CKD stages

The International Renal Interest Society (IRIS) has classified chronic kidney disease (CKD) in cats into four major stages based on blood creatinine concentration. In addition, it recognises three further substages based on the presence of proteinuria and four further substages based on blood pressure.

This classification facilitates diagnosis, communication and treatment of CKD in cats.

Cats are staged only after diagnosis of CKD has been , using the three diagnostic approaches mentioned above.

The main criterion, blood creatinine level, is evaluated by means of at least two tests 14 days apart. Staging requires a stable value. Serum creatinine is considered to be stable if the variation between successive measurements is less than 20%.

It is important to note that the first evidence of CKD is the increase in the cat’s personal creatinine level (dependent, for instance, on weight and muscle tone), which can be well below 140 µmol/l. You should therefore get your cat’s kidneys checked annually by your vet from the age of five years, so that an increase in the cat’s personal level can be detected early on, even before clinical symptoms arise.


Stage 1

Stage 1 CKD often goes unnoticed, because, at this earliest stage, clinical symptoms are not yet strongly present and often not recognised: the beginnings of a dull coat; increased drinking (Polydipsia); increased urine flow (Polyuria); and the first signs of fatigue. The blood creatinine level is still in the normal range as kidney function is only slightly impaired. IRIS guidelines suggest that the presence of proteinuria should be clarified, although it is rarely expected at this stage; in most cases, the urine protein-to-creatinine ratio (UPC) is below 0.2. High blood pressure may have already developed and should be tested for and then appropriately treated.

CKD-triggering factors should be identified at this stage and – as far as possible – eliminated. Caution is advised in employing kidney-damaging drugs. You should carefully monitor and encourage your cat’s drinking.

Stage 2

In this stage, CKD symptoms become more prominent. Lack of appetite, drowsiness, increased drinking, increased urination and weight loss are now more likely to be noticed by owners. Kidney function, now reduced by up to 33%, brings with it the first signs of changes in blood values. According to IRIS, blood creatinine at this stage is 140–250 µmol/l, and urea is also elevated, so that the onset of azotaemia can be detected. Proteinuria is usually present as evidenced (according to IRIS guidelines) by a UPC ratio of over 0.4. Blood pressure needs to be controlled and, if elevated, treated with appropriate medication. Phosphate levels in the blood need to be monitored and kept stable below 1.5 mmol/l through the use of phosphate binders and/or phosphate-reduced food (kidney-friendly diets). Reduced potassium blood levels (hypokalaemia) and metabolic acidosis should be appropriately treated. Treatment is therefore refocused to slowing CKD progression. At this stage, cats are still likely to survive for more than three years.

Stage 3

At this stage, the clinical symptoms aggravated by uraemia are usually so obvious that they no longer go unnoticed by many cat owners. Uraemia is caused by the kidney function being reduced to a maximum of 25%; the kidneys are no longer able to adequately excrete the waste products they would normally eliminate such as uraemic toxins, which is why they accumulate in the blood. Thus, creatinine and phosphate are also no longer sufficiently filtered out by the kidneys and their accumulation in the blood can be detected as moderate azotaemia. According to IRIS, at this stage creatinine levels are 251–440 µmol/l. Since the kidneys are no longer able to excrete enough phosphate, phosphate intake should be reduced. In most cases, a combination of phosphate binders and phosphate-reduced feed (kidney-friendly diets) is recommended, in order to keep the phosphate level at this stage below 1.6 mmol/l. If the blood becomes acidotic (metabolic acidosis – low blood pH), this should be treated, likewise high blood pressure and proteinuria. At this stage infusions may be necessary to ensure fluid intake. If the haematocrit falls below 20%, treatment of the anaemia is also advisable. The cat’s appetite, which is often significantly reduced due to vomiting, nausea and stomach acidity, should be stimulated by medication, where necessary, to keep up the cat’s energy levels. Measures to provide support for the kidneys are aimed at slowing down CKD progression and improving quality of life by alleviating clinical symptoms. Cats in stage 3 are still likely to survive for about two years.

Stage 4

This last stage is characterised by progressive clinical symptoms, which should be appropriately treated. The focus of treatment should be relief of clinical symptoms such as severe nausea, vomiting, loss of appetite, drowsiness, emaciation and dehydration, to increase or, at a minimum, stabilise the cat’s quality of life. High blood pressure (due to its detrimental effects on vision, such as retinal detachment and even blindness), proteinuria and anaemia must be treated with drugs, and metabolic acidosis must be managed. Kidney function is reduced to less than 10% of normal and is accompanied by a high degree of azotaemia (an increase in blood creatinine above 440 µmol/l) and increased blood urea and phosphate. Phosphate levels should be kept below 1.9 mmol/l with phosphate-reduced feed and phosphate binders. The aim is to make the remaining (on average) one to three months of the cat’s life as comfortable as possible.


Three substages based on the presence of proteinuria

For measuring it is important to repeat UPC tests at intervals of 10–14 days. Proteinuria may also be present in inflammation of the urinary tract or bladder. This should definitely be excluded.

Four substages based on blood pressure

High blood pressure can be accompanied by proteinuria, because high blood pressure damages the blood vessels of the glomerulus allowing more protein to pass out into the urine.

However, high blood pressure can also lead to organ damage, for instance of the eye (retinal detachment, which can lead to blindness), the brain (stroke), heart and kidneys. Defining the risk of such damage is the rationale for the different IRIS stages.

The individual stages are associated with specific diagnostic recommendations, which should be followed.

Finally, it should be emphasised once again that even if your cat is classified as late stage 3 or 4, this does not mean that their “fate is sealed”. Many cats lead a surprisingly good life with very little residual kidney function. However, staging facilitates standardisation of treatment options.