In addition to regular check-ups, which usually include a blood test, a urine test is also recommended for senior cats aged 9-13 years. Especially for the examination of the cat’s kidney health, it can provide valuable information, as it is produced directly in the kidneys. Urine production is a kidney filtration process, the reduction of which can indicate the possible presence of chronic kidney disease (CKD) in the cat.
It is not only from pregnancy tests that we know that urine is a good examination medium for detecting changes in the body. It is not only important which molecules and components or admixtures (e.g. blood) are contained in and in what quantity. The amount of water is also relevant. The water content often also determines its volume. In addition to crystals (indication of stone formation), bacteria (e.g. urinary tract infections, but the microbiological examination of the urine is also positive in 30% of CKD cats) and cells (e.g. bladder tumours), the urine can also contain proteins in the diagnosis of CKD. These can indicate the presence of proteinuriaIncreased protein in urine, leading to loss of protein. Normally, only a very small amount of protein is excreted in the urine. In chronic kidney disease, the amount lost may increase. High blood pressure damages blood vessels in the nephrons making them more permeable and... (loss of proteins through the urine).
Besides this, urine naturally contains so-called urinary substances. These are water-soluble harmful substances, such as uraemic toxinsToxic, nitrogen-containing urinary substances responsible for uraemia and kidney damage...., which are degradation products of protein metabolism. Urinary substances also include other degradation products. Be it from the metabolisation of food or, for example, pharmaceuticals. The kidneys are excretory organs that cleanse the blood and thus the body of these harmful substances and breakdown products. These substances leave the cat’s body through the urine.
Urine is not to be stored
However, urine is an unstable sample material. It makes special demands on the time of collection and testing. For instance, it can only be stored for about 30 minutes at room temperature.
Morning urine, for example, is well suited for assessing the concentration capacity of the kidneys. However, this requires several consecutive examinations. The concentration capacity of urine is a kidney function to recover as much water as possible from the produced urine at the place of its formation, i.e. in the kidneys. In the kidneys, when urine is produced, a lot of water is initially squeezed out into the filter units (nephronsNephrone sind die Filtereinheiten der Nieren....), which then has to be recaptured in subsequent sections of the kidneys. The fluid balance is very fragile. Even a 2% alteration of the body’s water level can be detrimental to the cat’s health.
Specific urine weight decreases in CKD
The urine’s ability to concentrate is measured as the specific urine weight. If the specific urine weight decreases, the urine becomes more pale as it contains more water. The volume of urine may be increased at the same time, so that the cat pees significantly more (polyuriaExcessive production of urine, potentially due to kidney failure....). The kidneys lose their ability to concentrate when kidney function tissue is deteriorated during the course of CKD. However, a low specific urine weight can also occur if the cat has been drinking a lot or has been receiving IV fluids. This must be taken into account when assessing the specific urine weight. However, specific urine weight is never assessed in isolation, but only in combination with a blood test.
If the cat loses a lot of water through the kidneys because the kidneys have lost their ability to concentrate in the course of CKD and the urine volume (polyuria) increases, the cat has to take in a lot of water at the same time. In the past, there was a misconception that cats “properly rinse” their kidneys when they drink a lot (polydipsia=PD) and pee a lot (polyuria=PU). Nowadays we know that this water loss via the kidneys is a sign of impaired kidney performance and is by no means to be judged positively. PU and PD are usually the first signs of kidney dysfunction. Although by this time more than 70% of the kidney function tissue has already been destroyed. It is important that the cat drinks a lot at the same time as it loses a lot of water. Otherwise it will become dehydrated.
Urine collection is difficult in cats
There is suitable litter for the cat’s litter tray to collect urine, which is then given to the veterinary practice for examination. In addition, the bladder is usually punctured by the vet so that fresh urine can be examined directly. This is a procedure that not every cat will readily endure. Therefore, urine testing is used far too infrequently to diagnose CKD.
Test strips that are held into the litter tray by cat owners can give very different and sometimes confusing results. In the case of proteinuria, however, they can sometimes be the first indication. Unfortunately, most of the time the result is not conclusive. Proteinuria is assessed with the so-called UPC (= urine protein-to-creatinine ratio), which is the ratio of urine protein to creatinineCreatinine is a breakdown product of muscle metabolism. Its blood level is dependent, among many factors, on age, weight, nutritional status and muscle mass. Hence, creatinine levels in the blood vary from individual to individual. Creatinine is continuously excreted in the urine. Increased blood creatinine.... The International Renal Interest Society (IRIS) has defined the following values for the assessment of proteinuria:
Proteins are increasingly squeezed out
Proteinuria occurs with high blood pressure (hypertensionHigh blood pressure.). In the filter units (nephrons), more proteins are then squeezed out of the blood into the urine, which otherwise would not be excreted. The reason for this is an increased filtration pressure due to the increased blood pressure. Physiologically, only very small amounts of protein can be detected in the urine. In the case of CKD, the impaired kidney tissue is also unable to retain these proteins, so that the proteins are lost to the body.
Increased protein excretion can occur with inflammation, urinary stones and also in relation to feeding or sex (male cats). Therefore, further urine tests for cells, bacteria and urinary stones are recommended to verify the diagnosis of proteinuria. In addition, 3 consecutive analyses of the UPC within 1-2 months are recommended to confirm the presence of potential proteinuria.
The proteins excreted are mostly albumins and globulins. Their significant loss can lead to oedemaExcess fluid collecting in the tissues of the body, which can cause them to become swollen...., i.e. water retention in the body. This is because proteins lead to oncotic pressure, which ensures that fluid is kept in the blood and does not leak into the tissues. If the body loses these important plasma proteins through the urine, water can leak into the tissues.
In addition, protein loss further contributes to the emaciation of a CKD cat.
Checking for the presence of proteinuria is therefore very important in the management of a CKD cat. Proteinuria is treatable (→ see treatment for more information).
Moreover, as a precaution, blood pressure should be checked every year in senior cats and every 3-6 months in cats diagnosed with CKD.
Bibliography:
- Sparks, A. H. / Caney, S. / Elliot, J. / Finch, N. / Gajanayake, I. / Langston, C./ Lefebvre, H. / White, J. & Quimby, J. (2016): ISFM Consensus Guidelines on the Diagnosis and Management of Feline Chronic Kidney Disease. Journal of Feline Medicine and Surgery,18, S. 219–239.
- IRIS-Staging: http://www.iris-kidney.com/pdf/IRIS_Staging_of_CKD_modified_2019.pdf
- http://www.iris-kidney.com
- ISFM Consensus Guidelines on the Diagnosis and Management of Hypertension in Cats: https://journals.sagepub.com/doi/pdf/10.1177/1098612X17693500
- Proceedings Pet-Vet-Thementag Niere & Harntrakt 12. Juni 2021 (Online)
