Urinary tract infections (UTIs) encompass a wide range of clinical and pathological conditions affecting different parts of the urinary tract. Each condition has its own epidemiology, natural history, and diagnostic and treatment considerations, highlighting the need for a standardized classification. Several classification systems are currently in use. Most guidelines for clinical practice [1–4] and research [5–7] are based on classification of UTIs as uncomplicated or complicated infections. This concept was first introduced by Lindemeyer et al [8] in 1963, and later refined by the Infectious Diseases Society of America (IDSA) and the European Society of Clinical Microbiology and Infectious Diseases in 1992 [9]. However, this distinction is often misunderstood and a persistent source of confusion, even among UTI experts, and especially among physicians who are not infectious disease specialists [10,11].
Re-classification of UTIs
Introduction:
Traditionally, UTIs have been divided into ‘complicated’ and ‘uncomplicated’ based on host and anatomical factors, however this binary classification has often been misinterpreted.
These definitions have been used for almost six decades, first described by Lindemeyer et al in 1963. They were subsequently refined by the IDSA (Infectious Diseases Society of America) to distinguish infections in healthy women from those linked to urinary tract abnormalities.
They have been defined as follows (EAU Guidelines 2024):
(European Association of Urology Guidelines on Urological Infections: Summary of the 2024 Guidelines, Kranz et al, PMID: 38714379)
This framework provided a broad way of differentiating ‘low-risk’ cases (such as acute cystitis in a young woman) from those at higher risk of treatment failure, recurrent infection or systemic illness. However, over time, the definition of ‘complicated’ expanded to cover a heterogeneous spectrum of conditions, ranging from mild infections in men to life-threatening urosepsis which consequently limited and complicated its clinical utility.
As such, the two terms have been open to misinterpretation with the potential to result in inappropriate patient care (i.e. undertreatment of systemic infections or overtreatment of low-risk cases) and also resulting in significant heterogeneity in clinical trials. This subsequently results in difficulty to formulate high level evidence-based guidelines.
In 2025, the EAU proposed a paradigm shift; to abandon the terms ‘complicated’ versus ‘uncomplicated’ in favour of a simpler, clinically intuitive classification of ‘localised’ versus ‘systemic’ infections.
These have been described as follows:
The recommendation is that once it has been agreed if the UTI is localised or systemic, the presence of any additional risk factors needs to be accounted for. This would then influence the treatment as certain risk factors may increase the likelihood of a more complicated clinical course.
This means that both a localised UTI and a systemic UTI can be considered ‘complicated,’ depending on the presence of additional risk factors.
Examples of such risk factors include:
One example of the potential for misinterpretation and inaccurate diagnosis/treatment using the old classification is pyelonephritis. The 2024 version of the EAU Guidelines clearly state that ‘uncomplicated pyelonephritis’ falls under the uncomplicated UTI category. As a result, some patients with pyelonephritis may be sub optimally managed with inappropriate antibiotic choices or shorter treatment courses, potentially resulting in poor patient outcomes. With the updated classification, pyelonephritis is considered to be a systemic UTI, which is more likely to result in appropriate treatment.
The updated classification also allows for UTIs in male patients to be treated as an uncomplicated infection (provided there are no additional risk factors or prostatic involvement).
Importantly, it remains to be seen whether this classification system can lead to clinically relevant improvements in management and treatment outcomes. This requires future studies to validate this classification system, and to assess clinical outcomes compared to the currently used classification.
Summary
This paper from the EAU Guidelines panel on urological infections provides a potentially more focused and clinically relevant framework for classifying UTIs.
It is suggested that the classification ‘complicated UTI’ and ‘uncomplicated UTI’ are now obsolete, and classification as ‘localised UTI’ (with/without risk factors) and ‘systemic UTI’ (with/without risk factors) should be used. The presence of these additional risk factors would then dictate which antibiotics are most appropriate, the duration and mode of administration of treatment and the need for additional diagnostic evaluation (such as cross-sectional imaging). The updated classification may provide both clarity and simplicity as well as potentially encouraging improved anti-microbial stewardship when managing patients with UTIs. Whether the use of this classification system leads to improved clinical outcomes remains to be seen.
With a more standardised definition that is less subject to misinterpretation, the issues related to heterogeneity in study populations within randomised controlled trials and basic research should no longer be an issue. This paradigm shift is necessary to help optimise interpretation, synthesis and comparison of results while reducing the risk of misclassification bias.