Epidemiology, risk factors, diagnosis, and microbiology of suppurative pyelonephritis: a systematic review and meta-analysis, Part 1

Introduction. The first part of a systematic review and meta-analysis addressing the problem of purulent pyelonephritis is presented in the article.

Aim. To analyze the epidemiology, predisposing factors, diagnostic approaches, and microbiological characteristics of purulent pyelonephritis, which is a complicated form of acute pyelonephritis characterized by renal parenchymal suppuration, sepsis, or localized abscess.

Materials and methods. A systematic review of 46 studies (1981–2024) on complicated pyelonephritis was performed. The review included clinical series (observational and one randomized trial) with ≥10 patients reporting on prevalence, risk factors, clinical course, diagnostic approaches, microbiology, treatment, and outcomes of severe pyelonephritis. Data were synthesized qualitatively, and a meta-analysis of key outcomes (mortality and need for surgical intervention) was carried out using a random-effects model.

Results. Acute pyelonephritis is one of the most common serious urinary tract infections, with an annual incidence of 15–17 per 10,000 women and 3–5 per 10,000 men. In most cases, the disease responds well to antibiotic therapy; however, in 20–30% of patients complicated pyelonephritis develops.

Predisposing factors include diabetes mellitus, urinary tract obstruction, advanced age, male sex, immunodeficiency, and pregnancy. Diabetes mellitus is present in 30–35% of hospitalized patients with pyelonephritis (vs. 10–15% in uncomplicated cases) and in 75–95% of patients with emphysematous pyelonephritis. Urolithiasis accounts for approximately 20% of cases with complicated pyelonephritis. Elderly (>65 years) and male patients are affected less frequently but experience more severe disease: men constitute only about 25% of acute pyelonephritis cases but have higher rates of abscess and sepsis.

Purulent pyelonephritis is typically associated with pronounced systemic symptoms: high fever (>39 °C in 70% of cases), chills (50–60%), and septic shock (25–30% upon admission). In 15–20% of severe cases, local urinary symptoms (flank pain, dysuria) are absent.

Laboratory findings usually demonstrate leukocytosis >15×109/L (80%) or, conversely, leukopenia <4×109/L (20–30%) in cases with disseminated intravascular coagulation, along with markedly elevated C-reactive protein levels (>100 mg/L).

Imaging plays a decisive role: ultrasound can detect hydronephrosis and pyonephrosis, whereas contrast-enhanced computed tomography is the gold standard for detecting abscesses and gas formation.

The main pathogens of complicated pyelonephritis are Gram-negative enteric bacteria, primarily Escherichia coli (60–75%), Klebsiella pneumoniae (10–15%), and Proteus mirabilis (5–10%). In 10–30% of cases, isolates exhibit multidrug resistance (e.g., ESBL-producing strains). Polymicrobial infection occurs in approximately 5–10% of severe cases.

Conclusions. Purulent pyelonephritis is a relatively uncommon but potentially life-threatening complication of renal infection, strongly associated with risk factors such as diabetes and urinary obstruction. Improved patient outcomes depend on early identification of high-risk individuals, timely imaging to detect purulent complications, and empirical antibiotic therapy that accounts for likely antimicrobial resistance (treatment strategies and outcomes will be discussed in Part 2 of this review).