Diagnostic Utility of Renal Hypertrophy on CT Imaging for Differentiating Acute Interstitial Nephritis from Other Causes of AKI

Background: Early treatment of acute interstitial nephritis (AIN) is effective; however, kidney biopsy, the gold standard for diagnosis, is often unfeasible in critically ill patients. Although kidney hypertrophy is a well-recognized feature of AIN, its diagnostic utility remains unclear. We evaluated CT-based kidney hypertrophy as a non-invasive AIN predictor. Methods: We retrospectively analyzed 40 adults who undergoing kidney biopsy for AKI (2010-2024) with pre- and post-AKI CT scans (AIN=16; non-AIN=24). Kidney hypertrophy rate was defined as the percentage increase in kidney volume. We evaluated five predictors: the classic triad, urinary white blood cells >10/high-power field, peripheral eosinophils >500/μL, drug exposure (antibiotics, nonsteroidal anti-inflammatory drugs, or proton pump inhibitors), and kidney hypertrophy >20%. A logistic regression model and weighted diagnostic score were constructed, and discrimination was assessed using the area under the receiver operating characteristic curve (AUC). Results: There was no significant difference in age (67.7 vs. 65.6 years) or estimated glomerular filtration rate (26.8 vs. 23.7 mL/min/1.73 m2) between the groups. The AIN group had higher eosinophils (477 vs. 153/µL, P = 0.02), triad positivity (56% vs. 25%, P=0.046), and kidney hypertrophy (140.0 vs. 108.8 %, P < 0.01). Stepwise variables inclusion increased the AUC to 0.85 with all five predictors. A scoring system (range 0–4.5; triad 0.5, eosinophils 1, drug exposure 1, hypertrophy 2) showed good discrimination. At a cutoff of 2.5, sensitivity was 75.0%, specificity 79.2%, positive predictive value (PPV) 70.6%, and negative predictive value 82.6%. A score ≥4.0 yielded 100% specificity and PPV (Figure). Conclusion: Kidney hypertrophy rate was 40% in AIN patients. A combined clinical-imaging score showed good discrimination, potentially aiding early diagnosis when biopsy is unfeasible.