal fibrosis in feline chronic kidney disease: known mediators and mechanisms of injury (2015) Lawson J, Elliott J, Wheeler-Jones C, Syme H & Jepson R The Veterinary Journal 203(1) pp 18-26 says “In most cases the underlying aetiology is unknown, but the most frequently reported pathological diagnosis is renal tubulointerstitial fibrosis. Renal fibrosis, characterised by extensive accumulation of extra-cellular matrix within the interstitium, is thought to be the final common pathway for all kidney diseases and is the pathological lesion best correlated with function in both humans and cats.”

Renal fibrosis in cats Brown SA International Renal Interest Society discusses factors that may contribute to the development of fibrosis and CKD.

In more simple terms, regardless of whatever is the initial cause of the CKD, inflammation and fibrosis (the formation of excess fibrous connective tissue) occur and the end result is a cycle of kidney damage. Cats with chronic interstitial nephritis have small, shrivelled kidneys with scar tissue, and this is what is seen in most CKD cats. Feline chronic kidney disease (2015) Grauer GF Today’s Veterinary Practice 5(2) pp36-41 says “the end point of irreversible glomerular or tubular damage is the same: fibrous scar tissue replacement of nephrons.”

Renal dysfunction in small animals (2016) Brown SA Merck Veterinary Manual has some information on interstitial nephritis and CKD.

Why CKD Occurs

There are a number of reasons why CKD occurs in cats, see above and Causes of CKD. However, in most cases there is little you could have done to prevent it because one of the main factors in the development of CKD in cats is aging. Studies indicate that around 10% of cats over the age of ten will develop CKD. Older cats are at even greater risk: Chronic Renal Disease in Cats (1989) Current Veterinary Therapy X, Krawiec DR & Gelberg HB, Ed. Kirk RW, WB Saunders Company p. 1170-1173 found that 30% of cats over the age of 15 had the disease.

A more recent study, Disease surveillance and referral bias in the veterinary medical database (2010) Bartlett PC, Van Buren JW, Neterer M & Zhou C Preventive Veterinary Medicine 94(3-4) pp264–271, found that 28% of the cats over the age of 12 who were examined at four US veterinary hospitals had CKD.

Therefore, if you have a cat aged 12-15 or over, s/he has a one in three chance of developing CKD.

Younger cats may also develop CKD but it is less likely. If they are very young (less than two years old), they may have a congenital problem. Other possible causes include kidney infections, blockages or exposure to toxins (though cats with these problems may have acute kidney injury).

The Cat Doctor mentions that CKD occurs twice as often in Siamese, Maine Coon, Abyssinian, Russian Blue and Burmese cats.

Risk factors in dogs and cats for development of chronic kidney disease (2024) Roura X International Renal Interest Society discusses the various factors that may lead to CKD.

Why CKD Cannot Normally Be Detected at an Early Stage

I often hear from people who are kicking themselves for not realising sooner that their cat was sick. But it is actually normal for CKD not to be diagnosed until as much as 66% of function has been lost, if not more.

The 170,000 – 190,000 nephrons found in a cat’s kidneys are many more nephrons than are needed for normal function; plus nephrons can increase their individual function to some extent when other nephrons die. This is why people can donate a kidney and still manage perfectly well with one kidney. In the case of a kidney transplant, if you remove one kidney from the donor, the donor’s glomerular filtration rate (GFR, a measure of kidney function) will immediately fall to half of what it was, but will then gradually improve as the remaining nephrons increase their function to compensate for the loss of one kidney. Eventually the nephrons in the remaining kidney will reach almost the same level of function as two kidneys.

It works in a similar way in a cat with kidney disease, i.e. as damaged nephrons die (they are often described as “scar tissue”), other nephrons take over their work. Eventually, however, all the remaining nephrons will be working fulltime (i.e. there will be no renal reserve left). It is at this point, when around 66-75% of function has gone, that you will probably start to see symptoms in your cat, as the remaining nephrons start finding it harder to cope with the workload.

Feline chronic kidney disease (2015) Grauer GF Today’s Veterinary Practice 5(2) pp36-41 states “Progressive diseases that slowly destroy nephrons allow intact nephrons to undergo compensatory hypertrophy, which can delay onset of renal failure. Therefore, when renal failure occurs (< 25% of the original nephrons functional), nephron hypertrophy can no longer maintain adequate renal function.”

So please do not feel guilty for not noticing sooner — there was probably nothing for you to notice, plus cats are very good at hiding signs of illness. CKD is not normally painful, so this makes it easier for the cat to hide what is happening.

There are a number of possible methods of Early Detection, but some of these are quite specialised, and most people wouldn’t know about them; in fact, not all vets are familiar with all of them. Even the SDMA test can only detect CKD when 40% of function has gone.

Don’t waste your energy beating yourself up. What you need to focus on is the fact that cats with CKD can often manage quite well on limited kidney function. For some cats, things only become critical when they have lost as much as 90% of function, and there are some cats who cope astonishingly well with even less function. Renal dysfunction in dogs and cats (2025) Van Vertloo L MSD Veterinary Manual says “With appropriate therapy, animals can survive for long periods with only a small fraction of functional renal tissue, perhaps 5%-8% in dogs and cats.”

So the goal is not to worry about the function that has already been lost, but to try to retain the remaining function for as long as possible, and keep your cat feeling as well as possible. This site aims to help you with both goals.

Links to More Information

Cornell University College of Veterinary Medicine has a helpful video overview of feline kidney disease. Click on Part V first, Understanding kidney disease.

Kidney disease (2007) Syme H Royal Veterinary College is a 22 minute long podcast which you can download. Scroll down to RVC10 if necessary.

This page last updated: 03 June 2025

Links on this page last checked: 03 June 2025