Definition of Complicated Urinary Tract Infection
A complicated UTI is a symptomatic urinary infection occurring in individuals with functional or structural abnormalities of the genitourinary tract, or in hosts with conditions that increase the risk of treatment failure or serious complications. 1
Core Definition
Complicated UTI encompasses any symptomatic urinary infection that occurs outside the narrow definition of uncomplicated UTI—essentially any UTI that is NOT in an otherwise healthy, nonpregnant woman with normal genitourinary anatomy. 2, 3
The infection may involve either the bladder (cystitis) or kidneys (pyelonephritis), but the distinguishing feature is the presence of complicating factors rather than the anatomical site. 1
Urinary infection in men is almost always considered complicated, as uncomplicated UTI occurs rarely in males. 1, 2
Anatomical and Structural Abnormalities That Define Complicated UTI
Urinary tract obstruction (e.g., stones, tumors, strictures, prostatic hypertrophy) classifies a UTI as complicated. 2, 4
Vesicoureteral reflux makes any UTI complicated. 2
Incomplete bladder emptying or elevated post-void residual volumes are structural/functional abnormalities that complicate UTI. 2, 4
Presence of foreign bodies, most notably indwelling urinary catheters, but also ureteral stents or nephrostomy tubes, defines complicated UTI. 2, 3
Bladder or urethral diverticula, fistulae, or cystoceles are anatomical abnormalities that make UTI complicated. 2, 4
Urinary calculi or infected urachal remnants create niches for bacterial persistence and classify UTI as complicated. 4
Host Factors That Define Complicated UTI
Pregnancy automatically classifies any UTI as complicated due to increased risk of progression to pyelonephritis and adverse pregnancy outcomes. 2, 3
Diabetes mellitus is a host factor that makes UTI complicated. 3
Immunocompromised states (e.g., HIV, transplant recipients, chemotherapy patients) classify UTI as complicated. 2, 5
Neurological diseases affecting bladder function (e.g., spinal cord injury, multiple sclerosis) make UTI complicated. 2, 5
Advanced age or institutional residence (nursing home) is recognized as a host factor for complicated UTI. 3
Recent urological instrumentation or surgery classifies subsequent UTI as complicated. 2
Microbiological Characteristics
Complicated UTIs demonstrate greater microbial diversity and higher rates of antimicrobial resistance compared to uncomplicated UTIs. 6, 7
While E. coli remains the most common pathogen (approximately 75% overall), non-E. coli organisms are proportionally more common in complicated UTI, including Enterococcus, Proteus, Klebsiella, Pseudomonas, and Enterobacter species. 4, 6
Clinical Implications of the Complicated UTI Designation
Complicated UTIs carry higher risk of treatment failure, recurrence, progression to chronic infection, and severe outcomes including sepsis. 3, 7
Treatment duration is longer for complicated UTI (7-14 days) compared to uncomplicated cystitis (3-5 days). 3, 6
Urine culture is mandatory before initiating antimicrobial therapy in complicated UTI, whereas it is optional in typical uncomplicated cystitis. 2, 7
Imaging studies are often indicated in complicated UTI to characterize the underlying abnormality, whereas routine imaging is not required for uncomplicated cases. 3
If the underlying genitourinary abnormality cannot be corrected, recurrence rates approach 50% within 4-6 weeks after treatment. 6, 8
Common Pitfalls
Failing to distinguish between uncomplicated and complicated UTI has critical implications for antibiotic selection, treatment duration, and need for imaging. 2, 3
Asymptomatic bacteriuria in patients with complicated host factors (e.g., catheter, diabetes) should generally NOT be treated, as treatment does not improve outcomes and promotes resistance—exceptions include pregnancy and before invasive urological procedures. 3, 7
Nitrofurantoin should be avoided in complicated UTI, particularly in patients with renal impairment or when tissue penetration is needed (e.g., pyelonephritis), as it achieves inadequate tissue levels. 3, 8