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 aa 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.