A patient shows one or more of the following: advanced bone loss, grade II and III furcation involvements, tooth mobility, inaccessible areas, systemic/environmental factors represents:
Questionable prognosis
Poor prognosis
Fair prognosis
Hopeless prognosis
Good prognosis: Control of etiologic factors and adequate periodontal support ensure the tooth will be easy to maintain by the patient and clinician.
Fair prognosis: Approximately 25% attachment loss or grade I furcation invasion (location and depth allow proper maintenance with good patient compliance).
Poor prognosis: 50% attachment loss, grade II furcation invasion (location and depth make maintenance possible but difficult).
Questionable prognosis: >50% attachment loss, poor crown-to-root ratio, poor root form, grade II furcation invasion (location and depth  make  access  difficult)  or  grade  III  furcation  invasion; mobility no. 2 or no. 3; root proximity.
Hopeless  prognosis:  Inadequate  attachment  to  maintain  health, comfort, and function.
Ref: Newman and Carranza’s Clinical Periodontology, thirteenth edition; page no 413