Chronic renal insufficiency - Diagnosis
Blood tests in conjunction with urine tests are the only way to definitively diagnose renal insufficiency. Affected cats will have an increase in the two main kidney markers - blood urea nitrogen (BUN) and creatinine (see the overview again to remember why these markers go up in renal insufficiency). When these two tests are elevated, we refer to the condition as azotemia. When we know that the azotemia is due to kidney disease, it is called renal azotemia.
We care about azotemia because urea in the blood is a toxin, and it makes the cat feel sick. And I mean literally sick - nauseated, and nauseated cats don't eat. As well, there are other disease conditions that result, including parathyroid hormone imbalances, toxic products from the abnormal breakdown of body protein, and other metabolic "derangements".
Other things besides kidney disease can cause azotemia (dehydration, for example), so a urine test is necessary to confirm that the kidneys are really the culprits. We measure the concentration of the urine in a test called a urine specific gravity, or USG. Distilled water has a specific gravity of 1.000, while very concentrated cat urine has a specific gravity of 1.080 or even higher. Normal cat urine should be above 1.040; if we see azotemia and a USG below 1.040, we make a diagnosis of chronic renal insufficiency (and start talking about renal azotemia).
Another test we will need to do on the urine is a bacterial culture. Some of these kitties have infections in the kidneys or bladder. these infections are not always easy to spot on a routine urinalysis. The urine is very dilute, and there is a lot of it. Finding bacteria in these conditions is often like looking for a needle in a haystack. If we culture the urine was can tell for certain whether there is infection present, and if there is we can tell what bacterial organism is causing it and what antibiotics will get rid of it.
We often recommend doing abdominal radiographs (x-rays) because some cats with renal insufficiency have kidney stones or bladder stones. While we do not always treat the stones themselves, it is important to be aware of their presence. The stones can move from the kidneys into the ureter, causing tremendous pain (ask any man who has had to "pass a stone"). If I am presented with a cat with acute abdominal pain, it certainly decreases the amount of testing I am going to do if I know that cat has kidney stones. I'm going to look for a ureter stone first, rather than wondering about cancer.
10-15% of cats will have high blood pressure (hypertension) at the time of chronic renal disease diagnosis, and another 10-15% will develop hypertension over the course of the disease. High blood pressure can cause further damage to the kidneys. Paradoxically, high blood pressure can temporarily improve kidney function by increasing the blood flow to the kidneys, but this effect is short-lived. We will need to make routine blood pressure measurements in all cats diagnosed with chronic renal insufficiency.
And finally, we will recommend a test called a urine protein : creatinine ratio. A long name with a short meaning for once. It basically gives us a way to quantify the amount of protein being lost in the urine. In normal urine the ratio of protein to creatinine should be low. Creatinine is a waste product and there should be lots in the urine, while protein is something we like to see remaining in the body. We like to see at least 4 to 5 times as much creatinine as protein. The ratio is usually written with the protein first and the creatinine as "1", so we like to see a ratio of 0.2:1 or less. When we start getting up to 0.5:1 we start to worry about excessive protein loss. And when the protein and creatinine are equal (1:1) or worse yet, there's more protein than creatinine (2:1 or 4:1) we know the kidneys are really falling down on the job.
The amount of protein in the urine is hard to pin down, because the urine concentration can vary so much from sample to sample. The ratio of protein to the normal waste material creatinine gives us a definitive way to measure the relative loss of protein.