I usually ask/consider whether a diagnostic will change the way a given illness will be treated- if treatment would be the same whether it's a mystery illness OR disease x, then it really doesn't matter much. If there's something they would do differently to treat disease x, then better to test so you can tailor treatment better. Example, when one of my dogs had a mast cell tumor removed, I had already decided not to pursue further treatment (chemo, radiation, etc), so didn't take her for staging. I did take her to see a specialist when she had multiple enlarged lymph nodes which had needle aspirates repeatedly negative for lymphoma- the lab had suggested several differentials (sterile abscesses, fungal) that might be treatable, and I wanted to rule them out and/or treat if she had something which was easily treatable. The specialist was able to get a better LN sample via ultrasound guidance, and she did have lymphoma, which I decided not to treat due to her other health problems.
There's also the not knowing factor, which can be both good and bad. A lot of people who I interact with decline a biopsy of a growth because they'd rather not know, and wouldn't treat if it IS cancer. Others want to know so they can pursue further diagnostics and treatment if there is a malignancy. Still others just have to know, and/or are undecided as to where they'll go if there's a "bad" diagnosis. I don't think any of those choices is "wrong".
I think, "how will it change the treatment plan?", is an important question to ask regarding a given test, particularly with older or otherwise infirm/sickly dogs. If it won't change the treatment, or the treatment for a given thing you are testing for isn't something you would choose to do, I don't think it's necessarily the worst thing to decline additional testing and just treat symptomatically. Maybe ask what the differentials would be for the area of concern on her lung, and what treatment options would be feasible for a dog of her age and health status- if there is minimal benefit to be gained from the tests, I would be reluctant, but if there's a possible "easy fix" that would make the dog feel better for a longer period of time, that would sway me more toward considering them. Will it drive you crazy not to know?
There's also the not knowing factor, which can be both good and bad. A lot of people who I interact with decline a biopsy of a growth because they'd rather not know, and wouldn't treat if it IS cancer. Others want to know so they can pursue further diagnostics and treatment if there is a malignancy. Still others just have to know, and/or are undecided as to where they'll go if there's a "bad" diagnosis. I don't think any of those choices is "wrong".
I think, "how will it change the treatment plan?", is an important question to ask regarding a given test, particularly with older or otherwise infirm/sickly dogs. If it won't change the treatment, or the treatment for a given thing you are testing for isn't something you would choose to do, I don't think it's necessarily the worst thing to decline additional testing and just treat symptomatically. Maybe ask what the differentials would be for the area of concern on her lung, and what treatment options would be feasible for a dog of her age and health status- if there is minimal benefit to be gained from the tests, I would be reluctant, but if there's a possible "easy fix" that would make the dog feel better for a longer period of time, that would sway me more toward considering them. Will it drive you crazy not to know?