What are the diagnostic criteria for a Urinary Tract Infection (UTI)?

Diagnostic Criteria for Urinary Tract Infections

UTI diagnosis requires BOTH clinical symptoms consistent with urinary tract infection AND laboratory evidence of pyuria plus pathogenic bacteria confirmed by quantitative urine culture—never diagnose or treat based on laboratory findings alone. 1

Essential Diagnostic Components

The diagnosis must combine three elements:

• Clinical symptoms that vary by infection location 1
• Pyuria (white blood cells in urine) must be present 1, 2
• Bacteriuria confirmed by urine culture with appropriate colony counts 1, 2

Critical pitfall: The presence of bacteria without symptoms is asymptomatic bacteriuria, which should NOT be treated (except in pregnancy) as treatment causes antimicrobial resistance, adverse effects, and C. difficile infection. 2

Clinical Symptoms by Location

Lower Tract (Cystitis)

• Dysuria (painful urination) 1
• Urinary frequency 1
• Urinary urgency 1
• Suprapubic pain 1
• Absence of fever or systemic symptoms 1

Upper Tract (Pyelonephritis)

• All cystitis symptoms PLUS: 1
• Fever 1
• Flank pain 1
• Systemic symptoms (malaise, vomiting) 1

Pediatric Presentation (Infants and Children)

• Non-specific symptoms: fever, vomiting, abdominal pain 1
• In infants 2-24 months, fever without obvious source qualifies as a clinical symptom 2

Important note: In women with typical symptoms and no vaginal discharge, self-diagnosis is accurate enough to diagnose uncomplicated UTI without further testing. 3

Laboratory Confirmation

Urine Culture Thresholds (Gold Standard)

Colony count requirements vary by collection method and population:

Adults

• Clean-catch/midstream specimen: ≥100,000 CFU/mL 1
• Catheterized specimen: ≥50,000 CFU/mL 1
• However, even growth as low as 10² CFU/mL with specific symptoms and pyuria can represent UTI 1, 4

Infants and Children

• Catheterized specimen: ≥50,000 CFU/mL 2
• Clean-catch/midstream: ≥100,000 CFU/mL (due to higher contamination risk) 2
• Suprapubic aspiration: ≥100 CFU/mL or any growth may be positive 2

Typical Uropathogens

• E. coli (causes ~75% of UTIs) 5
• Klebsiella 2
• Proteus 2
• Enterococcus 2
• Staphylococcus saprophyticus 2

Not considered uropathogens: Lactobacillus species, coagulase-negative staphylococci (except S. saprophyticus), Corynebacterium species. 2

Urinalysis Performance (Screening Tests)

Urinalysis supports but does not confirm diagnosis—understand its limitations: 1

Individual Test Characteristics

• Nitrites: 19-48% sensitivity, 92-100% specificity (highly specific but misses many infections) 1, 2
• Leukocyte esterase: 72-97% sensitivity, 41-86% specificity (more sensitive but less specific) 1, 2
• Combined positive (either test): 46-100% sensitivity, 42-98% specificity 1, 2
• WBC >5/μL: 90-96% sensitivity, 47-50% specificity 2
• Gram stain of uncentrifuged urine: 93% sensitivity, 4% false positive rate 2

Critical interpretation: In patients with high probability of UTI based on symptoms, negative dipstick urinalysis does NOT rule out infection. 4 Pyuria is commonly found without infection, particularly in older adults with incontinence. 4

When to Obtain Urine Culture

Culture NOT Routinely Necessary

• Simple uncomplicated cystitis in healthy nonpregnant patients 1

Culture IS Indicated

• Complicated UTIs 1
• Recurrent UTIs 1
• Suspected pyelonephritis 1
• Pregnant women 1
• Treatment failure 1
• History of resistant isolates 1
• Atypical presentation 1
• All men with UTI symptoms 3
• Adults ≥65 years (to adjust antibiotics after empiric treatment) 3

Classification Systems

Uncomplicated vs Complicated UTI

Uncomplicated UTI: 5, 6

• Cystitis in non-pregnant woman
• No immunocompromise
• No anatomical/functional urogenital abnormalities
• No signs of tissue invasion or systemic infection

Complicated UTI (any of the following): 5, 6

• Men
• Pregnant women
• Immunocompromised patients
• Anatomical abnormalities (cystoceles, diverticula, fistulae, indwelling catheters, obstruction) 5
• Functional abnormalities (voiding dysfunction) 5
• Diabetes 5
• Prior urinary tract surgery/trauma 5
• Repeated pyelonephritis 5

Recurrent UTI Definition

• ≥3 episodes within preceding 12 months 5
• OR: 2+ episodes of pyelonephritis 5
• OR: 1 pyelonephritis + 1+ cystitis 5
• OR: 3+ episodes of cystitis 5

Special Populations

Pediatric UTI

• UTI confirmed by ≥5 x 10⁴ CFU/mL in catheterized specimen or 10⁵ CFU/mL in voided specimen 5
• Between 50-64% of children with febrile UTI have acute pyelonephritis on renal cortical scintigraphy 5
• Peak incidence in first year of life, second peak at ages 2-4 years during toilet training 5

Elderly Patients

• Asymptomatic bacteriuria is common and should NOT be treated 2
• UTIs may present differently with atypical symptoms 4
• Pyuria commonly present without infection, especially with incontinence 4

Pregnancy

• Positive dipstick testing is specific for asymptomatic bacteriuria, but urine culture remains test of choice 4
• Asymptomatic bacteriuria SHOULD be treated in pregnancy (exception to general rule) 2

Role of Imaging

Imaging is NOT part of routine UTI diagnosis but indicated for specific scenarios:

• Ultrasound: 74.3% sensitivity, 56.7% specificity; first-line in children, pregnancy, transplant recipients 2
• CT scan: 81-84% sensitivity, 87.5% specificity; reserved for persistent symptoms beyond 72 hours, suspected abscess, or alternative diagnosis 2
• MRI: 100% sensitivity, 81.8% specificity; limited role 2

Molecular Testing

Molecular diagnostics have NO established role in routine UTI diagnosis because they cannot distinguish infection from asymptomatic bacteriuria, cannot determine bacterial viability, and cannot provide quantitation. 2