So what do I do if an owner can only do items one, two, and three on my diagnostic list? We treat presumptively. Then we rely on response to therapy to try and dial in on a diagnosis.
Maybe we change to a hypoallergenic diet. I'm a fan of the Royal Canin Hypoallergenic, but the novel protein diets like Venison and Potato are great ideas. They also have a product called Intestinal HE which can be fantastic for some chronic mal-digestion cases or during the healing phase of a severe gastroenteritis. It's a rich food, so it's not appropriate for long-term use in patients where obesity is an issue.
Maybe we treat with a broad spectrum de-wormer. Maybe we use an antimicrobial (like metronidazole) to help get bacterial populations under control. Maybe we use a pro-biotic or pre-biotic. Maybe we are able to get good enough response with these treatments and we can stop there.
Or maybe we don't.
If we want the answers then we move to the next step. Don't shy away from biopsy just because it sounds scary. It really is critical to making a good diagnosis.
Steroid use is something that should be considered ONLY after we have have ruled out as much as we can or as much as we can afford. Steroid trials can be a great diagnostic tool, but we have to keep in mind that LOTS of stuff will respond to steroids. Just because there is inflammation that responds to steroid doesn't mean that we know what has triggered it. Wouldn't we feel silly if we treat with a steroid and get a response only to find out later that the cat has hookwork infection or giardia and we just never did a proper exam to find that out!
Another unfortunately common error is to send a dog for colonoscopy for chronic large bowel diarrhea only to find out that there are whipworms causing the problem and we did a poor job or properly diagnosing them or failing to treat them. Don't jump ahead. Follow the advice of your veterinarian, work stepwise. If your veterinarian asks you to submit yet another sample of stool don't respond with, "If you didn't find it before, I'm not paying for you to not find it again." Remember that it often times takes multiple attempts to find a parasite. It's a ton cheaper to do another fecal exam and treat with fenbendazole (Panacur) than it is to jump to endoscopy.
Fire off questions to me. This is a long topic and I've hopefully not made it too confusing. I'd love to hear questions if there's something that doesn't make sense.