The progressive rising of multimorbidity, which has been always considered the hallmark of clinical complexity (CC), has made management of the "complex" patient one of the most topical and challenging issues in medicine. However, patient-related factors (multimorbidity, age, frailty, disease severity) pertain only to the biological complexity, while CC is the result of the dynamic interaction between biological complexity and a number of other coexisting factors (socio-economic, cultural, behavioural, environmental). Starting from these premises, the investigators designed a five-year observational prospective longitudinal study that aims to validate and compare a CC score system on a large cohort of patients (n=1000) admitted in internal medicine wards. Clinicians, biostatisticians and epidemiologists will cooperate into the project. A questionnaire that encompasses the main biological and extra-biological factors was designed (Clinical Complexity Index, CCI) by a multiprofessional consensus. This questionnaire will be administered by the investigators to the patients and validated. Consecutive patients will be enrolled every other week for two years and followed-up for 5 years. The primary endpoint will be the validation of the CCI. Thereafter, the investigators will evaluate the correlation between the CCI and the length of stay of the index hospitalization, assuming that a higher CCI score is associated with longer length of stay. The secondary endpoints will be the demonstration of the association between higher CCI score and more health resources utilization (i.e., evaluating occurrence of hospital readmissions, number of accesses to the emergency room, visits at the outpatient clinic, different drugs prescribed and hospital reimbursement according to the local diagnosis-related group [DRG] system) along with worse prognosis (mortality at 1 and 5 years).

The Clinical Complexity Index (CCI) that will be validated is made of 25 different variables, divided into 5 domains (biological, socioeconomic, behavioral, environmental, cultural). All variables were dichotomised and were coded -1 (absence of the variable) and +1 (presence of the variable). For each variable, the answer "yes" increases the degree of complexity. See Citations for further details.

Here is the CCI that will be used in the study:

BIOLOGICAL DOMAIN Age > 75 years yes☐ +1 no☐ -1 Intake ≥ 5 medications yes☐ +1 no☐ -1 Cumulative Illness Rating Scale (CIRS) > 3 and/or CIRS severity >3 yes☐ +1 no☐ -1 ↑ frailty (Edmonton Frail Scale > 5) yes☐ +1 no☐ -1

↓ mobilization (Barthel<60) yes☐ +1 no☐ -1

SOCIO-ECONOMIC DOMAIN Living alone yes☐ +1 no☐ -1 Income < 1000 €/month yes☐ +1 no☐ -1 Unemployment/precarious work yes☐ +1 no☐ -1 Dependent/disabled family member yes☐ +1 no☐ -1 Need for a caregiver yes☐ +1 no☐ -1

BEHAVIOURAL DOMAIN Inadequate adherence to medications yes☐ +1 no☐ -1 Active smoking of at least 4 cigarettes/day yes☐ +1 no☐ -1 Alcohol (>3 Alcohol Units/day) and/or drug abuse yes☐ +1 no☐ -1 Inappropriate diet yes☐ +1 no☐ -1 Cognitive impairment (Short Blessed Test > 9) yes☐ +1 no☐ -1

ENVIRONMENTAL DOMAIN Institutionalization yes☐ +1 no☐ -1 Difficult access to healthcare yes☐ +1 no☐ -1 Presence of home architectural barriers yes☐ +1 no☐ -1 Occupational exposure to toxins yes☐ +1 no☐ -1 Air pollution yes☐ +1 no☐ -1

CULTURAL DOMAIN Schooling < 8 years yes☐ +1 no☐ -1 Insufficient access to information yes☐ +1 no☐ -1 Lack of adherence to health screening programs yes☐ +1 no☐ -1 Language barriers yes☐ +1 no☐ -1 Perceived discrimination yes☐ +1 no☐ -1

The investigators will administer the CCI to all patients admitted to the wards (Internal Medicine and Subacute ward), according the study protocol. All the enrolled patients will be followed-up with a phone call after after discharge (at 4-8-12 months) for collecting the following data: occurrence of hospital readmission within 1 month after discharge; the number of readmissions; the number of accesses to the emergency room (ER); the number of visits at the outpatient clinic; the number of different drugs prescribed; mortality. Mortality will also be assessed once a year for 5 years.

Inclusion Criteria:

• Age ≥ 18
• Hospitalized in one of the participating Units (Internal Medicine wards or Subacute ward)

Exclusion Criteria:

• Already enrolled in the study during a previous hospitalization
• Denied informed consent

All patients admitted in the aforementioned wards will be enrolled in the study during the time of the study

All individual participant data (IPD), anonymized and aggregated, that underlie results in a publication.

Only on future articles that will be published.