Hi Dr Draper,
One thing that you might add ... though I have no experience with it since I've only had female cats, and I don't know details of what to look for ... but I know that a blocked urethra in males can be a life-threatening emergency fairly quickly. I know it is rare in females because they have a larger urethra, but if a male is straining in the litterbox and unable to produce urine, I think it is time to get to an emergency clinic. I've known a couple people who have lost their cats because they didn't realize it was such an emergency. I had one friend who lost two young male kittens (ca 4-5 months old). She recognized they were in trouble and took them to her vet, but the vet didn't believe male kittens that young would block and waited too long. A few hours later, my friend tried to rush them from her vet's to an emergency clinic with specialists and one died on the way and the other soon after arriving, from bladders that burst.
In my own experience, the only times I've taken my kitty to an emergency clinic was once for constipation because my cat was obviously upset, going in and out of the litterbox, straining to poop but not able to get it out, and crying. Right after I called the emergency clinic to ask if this was an emergency (of course they said, yes, and to bring her in) and was getting ready to go, she finally was able to get a small but thick and hard feces out. I took her in but she was fine and no more stool.
The other time I took her to emergency was when she was having trouble walking with her back legs and would start to fall over every few steps. I rushed her to emergency but, again, they didn't find any problem and chalked it up to an arthritis episode. She seemed okay a few hours later. I'm not sure if it was arthritis or maybe a mini-stroke, or maybe low potassium (she had CKD). But I think difficulty walking is a reason to get to emergency.
Another time I called the emergency clinic but decided not to go. My 18 yr old cat was obviously sick, vomiting and not feeling well. But we knew she had pancreatitis and she was a very complex case (also having CKD, hypertension, grade 1 murmur, IBD, poss cholangitis, etc). Of course, it was a Sunday and I couldn't reach her vet or Internist. But the vet tech that assisted my Internist was working a shift at the emergency clinic and knew my cat's case well. I talked to her and she advised against bringing my cat down because there was at least a 2 hour wait and vets who were unfamiliar with her case ... and my cat became very stressed at vet visits. But the tech was able to get word to her Internist who called me at home and advised me. The next day I was able to get her to her regular vet ... and two days later an ultrasound and lymph node aspirate diagnosed pancreatic cancer that had spread. While I think a vomiting, obviously sick cat is a good reason to seek emergency care, in my case it was better to wait a few hours for her Internist who knew her case.