LAUSR

Acute Kidney Injury (AKI) is a common syndrome associated with significant morbidity and mortality and is recognized as a risk factor for progression to chronic kidney disease (CKD). However, AKI encompasses a broad spectrum of underlying etiologies, making accurate differential diagnosis essential for guiding targeted treatment and follow-up strategies. Despite the established utility of kidney biopsy in evaluating renal pathology, its role in AKI remains underutilized. This study aims to describe the clinico-pathological characteristics, histological diagnoses, outcomes, and procedure-related complications in patients presenting with AKI who underwent native kidney biopsy. We conducted a retrospective review of 253 native kidney biopsies performed at our center between 2017 and 2018. At the biopsy, patients were classified according to KDIGO AKI stages based on serum creatinine and urine output criteria. Clinical and laboratory data, histological findings, and follow-up outcomes were collected. Among the 253 biopsies, 96 patients (38%) presented with AKI whose baseline characteristics are reported in Table 1. The cohort had a mean age of 60.2 years, with a slight male predominance (55%). The mean serum creatinine at presentation was 4.85 ± 3.36 mg/dL. The distribution of AKI severity was: 44.8% Stage 1, 26% Stage 2, 18.7% Stage 3, and 10.4% Stage 3D (requiring renal replacement therapy, RRT). The most frequent histological diagnoses (reported in Fig. 1) included crescentic glomerulonephritis (17.7%), monoclonal gammopathy of renal significance (MGRS), and cast nephropathy (12.5% each), followed by acute interstitial nephritis (AIN, 12.5%). Diagnostic yield was high, with only 2% of biopsies being insufficient for evaluation. Procedural complications were rare (2% requiring transfusion), and no biopsy-related deaths occurred. Based on clinical presentations, AKI patients were classified into three subgroups: AKI with urinary abnormalities (AKI-U; 49%), AKI with nephrotic syndrome (AKI-NS; 38%), and isolated AKI (AKI-I; 13%). The histological diagnoses according to the clinical presentation are reported in Fig. 2. In the AKI-U subgroup, crescentic glomerulonephritis (36%) and IgA nephropathy (13%) were the most prevalent diagnoses. In AKI-NS, MGRS, minimal change disease, and diabetic nephropathy were equally represented (16.3% each). In AKI-I, AIN was the predominant diagnosis (60%). Patients in the AKI-U group tended to have a more prevalent ANCA positivity (p: 0.013), while at biopsy indication, mean serum creatinine was higher in the AKI-I group (p: 0.0159). Following biopsy, 78 patients (81%) received a biopsy-directed immunosuppressive treatment. At 90-day follow-up, 60% of patients developed CKD, 10% achieved complete renal recovery, 5% died, and 15% required RRT. Finally, of the 10 patients requiring dialysis at presentation 4 patients (40%), all of whom were treated, were not dialysis-dependent at the last available follow-up. AKI presents with heterogeneous renal lesions, many of which are treatable but underrecognized without biopsy. Kidney biopsy has a high diagnostic yield and can guide treatment, particularly in severe or dialysis-dependent AKI. Expanding biopsy indications and standardizing diagnostic approaches may improve the identification of treatable conditions, ultimately enhancing patient outcomes.