Need more information- I see you are a nurse, and are likely coming at this from a human medicine point of view. Things are a bit different in the canine world, with different tests, and different liver responses to things.
First, need to know what the reference range on the machine used to analyze the blood was. Also, need the alkaline phosphatase, bilirubin, ggt levels to put things into better context.
Medication history- such as any regularly given medications for arthritis, etc., or other health conditions need to be taken into consideration.
What about clinical signs and symptoms? How is Aspen acting?
Aspen is asymptomatic here are her following lab values, ALT 293 high end of range 107, AST 74 high end of range 55, ALK Phos .35 range 10-150 Total Bilirubin 0.2 range 0.0 - 0.4 BUN 10 range 7-27, Creat., 1.6 range 0.5 - 1.8, on no regular meds except heart worm, no change to appetite or thirst, voiding fine bowel habits normal. Moderatly active. spay.
Very helpful. Thank you. An elevation of 293 is actually considered a reasonably mild elevation. All the other values actually look very good. The ALT indicates that hepatocytes are leaking the enzyme, but the enzyme can also come from muscle tissue, though it is not so common unless there has been known trauma. I assume that is why you asked about running a CK. Most likely your CK will come back normal. Typically when we see elevated ALT due to muscle/bone tissue, it is either a growing puppy, or is a patient who has undergone trauma such as being hit by a car. The ALKP being normal, and TBIL being normal means there is no liver congestion- so no swelling of the liver, no infiltration with fat or steroid. That makes some of the more common endocrine diseases, like Cushing's much less likely.
With veterinary medicine, liver values are of concern based on magnitude of elevation as multiples of the normal. That is why I say the elevation isn't so serious. I am assuming that since you already tried flagyl (hepatitis?) and Denamarin, that this isn't an isolated test result. If it is isolated, the best bet is to have it rechecked in a month to see if it was transient, which can happen, not uncommonly.
Beyond that, it depends on how far you want to go with diagnostics. The best next step is actually hepatic ultrasound, ideally with at least a needle guided biopsy. It is quite possible, in a dog that seems overall quite healthy, but is teetering on the transition to geriatric years, that this is an incidental finding, something to monitor, but not a true health problem you will be able to do anything about. The Denamarin supplement is an excellent idea. Vitamin C and Vitamin E are also great antioxidants and may help preserve liver health.
One other thing worth evaluating is dental health. If teeth are a problem there will be a constant source of bacteria to the blood stream and you could end up with a chronic active mild hepatitis. A good dental evaluation and cleaning, including dental radiographs to detect problems beneath the gum line, is probably an excellent idea if it hasn't been performed recently. I don't think the tests you inquired about would result in any useful information in this case.
I hope this is more what you were looking for...I should have disabled the rating feature earlier since I really needed more information on context before commenting further.