Understanding CT Resolution Timeline in Acute Pyelonephritis: A Framework for Clinical Practice

Abstract Purpose: Acute pyelonephritis is diagnosed based on clinical and laboratory signs. Computed tomography (CT) is typically reserved for detecting complications or alternative diagnoses. Current guidelines do not recommend routine follow-up imaging when abnormalities are found. The expected time to resolution (TTR) of CT abnormalities has not been clearly identified. This is the goal of our study. Methods: Retrospective study at an academic hospital in Harris County, Texas. Adult patients with a diagnosis of acute pyelonephritis who underwent at least two CT scans were included. Three board-certified radiologists independently reviewed CT studies performed within one year of the index scan and assessed for striated nephrogram, parenchymal edema, patchy hypodensities, perinephric fat stranding, and urothelial thickening. TTR was defined as the interval between the first positive CT sign and the first subsequent CT in which the sign was absent. TTR was calculated for individual signs and globally, provided clinical criteria for pyelonephritis were met. Sensitivity, specificity, and inter-rater agreement were also evaluated. Results: 288 CT scans from 93 patients were reviewed; 252 (88%) were of sufficient quality for analysis. The mean global TTR was 149 ± 100 days, with wide variation across individual signs. Moreover, imaging abnormalities persisted for 105 days despite clinical resolution. Conclusion: CT signs consistent with pyelonephritis may persist for several months after clinical recovery. Clinicians should interpret persistent imaging abnormalities appropriately to avoid unnecessary diagnostic or therapeutic interventions.