Do not panic. There are two values on blood work usually primarily used to assess the kidneys - BUN (blood urea nitrogen) and creatinine. They are nitrogen wastes that result primarily from protein metabolism and normally are excreted in the urine.
If they are high on bloodwork there are three very general categories of causes:
1. Pre-renal = "Before the kidneys" - most commonly dehydration that can range from "I just didn't drink much water today" to serious dehydration. Some people also put a recent meal (especially one high in protein) in this category but you may also see a meal classified as "extra-renal."
2. Renal - a wide variety of kidney-specific diseases.
3. Post-renal = "After the kidneys" - most commonly a urinary obstruction which should be obvious so... not in this case
To differentiate between #1 and #2, you really need to look at a measurement of the concentration of urine called a urine specific gravity or Usg. A normal, functioning kidney will typically produce a concentrated urine in dogs and cats and especially to conserve water in the face of dehydration, while an unhealthy kidney loses the ability to concentrate urine properly. So if the values are high on blood work but the urine is very well concentrated, the kidneys are responding appropriately and you're looking at pre-renal causes which are not generally concerning. If the values are high on blood work and the urine is dilute, the kidneys are not responding appropriately and something is probably wrong with the kidneys themselves - but there are a million specific things that can go wrong, some of which are permanent and some of which are temporary/reversible (like a bad infection) so at that point it might take more investigation to get a more specific answer.
Make sense?
