How do you test for urinary tract colonization in a patient?

Testing for Urinary Tract Colonization

The key distinction is that urinary tract colonization (asymptomatic bacteriuria) should not be routinely tested for or treated in most populations, and when testing is indicated for suspected infection, proper specimen collection with urinalysis and culture is essential. 1, 2

Understanding Colonization vs. Infection

• Asymptomatic bacteriuria (colonization) is extremely common, affecting 10-50% of long-term care facility residents and 15-50% of older adults in the community, and represents bacterial presence without inflammation or symptoms 1, 2
• The absence of pyuria (negative leukocyte esterase AND no microscopic WBCs) has excellent negative predictive value approaching 100% for excluding true infection, helping distinguish colonization from infection 1, 2
• Pyuria alone does not distinguish infection from colonization - many colonized patients have pyuria without infection, particularly elderly patients with incontinence or other lower urinary tract symptoms 1, 3

When NOT to Test

Do not perform urinalysis or urine cultures in asymptomatic individuals, including:

• Older adults living in the community or long-term care facilities without acute urinary symptoms 1, 2
• Patients with indwelling catheters who lack fever, hypotension, or specific urinary symptoms 1, 2
• Elderly patients with only non-specific symptoms like confusion, functional decline, or falls without dysuria, frequency, urgency, fever, or gross hematuria 1, 2

When Testing is Indicated

Testing should only occur with acute onset of specific UTI-associated symptoms:

• Dysuria, frequency, urgency, fever >38°C (100.4°F), gross hematuria, or new/worsening urinary incontinence 1, 2
• Suspected urosepsis with fever, shaking chills, hypotension, or hemodynamic instability 1, 2

Proper Specimen Collection Methods

Specimen collection technique is critical to avoid false positives from contamination:

For Non-Catheterized Patients:

• Midstream clean-catch urine in cooperative adults - process within 1 hour at room temperature or refrigerate if delayed 1, 2
• In-and-out catheterization for women unable to provide clean specimens to avoid contamination 1, 2
• Clean condom catheter with frequent monitoring for men when midstream collection is not feasible 1, 2

For Catheterized Patients:

• Never collect from the drainage bag - bacteria multiply to high levels in the bag, causing false positives 1
• Aspirate from the catheter sampling port after cleaning with 70-90% alcohol 1
• Consider replacing the catheter and collecting from the newly placed catheter if strong clinical suspicion exists 2

For Infants and Young Children:

• Catheterization or suprapubic aspiration is mandatory - bag-collected specimens have only 15% positive predictive value with 85% false positives 1

Diagnostic Testing Algorithm

Step 1: Urinalysis (dipstick and/or microscopy)

• Leukocyte esterase: 83% sensitivity, 78% specificity; negative result helps rule out infection 1, 2
• Nitrite: 19-48% sensitivity but 92-100% specificity; positive result strongly suggests infection 2
• Combined leukocyte esterase + nitrite: 93% sensitivity, 72-96% specificity 1, 2
• Microscopic pyuria threshold: ≥10 WBCs/high-power field in spun urine 1, 2

Step 2: Urine Culture (when indicated)

• Obtain culture before antibiotics if:

  • Either leukocyte esterase OR nitrite is positive with symptoms 1, 2
  • Suspected pyelonephritis or urosepsis regardless of urinalysis 2
  • Febrile infants <2 years (10-50% have false-negative urinalysis) 2

• Culture thresholds for diagnosis:

  • ≥50,000 CFU/mL for catheterized specimens (updated from older 100,000 threshold) 1
  • Lower thresholds acceptable for suprapubic aspiration 1
  • Even 10,000 CFU/mL may be significant with symptoms and pyuria in some contexts 1, 3

Critical Pitfalls to Avoid

• Do not treat positive cultures without symptoms - this represents colonization, not infection, and treatment provides no benefit while increasing resistance 1, 2
• Do not use bag-collected urine for culture in children - the false positive rate makes results uninterpretable 1
• Do not delay specimen processing - refrigerate if transport exceeds 1 hour to prevent bacterial multiplication that causes false positives 1, 2
• High epithelial cell counts indicate contamination - repeat collection with proper technique rather than treating based on contaminated results 2
• Mixed bacterial flora suggests contamination, not polymicrobial infection - obtain a clean specimen before making treatment decisions 2

Special Population Considerations

Long-Term Care Facilities:

• Evaluate only with acute onset of specific urinary symptoms - presence of pyuria has low predictive value due to 10-50% asymptomatic bacteriuria prevalence 1, 2
• Prospective studies show untreated asymptomatic bacteriuria persists 1-2 years without increased morbidity or mortality 1

Critically Ill/ICU Patients:

• Pyuria may be absent in catheter-associated infections despite significant bacteriuria, unlike community-acquired UTI where pyuria is highly predictive 1
• Infections are often polymicrobial with multiresistant nosocomial organisms 1

Pediatric Patients (2-24 months):

• Both abnormal urinalysis AND positive culture (≥50,000 CFU/mL) required to confirm true UTI rather than asymptomatic bacteriuria 1
• Obtain specimens before administering antibiotics in ill-appearing children 1