Acute kidney injury: Etiology and disease course

Acute kidney injury (AKI) is a serious clinical condition characterized by sudden and often reversible loss of kidney function. In recent years, the concept of acute kidney disease (AKD), which can develop in the post-AKI period and lead to both short and long-term consequences, has been defined. AKD is considered a transitional phase between AKI and chronic kidney disease (CKD) and plays a determining role in prognosis. However, the incidence, risk factors, and long-term clinical outcomes of AKD are still not fully elucidated. On the other hand, cases of AKI developing on a background of existing CKD, i.e., acute-on-chronic kidney disease (A-CKD), also represent a high-risk group for AKD development and poor prognosis. This retrospective study included 228 patients followed in the internal medicine ward of our hospital between January 1, 2022, and December 31, 2024. AKI and A-CKD were defined based on KDIGO guidelines; patients were etiologically divided into prerenal, intrarenal, and postrenal groups. The diagnostic criteria for AKD were the presence of AKI, GFR <60 mL/min/1.73 m², a greater than 35% decline in GFR, or a greater than 50% increase in serum creatinine level. Risk factors including advanced age, diabetes mellitus, hypertension, cardiovascular disease, malignancy, sepsis, and nephrotoxin exposure were examined. Kidney functions were assessed at diagnosis, on day 7 (±2 days), and on day 90 (±15 days); temporary/permanent dialysis requirement, new-onset CKD, CKD progression, and 90-day mortality rates were determined. A total of 228 patients were included in the study; 145 (63.6%) were acute kidney injury (AKI) cases, and 83 (36.4%) were AKI cases developing on a background of chronic kidney disease (A-CKD). The mean age was 69.6 ± 13.2 years, with patients in the A-CKD group being significantly older (72.7 ± 11.0 vs. 67.7 ± 14.4 years). The gender distribution was 54.8% male (n=125) and 45.2% female (n=103). The most common etiology of AKI was intrarenal (42.1%; n=96), followed by prerenal (38.2%; n=87) and postrenal (19.7%; n=45) causes. According to KDIGO staging, 34.2% of cases were stage 1 (n=78), 27.6% were stage 2 (n=63), and 38.2% were stage 3 (n=87). The primary outcome, acute kidney disease (AKD), was detected in 46.1% (n=105) of the entire cohort. When subgroups were analyzed, the frequency of AKD was 59.0% (n=49) in A-CKD, compared to 38.6% (n=56) in isolated AKI, and this difference was statistically significant (p=0.003). Among secondary outcomes, the need for temporary hemodialysis increased in the presence of AKD in both groups. The need for permanent hemodialysis was significantly higher in the A-CKD group compared to the AKI group (12.0% vs. 2.8%; p=0.005). At the 90-day follow-up, new-onset CKD was observed in 42.9% of the AKI group, while CKD progression was observed in 72.5% of the A-CKD group (p<0.001 for both). Furthermore, the 90-day mortality was 19.3% in A-CKD and 9.7% in AKI, a difference that was also statistically significant (p=0.039). These findings reveal that AKI developing on a background of CKD is associated with a higher frequency of AKD, a greater need for permanent dialysis, and higher mortality over a three-month period compared to isolated AKI. Notably, the development of AKD emerges as a strong predictive factor for both new-onset CKD and CKD progression. Therefore, standardizing secondary prevention strategies targeting close nephrological follow-up in the early post-discharge period (first week) and at the 3rd month, avoidance of nephrotoxins, and hemodynamic optimization is of great importance.