The fact that she is vomiting so much and cannot even keep water down is concerning. There are a number of problems that can cause dogs to vomit frequently despite treatment, these can include:
*obstructions - this either be due to a piece of foreign material (some foreign material will not show up on x-ray) or a physical obstruction due to a swelling of the intestinal tract (tumor) or abnormal function of the gastrointestinal tract e.g. a gastooesphageal hernia.
*hormonal problems - less likely but Addisons diesease and diabetes may be a possibility.
A lot of these problems may not be detected on the tests that have been run already. Ultimately the fact that she continues to vomit and loose weight (despite the fact you have already done some good tests and tried some treatments for her) means that Abby should be seen by a vet again to do some additional tests. In this situation I recommend that you take Abby to see an internal medicine specialist ASAP. I realize that a visit with a veterinary internal medicine specialist can seem a bit daunting but they are in the best position to use their advanced training to diagnose problems quickly. They are also able to tell you what tests they think will be most helpful so you can avoid spending money on less useful tests. Depending on what they notice on their physical exam it is likely that the specialist will suggest some of the following tests for Abby: pancreatic lipase test, electrolyte panel, abdominal x-rays and ultrasound, they may even suggest endoscopy to assess her esophagus and stomach.
Since Abby is vomiting so often and has lost weight (likely due to dehydration) I suggest that you consider contacting local hospitals tonight, it would be safest to have her hospitalized and on fluids through the night until the additional tests can be ran.
Please let me know if you have any follow up questions. I wish you and Abby the best of luck.
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