**5. Conclusions**

In recent years, numerous studies joined the challenging quest for urinary biomarkers in diagnosis and prediction of acute kidney allograft rejection. Authors must face the difficult task to allow for mediating between the need for a precise setting and reference standard diagnosis (to develop the most precise biomarkers), and the need for their validation in the most heterogeneous population of kidney allograft patients (to increase clinical utility). Urinary chemokines CXCL9 and CXCL10, alone or in combination with others, are the most frequently used and the most promising biomarkers, but multi-parametric clinical and laboratory models could represent the best strategy for future studies. Remarkable advances have been made on the path of allowing a more precise allocation of resources, helping clinicians to move from the standard protocol/indication biopsy dichotomy, to reduce unnecessary immunosuppression, and to improve kidney allograft outcomes in the long-term.

**Author Contributions:** Conceptualization, investigation, resources, F.G. and L.C.; methodology, data curation, formal analysis, visualization F.G.; writing—original draft preparation, F.G. and L.C.; writing—review and editing, E.B., F.B., C.E., R.M.R., J.P.H.; supervision J.P.H., P.R.; funding acquisition, P.R. All authors made a significant contribution to the content of this manuscript as per ICJME recommendations. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** We thank the Meyer Children's Hospital and the Meyer Children's Hospital Foundation.

**Conflicts of Interest:** The authors declare no conflict of interest.

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