the amount of suffering and pain
the capability to keep clean
the capacity to interact enthusiastically with family members
the strength and flexibility to be somewhat mobile
the ratio of "good days" to "bad days," as characterized by weakness, vomiting, depression, etc.
Work closely with your veterinarian to determine when a reasonable quality of life can no longer be maintained, despite your best efforts of intervention. At that time, feelings of denial and guilt must be put aside in order to honor the dignity of the animal. If death comes quickly and naturally, without suffering, accept it as a blessing. Whatever the situation, your veterinarian can support you in assessing, facing, and managing the gamut of end-of-life decisions and provide professional support when the time comes to "let go."