To answer your question....it depends upon what is causing the kidney failure as to whether there is severe pain or just a feeling of malaise that will gradually worsen into pain as the kidneys shut down.
Signs of pain from kidney problems will be a piggie not eating, hunched with spikey fur and often facing into the corner. ..sometimes they will have blood in the urine or not be peeing at all.
However, having gone through your previous threads I would comment as follows:
Kidney failure is an end result of either
a) stones in the kidney (or other part of the urninary tract) stopping it functioning properly and preventing proper excretion - seen on Xray and yes, this is very painful.
b) auto-immune reaction where the immune system attacks kidney cells and they cease to function properly- I've not heard of this in guinea pigs so can;t comment
c) an adverse (eg certain antibiotics) or long term (eg metacam) drug reaction
d) kidney tumour
e)pressure put on the kidneys from heart problems - again often diagnosed on Xray as an enlarged heart with fluid in the abdomen and/or around the lungs...often displayed more as lethargy, breathing issues and not eating rather than pain
From your earlier threads it sounds like your Sooty has been diagnosed with fluid in the abdomen (ascites). It also sounds like she has been given the diuretic frusemide by the vet to try and get rid of it and the tablets you have been given could be 20mg frusemide tablets? If you are giving her a whole one of these at that dose per day then that is a hefty dose and you should start to see some improvement provided she is not too far advanced. You could try grinding the tablet between two teaspoons into a fine powder and mixing it with 1.0ml of water and syringing the correct dose to her (that's how I give my pigs their frusemide). Incidentally, she should be given the diuretic twice or three times a day rather than just once for it to have a proper effect but the total dose in any 24hours should not exceed 20mg/kg...and you have to be careful that she doesn;t deydrate. (I know, it sounds silly when she has all that fluid but the diuretic also saps water from all the other cells)
Your vet has decided the ascites is due to kidney failure. How did they reach that decision? Did they perform an Xray or ultrasound to check for stones/blockages/tumours?
Ascites can be caused by heart problems, lymphoma/tumour in the abdomen or kidney failure. I had a piggie with ascites..(also erratic breathing and swollen front paws)..she was found to be a heart pig on Xray. We gave her frusemide and fortekor and she went on to live a very happy extra 14 months before she was helped to The Bridge because of a huge bladder stone that we couldn;t operate on.
I also had a piggie diagnosed with a urethral stone (which was removed) and subsequently found on Xray to have kidney's chock full of stones which were inoperable. She didn;t have ascites, she just stopped eating, lost weight and sat hunched in the corner with spikey fur. She was maintained on painmeds (dosage increased over time) and again spent an extra 6 months happy life eating, drinking and popcorning with her mate before she finally indicated she was ready to go to The Bridge, preceded by very bloody wee.
I really feel for you as personally I don;t think you can make an informed decision at the moment because you don;t know the real cause ....so unless you can see that Sooty is in pain from the signs already posted for you in the link to guinea lynx I think you need to keep force feeding her to keep her guts moving and also consider having an Xray done to check it isn;t just a stone blocking the urinary tract that could be removed...(bladder or urethra as opposed to ureter or kidney as the latter two are inoperable) ....or a heart issue. It may be that your vet has already considered these other possibilities and can explain why they feel that she definitely has kidney failure. If you feel satisfied by the vets explanation, and there is no improvement over the next 24 hours with the frusemide, then sadly I think you will have to consider helping her to The Bridge before the kidneys shut down completely and she suffers end stage renal failure because it's not just about pain, it;s about the other symptoms that can be associated with it....
I hope the above helps you reach a decision that you feel comfortable with at this difficult time
My thoughts are with you
x