Creatinine is a by-product of muscle metabolism and is eliminated by the kidneys. Elevated levels can indicate early stages of kidney disease , muscle disease, arthritis, hyperthyroidism, and diabetes. Mild elevations can be seen with dehydration.
The most common reason for elevation of CK with normal BUN levels are muscle disorders. even recent exercise prior to the blood test can cause mild elevations, In some cases selenium/Vitamin E deficiencies and trauma to the muscles can also cause elevations in the CKHere is additional information
If this is the only thing that was elevated and the dog is not having any major problems Then I would not be to concerned. You mentioned she was on pain meds and anti inflammatory so if being treated for a muscle related injury this could be the cause of the elevation.
I hope this helps. please reply back if you have additional questions or concerns
That would make absolute sense if your vet is thinking about MMM (Masticatory Muscle Myositis )
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I would be sure they removed all grass seeds as well as these can migrate through the body ending up in the internal organs or the brain.