Long‐term renal recovery after acute kidney injury in hospitalized patients: Evidence for significant improvement?

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The Iceland national epidemiological study1 suggests a rise in hospital‐acquired acute kidney injury (AKI), a decline in long‐term mortality and an improvement in renal recovery over a 20‐year period. However, the datasets used by the authors have significant weaknesses, in spite of the large number of participants included and the complex statistical analyses.
There are serious doubts about the reliability of criteria defining AKI incidence and its increase over time. Serum creatinine (SCr)‐based definitions of AKI underestimate the true disease burden and misclassify the severity of AKI. Changes of serum creatinine do not distinguish prognostically relevant causes of AKI nor differentiate AKI from acute or chronic kidney disease (CKD). The importance of two recognized AKI diagnostic criteria (changes in serum creatinine, changes in urine output) was confirmed by a recent large scale study in critically ill patients. The study showed an increased short‐ and long‐term risk when patients met both criteria and when these abnormalities persisted longer than 3 days.2 Whether the notable rise in AKI incidence over time results from more aggressive diagnostic or therapeutic procedures or simply reflects the increased awareness of AKI remains speculative.
The authors admit that the classification of AKI severity was based on different numbers of SCr measurements throughout the study period. Thus, a median number of two SCr measurements per patient (IQR 1–4) in the first 5 years period was replaced by eight SCr measurements per patient (2–21) in the last 5‐year period. Underreporting of AKI in the first period may have contributed to changes in AKI incidence and severity over time.
There are major concerns regarding the improvement of renal recovery during the study period. The data show improved recovery for patients with AKI stage III only. Generally speaking, recovery of renal function from severe AKI depends on pre‐existing chronic kidney disease, cause, severity and duration of AKI, timing and choice of renal replacement, definition of renal recovery (baseline serum creatinine +10%), and care of surviving AKI patients by nephrologists.3 Regrettably, the authors provide no information on these determinants. Of importance, statistical analysis does not allow to control for the multiple known or unknown confounding variables of renal recovery. Furthermore, renal recovery should not be evaluated earlier than 12 months after an episode of AKI. Unfortunately, the authors followed up the patients with stage 3 only for a median of 5 (1–20) months.
Recovery of kidney function is an underappreciated, but fundamental long‐term outcome parameter for survivors of AKI. De novo post AKI‐CKD or progression of pre‐existing CKD portends significant morbidity, including initiation of dialysis, rehospitalizations, poor quality of life and earlier death.4 To date there is no therapy available to improve renal recovery from severe AKI. Additional studies on long‐term outcomes of severe AKI (stage III) are needed, to further explore the natural history of renal recovery. The Iceland study results are intriguing but do not answer this question.
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