A Cross-Sectional Analysis of Clinical and Biological Characteristics of Inpatients with Complicated Acute Pyelonephritis

Background: Complicated acute pyelonephritis (AP) is a severe upper urinary tract infection associated with systemic inflammation, organ dysfunction, and the risk of sepsis. The increasing prevalence of antimicrobial-resistant (AMR) organisms can alter clinical management. This study aimed to characterize the biological profile of inpatients with complicated AP and to eventually identify laboratory markers associated with risks of sepsis and AMR infections. Material and Methods: A retrospective observational analysis on 553 adult inpatients diagnosed with complicated AP between 2021 and 2025 was conducted in a tertiary center. Demographic, clinical, and biological parameters were analyzed, including inflammatory markers and renal and hepatic markers. Results: Group characteristics included a mean age of 63.82 ± 15.67 years, and 63% were female. At admission, inflammatory markers were raised, with leukocytosis (15.6 ± 5.8 × 103/µL), neutrophilia (10.1 ± 4.7 × 103/µL), and elevated C-reactive protein (CRP) (median 43.2 mg/dL). Coagulation activation was significant with elevated fibrinogen of 747 ± 145 mg/dL and D-dimer with a median level of 1249 ng/mL, of which 58% exceeded 1000 ng/mL. Mild to moderate renal impairment was frequently observed (creatinine 1.69 ± 0.76 mg/dL). In multivariate analysis, no biological parameter proved to be an independent predictor of AMR status among organisms. Discussion and Conclusions: Inpatients with complicated AP showed a pronounced inflammatory and procoagulant biological profile that did not vary between AMR pathogen and non-AMR pathogen infections. This suggests that the clinical value of biomarkers, such as leukocyte and neutrophile, CRP, D-dimer, fibrinogen, procalcitonin, urea, and creatinine, lies primarily in assessing disease severity rather than predicting antimicrobial resistance. The microbiological profile was dominated by Gram-negative pathogens, particularly Escherichia coli, although a heterogeneous spectrum of microorganisms was identified.