Temporal Changes in Incidence of Acute Pyelonephritis: A 19-year Population-based Danish Cohort Study

Background: Acute pyelonephritis (APN) is a severe infection of the upper urinary tract and renal parenchyma. Increasing comorbidity burden and antibiotic resistance along with improving diagnostics may contribute to heightening the risk of APN hospitalization. Yet, few population-based studies have described APN incidence over time and according to age groups.

Objectives: To examine temporal changes in APN hospitalization rate in Denmark. Furthermore, we analyzed the impact of changing coding practices and population demographics.

Methods: A nationwide register-based cohort study including Danish patients hospitalized for APN during 2000-2018 (identified using a composite of International Classification of Diseases codes, 10th Revision). Diagnoses registered less than 30 days apart were considered a single hospitalization. Patients were characterized according to sex, age, year of diagnosis, comorbidity, and region of residence. We estimated annual sex- and age-standardized APN hospitalization rates per 10,000 person years with 95% confidence intervals (CIs), and stratified by sex, age group, diagnosis code, and region of residence. Hospitalization rate for urinary tract infection and sepsis diagnoses were investigated to address concomitant changes.

Results: We identified 61,439 APN hospitalizations in 52,479 patients (73.6% female). Median age at diagnosis was 28 years (IQR, 11-54) in female and 57 years (IQR, 8-73) in male patients. From 2000 to 2018, the sex- and age-standardized hospitalization rate per 10,000 person years increased from 6.4 (95% CI: 6.1-6.7) to 12.9 (95% CI: 12.5-13.3) in the female population and from 2.6 (95% CI: 2.4-2.8) to 4.0 (95% CI: 3.8-4.3) in the male population. Among infants, the rate increased from 7.4 (95% CI: 4.4-10.4) to 63.4 (95% CI: 54.4-72.4) in girls and from 17.1 (95% CI: 12.7-21.5) to 51.2 (95% CI: 43.3-59.1) in boys. The findings were associated with increased use of the unspecified pyelonephritis diagnosis and revealed interregional differences. A concomitant decline in hospitalization rates for unspecified urinary tract infection and sepsis diagnoses was observed in the Danish population in the most recent years.

Conclusions: The incidence of hospitalized APN in Denmark nearly doubled during 2000-2018. This was evident in all age groups and both sexes, yet largely driven by a prominently increasing hospitalization rate among infants. These temporal changes may partially be attributed to improved diagnostic specificity and changes in coding practice.