Initial ACTH Stimulation Test- When?

We recently took our 4 year old dog to the vet for a bad bout of GI upset (vomiting, inappetence and diarrhea with mucus and frank blood ). We did bloodwork and X-rays and was diagnosed with acute gastroenteritis with hematochezia. He was given a 5-day course of metronidazole and his symptoms have resolved.

The vet added a random cortisol to the blood test because we mentioned he has always had a sensitive stomach with a history of GI issues— he gets diarrhea every few weeks at least, and we aren’t always able to link it to dietary indiscretion. He has never had frank blood in his stool before though, which is why we took him in for an exam this time.

His cortisol came back as low, <13.8 nmol/L. The only other deranged lab value was mildly elevated ALT at 202U/L.

The vet advised us to consider an ACTH stimulation test to rule out Addison’s disease if he has another bout of illness. My question is whether it’s better to do the ACTH stimulation test when he is feeling well, or if we should wait until he has another flare-up of symptoms. The vet said wait until he’s sick, but I’m not confident that makes sense.

We plan to start seeing a different vet going forward anyway, because communication and follow up with our current very has been very poor. But I’m wondering if we should take him to a different vet now (while he is well) to request the test, or if we really should wait until another flare of up of diarrhea occurs (which, given his history, will probably happen within the next few weeks anyway.)

Thank you for your advice!