Determining quality of life for your pet can be very difficult. And to get myself off the hook I could say that only the owner can really make that call for sure. And while this is true, it is sometimes difficult for an owner to be objective and unemotional so there are some guidelines I use to help them figure it out.
As our pets age we can obviously expect changes. Some of the things we see frequently are decreased or total loss of vision and hearing. But these are typically of slow onset which allow for the pet to adapt, which they do very well. Also, these conditions (unless associated with untreated glaucoma, etc) are not painful and for most pets do not adversely affect quality of life. I often hear people "looking" for a reason to euthanize their pet talking about blindness and/or deafness as one of their reasons, but for me, I don't really think so.
Chronic pain is one of the other conditions we see in aging pets. Fortunately there are now many modalities for treating pain allowing for choices in every budget and with many concurrent medical conditions. Granted there comes a time when you run out of options and pain becomes unmanageable, then you must start to consider other factors and come up with a quality of life determination for your pet.
Dogs and cats are very good at masking pain, discomfort, and illness. But generally when I advise owners in determining quality of life I have them think about whether or not their pet pursues normal daily activities and functions. While we can obviously expect changes like sleeping more, stiffness, playing less, etc. an older pet with a good quality of life should still be interested in eating and drinking. They should be eating something regularly, even it's not what they have been eating their whole life. If you need to hand feed, or change foods frequently then it's time to see what's going on or re-evaluate the situation. They should be willing and able to go outside to eliminate a few times a day. If they can't, or won't, then that's another sign to either investigate or re-evaluate. Meeting you at the door, showing interest in playing games or going for walks-things they enjoyed when they were younger-are positive indicators, even if they can't play or walk for as long.
One thing you can not go by is how they act in the clinic. A pet will often perk up when in a new location, either from stress, excitement, adrenaline, etc. Just they way they stop limping or itching once in the exam room, a pet who is very lethargic and depressed at home may seem more lively upon exam. You need to go by what you see at home-this can be hard, but you have to base your decision off the overall condition of your pet.
Another good suggestion (and I'm sorry to whoever gave me this idea because I can't remember who it was to give them credit) is to use the two jar method. The idea behind this is you have a "good day" jar and a "bad day" jar. Every day you put a penny, marble, or whatever, into the appropriate jar depending on how your pet did that day. When the bad days outnumber the good, then you know you are losing the battle.
Now for the second part of the equation. When is it "time" and how far should I go? We all hope our pets will grow old gracefully and then one day simple fall asleep and not wake up. I can guarantee you it almost never, ever happens that way. And of the pets that do die at home, I can tell you that by their condition (when brought in for cremation) that they probably should not have been allowed to pass at home and that it most certainly was not painless. The ability to end pet suffering is a great gift, but it is also one of the most gut wrenching decisions we ever have to make. So many people will ask did I wait too long, am I being selfish, is it too soon, could I have done more, or did I miss something? And while there is no way to make this an easy decision, it can help to think about some of the factors involved ahead of time.
One of the first things I ask when counseling people with an older, ailing, or terminally ill pet is what are their goals? Some people don't want their pet to ever suffer or even feel sick for a day. These people will request euthanasia at the time of a terminal diagnosis, or shortly thereafter. It's not a common request, but it's not wrong either and you shouldn't feel bad about choosing that option. Some simply want to keep their pet comfortable for as long as possible without pursuing drastic measures. Others want to keep their pet around for as long as possible as long as discomfort is minimal and quality of life is acceptable.
When faced with decisions of how and if to treat your pet it's best to consider your individual pet's personality, concurrent conditions and what the treatment will involve. For example a very nervous pet who hates going to the vet and getting poked with needles may be one you don't want to put through chemotherapy for the addition a few more months of life. A dog with terrible hip dysplasia may not be a candidate for front leg amputation in the case of bone cancer. A dog that is difficult to medicate won't be a good candidate for treatment of heart failure (although I believe if you have a dog you should be able to give it a pill!) It's always best to know what the treatment will involve and what the expected prognosis is when making these decisions. Just because we can prolong life doesn't always mean we should. And it's also not wrong to decline invasive treatments like chemo, surgery, and radiation even if your pet is a good candidate. No one will fault you not wanting to put your pet through something potentially stressful or painful just to gain more time. But no one will fault for the opposite either. I have seen plenty of happy, tail wagging dogs receive these same treatments like troopers. We have great medications to manage pain, nausea and other side effects these days and with more and more research we hope to have better treatments and cures for many diseases making treatment even more desirable.
A final consideration in determining a treatment plan is cost. Although not ideal in factoring into the decision, it is very real world. And it's not wrong to decline expensive medications or procedures based on cost. I certainly see plenty of young treatable animals euthanized based on expense so declining major surgery on a terminally ill 14 year old dog is not an unacceptable choice.
This is such a gray topic, with so many factors, and so many different individual situations, but it does come up often. Hopefully reading this will help people know some things to watch for, and give them some things to think about ahead of time so that decisions are a little less confusing when the inevitable time comes to make them.