What are the markers for urinary tract infection (UTI) in urinalysis (UA)?

Urinary Tract Infection Markers in Urinalysis

The key markers for urinary tract infection (UTI) in urinalysis include leukocyte esterase, nitrites, pyuria (≥10 WBCs/mm³ or ≥5 WBCs/HPF), and bacteriuria, with leukocyte esterase and nitrite tests having sensitivities of 83% and 53%, and specificities of 78% and 98%, respectively. 1

Primary Diagnostic Markers

Leukocyte Esterase and Nitrite Tests

• Leukocyte esterase:

  • Sensitivity: 72-97%
  • Specificity: 41-86%
  • Positive predictive value (PPV): 43-56%
  • Negative predictive value (NPV): 82-91% 2
  • Excellent negative predictive value for ruling out UTI 1

• Nitrite test:

  • Sensitivity: 19-48%
  • Specificity: 92-100%
  • PPV: 50-83%
  • NPV: 70-88% 2
  • High specificity makes positive results more reliable 1

• Combined (leukocyte esterase or nitrite):

  • Sensitivity: 46-100%
  • Specificity: 42-98%
  • PPV: 52-68%
  • NPV: 78-98% 2
  • Provides better diagnostic accuracy than either test alone 1

Microscopic Examination

• Pyuria (WBC count):

  • Significant pyuria defined as:
    • ≥10 WBCs/mm³ on enhanced urinalysis or
    • ≥5 WBCs/HPF on centrifuged specimen 1
  • Sensitivity and specificity vary by threshold:

    • 5 WBCs/μL: 90-96% sensitivity, 47-50% specificity

    • 100 WBCs/μL: 93% sensitivity, 71% specificity
    • 200 WBCs/μL: 89% sensitivity, 86% specificity 2

• Bacteriuria:

  • More specific and sensitive than pyuria for detecting UTI 3
  • Traditionally, ≥100,000 CFU/mL has been the threshold, but lower counts can indicate infection in symptomatic patients 2

Interpretation and Clinical Context

1. Diagnostic Algorithm:

   • If leukocyte esterase and nitrite are both negative with no pyuria, UTI is unlikely 1
   • If either leukocyte esterase or nitrite is positive with pyuria, UTI is likely in symptomatic patients 2
   • If microscopic examination shows no pyuria (negative leukocyte esterase by dipstick), no urine culture is needed 2
   • If pyuria or positive leukocyte esterase is present, proceed with urine culture 2

2. Important Considerations:

   • UTI diagnosis should be primarily based on symptoms with urinalysis as supporting evidence 1
   • Relying solely on urinalysis without clinical context should be avoided 1
   • Urine should be processed within 2 hours or refrigerated to prevent false results 1
   • Collection method can affect test results (bag collection vs. catheterization) 1

Clinical Symptoms to Correlate with UA Findings

• Typical UTI symptoms:

  • Dysuria
  • Frequency
  • Urgency
  • Suprapubic pain
  • New or worsening urinary incontinence
  • Hematuria 2, 1

• Atypical presentations (especially in older adults):

  • Altered mental status
  • Functional decline
  • Fatigue
  • Falls 1

Common Pitfalls to Avoid

1. Overreliance on UA without symptoms:

   • Asymptomatic bacteriuria is common, particularly in older adults, and should not be treated with antibiotics 3
   • Positive dipstick tests have low positive predictive value when used alone 1

2. Improper specimen collection:

   • Contaminated specimens lead to false positives
   • For catheterized patients, obtain urine by aspiration of the catheter port, not from the drainage bag 2
   • For non-catheterized patients, midstream or clean-catch specimens are preferred 2

3. Delayed processing:

   • Specimens not processed within 2 hours or refrigerated may yield false results 1

4. Misinterpretation of pyuria:

   • Pyuria is commonly found in the absence of infection, particularly in older adults with lower urinary tract symptoms 3

By combining these urinalysis markers with clinical symptoms, clinicians can more accurately diagnose UTIs while avoiding unnecessary antibiotic treatment of asymptomatic bacteriuria, thus promoting antimicrobial stewardship.