What urinalysis findings are indicative of a Urinary Tract Infection (UTI)?

Urinalysis Findings Indicative of a Urinary Tract Infection (UTI)

The most reliable urinalysis findings indicative of a UTI are pyuria (≥10 WBCs/high-power field), positive leukocyte esterase, and positive nitrite tests, with the combination of these markers providing the highest diagnostic accuracy. 1, 2

Key Diagnostic Findings

Primary Urinalysis Markers

• Pyuria: ≥10 WBCs/high-power field is a strong indicator of UTI 1
• Leukocyte esterase:
  • Sensitivity: 83% (67-94%)
  • Specificity: 78% (64-92%) 2
• Nitrite:
  • Sensitivity: 53% (15-82%)
  • Specificity: 98% (90-100%) 2
• Combined leukocyte esterase or nitrite:
  • Sensitivity: 93% (90-100%)
  • Specificity: 72% (58-91%) 2

Additional Supportive Findings

• Presence of bacteria on microscopic examination
• Hematuria (blood in urine)
• Proteinuria (may be present but less specific)

Diagnostic Algorithm

1. Initial Screening: Perform urinalysis with dipstick testing for leukocyte esterase and nitrite, plus microscopic examination for WBCs 1

2. Interpretation of Results:

   • High probability of UTI: Positive nitrite OR pyuria with positive leukocyte esterase
   • Moderate probability of UTI: Positive leukocyte esterase alone
   • Low probability of UTI: Negative for both leukocyte esterase and nitrite, no pyuria

3. Confirmatory Testing:

   • If pyuria (≥10 WBCs/high-power field) OR positive leukocyte esterase OR positive nitrite test is present, proceed with urine culture 1
   • Negative urinalysis for WBCs and negative dipstick for leukocyte esterase are useful to exclude a urinary source for suspected infection 1

Clinical Considerations

Specimen Collection

• Proper specimen collection is crucial for accurate results:
  • For men: Clean-catch midstream specimen or condom catheter collection 1
  • For women: Clean-catch midstream specimen or in-and-out catheterization if clean catch not possible 1
  • For patients with indwelling catheters: Change catheter prior to specimen collection 1

Important Caveats

1. False Negatives: Approximately 10% of children with UTIs have negative rapid diagnostic tests 1, and this can occur in adults as well.

2. Organism-Specific Variations:

   • E. coli infections more commonly present with pyuria (80.6%) and positive leukocyte esterase 3
   • Klebsiella and Enterococcus infections may present without pyuria (only 53.3% and 52.0% show pyuria, respectively) 3

3. Special Populations:

   • In elderly patients, UTI may present with non-specific symptoms and signs (confusion, incontinence, anorexia, functional decline) 1
   • In diabetic patients, WBC sensitivity (65.7%) and specificity (100%) are slightly different from non-diabetics 4

4. Asymptomatic Bacteriuria:

   • Common in elderly (10-50% prevalence) 1
   • Should not be treated unless in specific populations (pregnant women) 2
   • Do not perform urinalysis or urine cultures in asymptomatic patients 1

Common Pitfalls to Avoid

1. Relying solely on a single marker: The combination of tests provides better diagnostic accuracy than any single test 2

2. Ignoring clinical presentation: Laboratory findings should always be interpreted in the context of symptoms 1

3. Treating asymptomatic bacteriuria: This can lead to unnecessary antibiotic use and resistance 2

4. Assuming absence of pyuria rules out UTI: Especially with certain organisms like Klebsiella and Enterococcus 3

5. Contaminated specimens: High squamous epithelial cell count (10-20/HPF) indicates contamination 2

By systematically evaluating these urinalysis findings and understanding their limitations, clinicians can more accurately diagnose UTIs and initiate appropriate treatment to reduce morbidity and mortality associated with urinary tract infections.