Dr Fiona Gilston BVMS MVetSci (Cons. Med.)
Advancements in veterinary medicine allow vets to screen and monitor for many health conditions. An increase in available blood tests to diagnose conditions also improves the chances of catching some diseases at an earlier stage than before. Kidney disease is one of the commonest diseases to affect our pet cats but detecting it is not always straightforward. One test your vet may run as part of a blood screen is known as SDMA. But what does it really mean if your cat’s level is found to be high?
Function of the Kidneys
As with us, cats have two kidneys, one on either side of the body, tucked up by the base of the ribs, close to the backbone. Blood is pumped through the kidneys via a series of branching blood vessels from where the main role of blood filtration occurs. Byproducts of bodily function as well as some toxins and drugs, are filtered out of the blood here. The product of this filtration process is urine, which is ejected out of the kidney, down a tube called the ureter, and into the bladder where it is stored.
This mechanism is an important part of how the body balances electrolytes such as sodium and potassium. Water is also filtered out or recycled here to manage hydration. Additionally, the pH of the blood can be altered, as needed, by the kidneys. This all influences the concentration of urine, acts to prevent both dehydration or overhydration, and regulates the salt levels within the body.
The kidneys have other roles beyond waste management. They produce multiple hormones which lead to various effects. They regulate blood pressure or send a message to the bone marrow to create new red blood cells. They also play a role in the activation of vitamin D, which is necessary for absorbing calcium from the diet.
What is Kidney Disease?
When the function of the kidneys is damaged or decreased, this is typically referred to as kidney disease, or kidney insufficiency. Some vets may refer to it as renal disease which is the medical term for the same condition. The term kidney disease accounts for any temporary or permanent change in function and describes any stage of the spectrum of disease. When disease and loss of function are severe or advanced, this is then referred to as kidney failure.
Causes of kidney disease can be sudden (acute) or longstanding (chronic). In cats, acute kidney disease can occur with toxin exposure. Lily plants and anti-freeze are two of the more common kidney toxins that are consumed by cats. Accidental overdoses of prescribed medications, or accidental ingestion of some human painkillers or topical creams can also result in acute kidney disease.
Acute disease can occasionally occur from impact trauma as a result of bruising, swelling, or damage to the blood supply or outflow. Other acute onsets may be the result of more chronic conditions elsewhere, such as blood clots resulting from heart disease. Infection both directly within the kidneys, or spreading from elsewhere, can also reduce kidney function.
On the other hand, chronic kidney disease is a very common degenerative condition that can occur as cats age. Although estimates vary between studies, it is likely that at least 40% of cats over the age of 10 will have some form of kidney disease, with over half of cats over the age of 15 expected to be affected.
Chronic changes can also occur secondary to high blood pressure (hypertension) or following prolonged exposure to toxins or infection. Cats that may appear to have recovered from an acute assault on the kidneys can be left with chronic insufficiency. Whereas the acute form will usually have sudden and dramatic signs of illness, the chronic form may not involve obvious or outward symptoms until quite advanced. Symptoms include vomiting, increased thirst and urination, weight loss, lethargy, and loss of appetite.
What is SDMA?
Symmetric dimethylarginine (SDMA) is a form of amino acid that is produced by many cells within the body during the breakdown of protein. It is filtered from the body by the kidneys, but unlike electrolytes or water, it is not reabsorbed for recycling by the kidneys. This means that the level of SDMA in the blood can give an indication of how well the kidneys are doing their job of filtering waste products.
SDMA can be assessed in your cat via a blood test, which your vet will either be able to test in the clinic, or through a local veterinary laboratory. The result is a numerical reading and will be classed as normal when less than 15 (micrograms in a decilitre), mildly elevated when 15-19, or elevated when 20 or more.
How is the SDMA Test Used?
SDMA is not typically run as a solitary test. Your vet will usually include it in a broader blood screen for senior patients or sick cats. This will usually include other kidney markers such as blood urea nitrogen (BUN) and creatinine (CREA), blood phosphorous levels, and urine analysis. Blood markers vary with how sensitive or specific they are to reflecting reduced function, so by testing for several possible indicators, there is a higher chance of an accurate diagnosis.
Although higher levels of BUN and CREA (known as azotaemia) can suggest kidney disease, both are affected by non-kidney factors. For instance, dehydration, bleeding within the digestive system, or recent ingestion of a high protein meal can affect the BUN levels. Muscle loss from old age, or other older cat diseases such as hyperthyroidism, can alter CREA levels. Even where azotaemia reflects kidney disease, these markers may not be abnormal until at least 75% of total kidney function has been lost.
High levels of phosphorous can be detrimental to the health of kidneys, and this can both result from reduced kidney function, and hasten any future decline. Urine is assessed by microscope for evidence of infection, crystals (the building blocks of stones) or shedding from the kidneys (known as casts). It is also assessed for concentration (which can be related to the cat’s hydration to see if it is appropriate), and evidence of bleeding or abnormal pH.
SDMA is far less affected by non-kidney factors, and it has also been shown to reflect kidney disease when as little as 25% of total kidney function has been lost. Even so, an elevated level does not immediately mean that your cat has, or will have, long-term insufficiency.
Determining Kidney Health
If your cat has a mildly elevated SDMA, but there is no other evidence of kidney disease on blood and urine tests, and your cat has no symptoms, your vet will likely recommend monitoring this level with repeat testing several weeks later. If it has reduced to normal at the follow-up, it is recommended to recheck the level 6 months later.
If SDMA remains elevated at the follow-up, is greater than 20 from the outset, or other blood and urine tests are abnormal, then there is likely reduced kidney function. This is also likely if your cat has relevant symptoms even if only the SDMA is high. In these cases, your vet will likely discuss measuring blood pressure, checking thyroid function, and possibly performing a urine culture to ensure other diseases are not putting a strain on the kidneys. They may even recommend having an ultrasound performed to visualise the kidneys.
Management of Kidney Disease in Cats
If an underlying disease is detected, then this needs to be treated in addition to supporting kidney health. Correction of high blood pressure, elimination of infections, or control of hyperthyroidism may allow the kidney function to improve. If so, SDMA and other blood/urine markers can be monitored over time to gauge the trend in results and assess for reduction or stabilisation.
The severity of kidney disease is graded according to the International Renal Interest Society (IRIS). The range of abnormalities across the blood and urine tests are used to assign a grade of 1 (early stage/mild) to 4 (late stage/severe). The IRIS grade determines how the disease should be managed. The main stay of support at all grades involves dietary adjustments and improved hydration. For mild cases, this will occur at home.
With more severe, or advanced disease, hospitalisation may be required to combat dehydration or flush out toxins. Cats or more likely to be vomiting and have an absent or low appetite at this point. These require medications to prevent nausea or increase appetite, or a feeding tube may be necessary to get an adequate number of calories into their body.
Treatment also needs to be individualised to you and your cat. Not every cat can be given pills, different cats will have differing tolerances to diet adjustments, and not every cat will cope with being hospitalised for care. Your vet can also discuss the financial involvement of repetitive blood tests and ongoing treatments.
Conclusion
Kidney disease is a commonly diagnosed condition in our pet cats, but it is not always straightforward to confirm via tests. SDMA is one of several tests that your vet can utilise to help diagnose the disease, and it has the potential to pick up reduced function at an earlier stage than some other tests. It is also useful for monitoring the disease over time by checking the trend in results. Alongside other blood and urine tests, SDMA allows grading of kidney disease, leading to a standardised management plan that can be adjusted to your cat’s tolerances.
Further Reading:
IRIS treatment recommendations
IDEXX, SDMA and hyperthyroid cats
Discussion