Congratulations to all those participating in the Olympics (yeah women’s hockey!). Here at Pleasant Valley Animal Hospital, we go for the gold all the time. I remember one of my teachers in vet school, Craig Greene, always said "you have to go for the gold". No work-up was complete without it.
Quite often he would refer to “Ludowici Logic”. It was his way of comparing what we would do in private practice in a small town to the ivory tower of veterinary school. (Ludowici is a small town toward the Georgia coast.) Getting the urine was his standard no matter where in the sticks one could practice, though. Too much potential information was being wasted without a standard urinalysis being performed.
Granted, he was an internist with a special interest in the kidneys, especially since his internist wife was also focused on the kidneys, but he was right. The urine may sometimes tell us more than the blood work we run.
Why would we pass up a good simple screen?
As an odd side note, I went to a human urologist who never once wanted my urine. I thought it was very suspect that he showed no interest in my liquid waste and wanted to run all sorts of other tests instead. And I was, let’s face it, more cooperative than the average patient I see. So it wasn’t a matter of difficulty in obtaining a sample, which is often the case in my field. (This is yet another case in point of trying not to emulate human medicine all the time.) My goal here is to tell you briefly how the urine helps us in a few specific ways, not to be exhaustive in all the things it can tell us.
Our urinalysis strips are the same as the human ones.
It covers the pH, (a measure of acid/base level), red and white blood cells, protein, bilirubin, urine specific gravity (concentration), glucose, ketones, nitrates, urobilinogen and ascorbic acid. Not all of these are that useful for us, and some are suspect. I never trust the USPG on a test strip. A refractometer is the best way to judge urine concentration, and we do that automatically. I also have yet to see urine strip coloring match what the color coding for bilirubin is supposed to be, normal or abnormal. This is a big shame. The strip is also not that useful for white blood cells, which is why we run a sediment to look at all the cells. The sediment also picks up crystals, which could be from stones, casts, which can indicate kidney damage, and bacteria.
The area where the urine beats the blood work the best is the protein.
The kidneys compensate really well for disease, so much so that a lot can be damaged and the blood work can still be normal. Typically, there will still be protein loss that precedes changes on the blood chemistries. (There is a newer test called an SDMA that is standard on our blood going out that is more sensitive than the old standbys of creatinine and BUN, but urine protein is still more sensitive.) The sediment is still needed though. Protein alone does not indicate a kidney problem. It goes up with infection as well. A further testing of proteinuria may be needed called a protein creatinine ratio to distinguish where in the kidneys the problem lies, but in the face of an infection, its information is suspect.
Ketones are also extremely helpful, since they aren’t on our blood work.
We can figure out that the patient is diabetic, but the ketones let us know just how serious the crisis is. It can make the difference in whether to hospitalize or not. Glucose on bloodwork is just a snapshot and may be normal at any given point. The urine collects over time and may let us know that we may have a diabetic on our hands where the blood sugar is getting over the renal threshold at least some of the time. Occasionally though, an infection with some bacteria may spike the sugar as part of their metabolism. There are also critters out there that are not diabetic that spill urine out of their renal tubules with a disease called Fanconi syndrome. (Think Basenji)
The point is that blood work doesn’t catch everything, nor does urinalysis.
They are complementary and each have their strengths and weaknesses. Clinical pathology is a matter of detective work, and any detective needs informants. We go to the most reliable informants that usually provide good info, before we go to the more specific ones. They may not give us the complete answers, but they quite often provide leads, if you will, of where to go next for more information, such as radiographs for bladder stones or a urine culture in the case of a urinalysis. Dr. Greene called urine “the window to the soul”. I wouldn’t go that far, but if we don’t attempt to look through it, we are missing a part of the big picture.