She's 14 and her body is frail

Had Milly at the vets tonight we known for some Time that she has had renal problems and she is loosing weight and for a small dog anyway she can't afford to, but no matter what we feed her she doesn't gain weight she is now only 3.5kg  . Some days she perky and wants to join in , others she will just lies down and sleeps. The vet wants to take blood , but I know in my heart we will not have much longer. I hope that she slips away in the night and we don't have to make that decision but if to save her from further discomfort we may have to take that path .