Blood tests


The usual tests to diagnose CKD measure the levels of creatinine and urea in the serum. In chronic kidney disease (CKD), both values are elevated (azotaemia is an increased concentration of breakdown products, which contain nitrogen, caused by protein metabolism in the blood.

Diagnosis of CKD, however, requires more than just a finding of elevated creatinine and urea in the blood or a positive SDMA test. According to IRIS guidelines, a CKD diagnosis must be corroborated by further findings.

Creatinine is a breakdown product of muscle metabolism and is one of the Creatinine itself is regarded as an indicator of the filtration capacity of the kidneys, as it is only via the kidneys that it can be excreted. However, several factors can affect creatinine blood levels. These include periods of hunger, fluid intake, muscularity and nutritional status of a cat, and this is precisely the case in lean and emaciated cats, in whom the creatinine level may be too low for the aforementioned reasons.

Each cat therefore has its own normal values. The normal (reference) value for creatinine is relatively wide in cats: approximately 40–170 µmol/l. That’s why, even if your cat is healthy, it’s a good idea to take your cat for an annual blood creatinine test; you’ll find out its individual normal value. This is important, because an increase within the normal range, along with specific urine tests and the SDMA test, can give you an early warning for CKD. The International Renal Interest Society (IRIS) regards creatinine as the main criterion for diagnosis and classification into stages.

SDMA test
Symmetric dimethylarginine (SDMA) is a breakdown product of the amino acid arginine and is excreted exclusively via the kidneys. SDMA is a highly specific biomarker for cat’s kidney function. According to the manufacturer IDEXX, it can identify CKD in cats at an early stage, when 60% of the nephrons are still functional and creatine elevation or clinical symptoms are not yet present.

The advantage of SDMA compared to creatinine is that it isn’t influenced by other factors such as muscle mass. It also increases at a very early stage of CKD, because it is highly correlated with a decrease in the functional capacity of the kidneys – the glomerular filtration rate (GFR).

A study on the SDMA test (Hall et al. 2014) demonstrated that it was able to identify CKD on average 17 months earlier than creatinine blood levels.

The SDMA test can identify CKD at an early stage.

An SDMA level of >14 µg/dl is indicative of CKD.

Earlier detection of CKD may contribute to better and more targeted treatment, with the potential of prolonging and improving the quality of life of a cat. I, therefore, personally recommend taking your cat for annual kidney check-ups from the age of 5 years.

A blood urea nitrogen (BUN) test measures the amount of urea nitrogen in the blood. Urea is a nitrogenous breakdown product of protein metabolism that must be excreted in the urine. While urea is in itself not a toxin, it is an indicator for poisonous (toxic) breakdown products of protein metabolism (uraemic toxins), which are more difficult to measure. If the kidneys are no longer able to fully filter out urea, it accumulates in the blood. This is what you would expect in CKD. Blood urea levels, however, also depend on other factors, for instance a cat’s nutritional status, dehydration, high blood pressure, gastrointestinal bleeding, diabetes and kidney stones. Hence, an elevated urea level alone is not a clear indication of the presence of CKD. Urea levels should always be considered together with creatinine levels. If creatinine is also elevated, this suggests a kidney disorder.

Increased urea levels alone can also be due to other causes, such as a high protein meal. Therefore, cats need to fast for about 8–12 hours before undergoing a urea test. Elevated urea and creatinine levels are also known as azotaemia (an increase in the levels of nitrogen-containing urinary substances in the blood). The International Renal Interest Society (IRIS) classifies CKD into four stages based on the presence and severity of azotaemia as measured by creatinine levels. Normal blood creatinine is defined as concentrations of 3.5–8.0 mmol/l, while a normal BUN level is 9.8–35.0 mg/dl.

An elevated blood phosphate level (hyperphosphataemia) also indicates the presence of CKD, as the chronically damaged kidneys can excrete less phosphate. However, other factors can also influence phosphate levels, for instance the meat content of food and time of last meal. In addition, phosphate levels in young cats are always higher. In isolation, an elevated phosphate level is as inconclusive as an isolated elevated urea level. Only a combination of elevated creatinine, urea and phosphate blood levels indicates the presence of CKD.

Further tests, such as urine analyses, then need to be carried out for verification. In the context of CKD management, phosphate levels provide a good indication of whether phosphate-reduced therapy has been effective.

Phosphate concentrations should be below 1.9 mmol/l, otherwise an appropriate phosphate-reducing therapy should be undertaken. The normal phosphate range is approximately 0.8–1.9 mmol/l.

Blood phosphate should be below 1.9 mmol/l.