**5. Conclusions**

The global population is ageing, and the prevalence of elderly subjects is increasing. Older adults are projected to increase enormously by 2050, rising to more than 400 million [42]. Chronic illnesses and disability causing hospitalization are frequent in the last decades of life and AKI is a frequent cause of morbidity and mortality as shown by the U.S. Renal Data Services (USRDS) 2018 [43]. The latter data demonstrated an increasing incidence rate of AKI over the past several years, in the elderly population. Patients over the age of 65 who required dialysis continued to have substantially higher mortality compared to general population [43]. Last year, our group demonstrated that in-hospital mortality was a frequent complication in elderly subjects with AKI discharge codes, involving more than a quarter of admissions. The increasing burden of comorbidity, dialysis-dependent AKI, and sepsis were the major risk factors for mortality [16]. Comorbidity is a well-known risk factor affecting survival in dialysis patients [44]; however, predictors of short-term survival in renal patients are still a matter of debate.

Multi-morbidity is crucial for defining the prognosis of the aged population [45], and our findings sugges<sup>t</sup> that pre-existing diseases diagnosed prior to admission may be associated with the outcome of an acute condition such as AKI (especially if AKI needs dialysis treatment). In elderly hospitalized subjects with AKI, WE effect seems to be a risk factor for IHM, even adjusting for comorbidity and advanced AKI stage. Thus, elderly patients admitted on Saturday or Sunday should deserve careful attention and evaluation, and consideration should be taken of their higher risk of IHM.

**Author Contributions:** Conceptualization, A.D.G., F.F., and R.M..; methodology, F.F, A.D.G., E.D.S., N.L. and F.M.; software, A.D.G., and E.D.S.; validation, E.D.S., N.L., F.M, and A.S.; formal analysis, F.F., A.D.G., N.L., and F.M.; investigation, E.D.S., B.B.; and A.S.; resources, R.C., A.S., and R.M.; data curation, A.D.G., E.D.S., R.C., N.L., and F.M.; writing—original draft preparation, F.F., and B.B.; writing—review and editing, F.F., and R.M.; visualization, A.D.G., R.C., and B.B.; supervision, F.F., B.B., A.S., and R.M.; project administration, R.C., B.B., and R.M; funding acquisition, F.F., and R.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work has been supported, in part, by a research gran<sup>t</sup> from the University of Ferrara (Fondo Ateneo Ricerca—FAR 2019, Fabio Fabbian).

**Acknowledgments:** We thank Massimo Gallerani, head of Medical Department, Azienda Ospedaliero-Universitaria "S.Anna", Ferrara, Italy, for precious help in obtaining data from the Italian Ministry of Health.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
