Acute‑on‑chronic kidney disease (AoCKD): a critical appraisal and call for action to define boundaries and optimize management

Acute‑on‑chronic kidney disease (AoCKD) refers to an acute decline in renal function occurring in individuals with established CKD. Despite its major clinical impact—including increased mortality, accelerated CKD progression, and higher cardiovascular risk—AoCKD remains underrepresented within major guideline frameworks (KDIGO, ADQI, NICE). Our review aims to critically appraise existing guideline frameworks, definitions, biomarkers, imaging recommendations, and research gaps related to AoCKD, and to highlight priorities for future harmonization and clinical integration. We conducted a targeted literature search of PubMed, Embase, and the Cochrane Library, to identify guidelines, consensus reports, observational studies, and systematic reviews describing acute deterioration in CKD. Extracted data included definitional criteria, epidemiology, biomarker performance, imaging guidance, and prognostic outcomes. Current KDIGO, ADQI, and NICE frameworks were evaluated in parallel. We report marked heterogeneity in definitions describing acute deterioration in CKD, with terms including AoCKD, AKI‑on‑CKD, acute pre‑existing kidney dysfunction, and cROCK‑based definitions. Reported incidence varies widely (1–33%) due to inconsistent criteria. Creatinine‑based definitions show reduced prognostic accuracy in CKD populations, whereas cystatin‑C–based eGFR demonstrates superior discrimination for mortality and progression. We propose an operational definition of AoCKD that includes functional criteria and biomarker-based criteria. The definition reflects the increased mortality in this phenotype even before reaching the KDIGO-AKI threshold. In conclusion, AoCKD is a clinically significant but poorly defined state within the kidney disease continuum. Harmonized definitions, incorporation of cystatin‑C–based markers, structured imaging pathways, and integration of early biomarkers are urgently needed.