Thank you for answering the questions for me!
A little bit more information about grapes and raisins just to get you updated on what you're looking at in regards ***** ***** exposure posing challenges later on down the road:
Currently, there is no known mechanism of action for grapes and raisins in dogs. It seems like the cause of renal damage after exposure may be due to an individual sensitivity in each particular dog, so even some dogs who ingest a lot of raisins or grapes may not show signs. They've been able to rule out pesticides and grape type as possible causes separate from whatever is in the grapes. Raisins tend to be a lot more toxic than grapes, and they tend to have a lower margin of safety. So far, we have not seen instances of -cooked- grapes/raisins causing an issue, but we still treat it with diuresis, as we don't want to take the risk that one day a dog will develop kidney damage from them. Grape juice also does not seem to cause the same issues.
The first thing we start to see is gastrointestinal upset, usually within the first 24-48 hours of ingestion. Dogs will vomit, stop eating, stop drinking, and gradually their kidneys stop successfully concentrating the urine, urination becomes scanty, and then they'll stop urinating altogether. Once kidney damage has developed, these signs can last for several weeks after exposure.
Now, if you had told me that you didn't test her blood until recently and that it was her BUN/Creatinine that was elevated, I would also be concerned about kidney damage due to the raisins. However, with an exposure to grapes and raisins, we see an elevation in those kidney values within a few days (48 hours is the most common time-frame).
But since you ran blood chemistries near to the exposure and again when you started to see blood in the urine and they came back normal, this leads me to believe that she either did not get enough of them to cause a kidney problem, or she is is one of those dogs who is just not as sensitive. Since it has been two months since the exposure (with those normal kidney values), the signs that you are seeing now are unrelated to the grapes.
Blood in the urine is also not super common with grapes and raisins, we tend to see an inability of the kidneys to properly concentrate the urine which eventually leads to urine cessation. This would have been seen within several days of ingestion, and unless she had sustained damage to the kidneys which would have been seen in those blood tests. After a grape/raisin exposure causes an noted elevation to the kidney enzymes, this does not go down on its own and requires 48+ hours of diuresis to get under control.
So it does sound like your vet has settled on a urinary tract infection as the cause of the blood in her urine. This is a VERY distinct possibility, as red blood cells in the urine coupled with bacteria seen via urinalysis is a very strong indicator of a urinary tract infection. Now, we CAN see urinary tract infections that can become more severe and lead to kidney infections. These are usually also treated with antibiotics, but stronger ones.
As UTIs that causes bleeding can cause dips in the RBC and platelet count in dogs, this would match with her blood work far better than renal damage from the raisins as well.
Antibiotics are the number one drug of choice here. If these infections aren't treated with an antibiotic to which they are susceptible, we see a urinary tract infection become a kidney infection, which can very easily lead to sepsis, stones, and other complications.
That being said, Penicillin itself is a very weak antibiotic when it comes to a UTI in dogs. Gentamicin is sometimes used as a "stronger" antibiotic, but may not always be the best "first try" antibiotic for a urinary or kidney infection, especially as there are other drugs that work and have fewer associated side-effects.
So given where you are, I would ask if your vet performed a urinalysis? If so, and bacteria were seen, then antibiotics are absolutely appropriate, though I would discuss your concerns about the gentamicin leading to kidney damage with your vet and ask if there is an alternative (such as Clavamox or Baytril, both of which work really well. Clavamox tends to be something they try first, then move on to Baytril as a more broad-spectrum antibiotic).
Another option would be to have them send of a sterile urine sample to have the bacteria grown and susceptibility tested. This will tell you exactly what you're dealing with and exactly which antibiotic is going to be the most effective.
Similarly, did your vet perform radiographs to check the kidneys and look for stones? These can complicate a urinary tract infection and may need further treatment on top of the antibiotics. Kidney infections are more commonly treated with Baytril, and sometimes stones even need to be surgically removed to prevent continued irritation and recurrent infections.
Hopefully this will get you started on the right track, and has addressed your concerns about the kidneys and the raisins a bit as well. If you have any further follow-up questions, please get back with me via reply!