Is an elevated leukocyte (white blood cell) esterase level indicative of a urinary tract infection (UTI)?

Leukocyte Esterase 500 and UTI Diagnosis

Leukocyte esterase alone is not sufficient to diagnose a urinary tract infection (UTI), as it has good sensitivity but limited specificity, and must be interpreted alongside clinical symptoms and other diagnostic findings. 1

Diagnostic Value of Leukocyte Esterase

• Leukocyte esterase is a marker for pyuria (white blood cells in urine) and has the following diagnostic performance:

  • Sensitivity: 72-97%
  • Specificity: 41-86% 1

• While leukocyte esterase has high sensitivity, its relatively low specificity means it can be positive in conditions other than UTI, including:

  • Asymptomatic bacteriuria (common in 10-50% of long-term care facility residents)
  • Contaminated specimens
  • Non-infectious inflammatory conditions 2

Proper Diagnostic Approach

Step 1: Assess for Clinical Symptoms

• UTI symptoms include:
  • Dysuria, frequency, urgency, nocturia
  • Suprapubic pain
  • Fever (in cases of upper UTI/pyelonephritis)
  • In elderly: may present with confusion, falls, or increased incontinence 2, 1

Step 2: Urinalysis Interpretation

• Microscopic pyuria (≥10 WBCs/high-power field) or positive leukocyte esterase has high sensitivity but limited specificity for bacteriuria 2
• The absence of pyuria or a negative leukocyte esterase test is highly valuable to exclude UTI (negative predictive value approaches 100%) 2
• Combined leukocyte esterase and nitrite testing offers the best diagnostic performance:
  • Combined sensitivity: 46-100%
  • Combined specificity: 42-98% 1

Step 3: Urine Culture

• Only order urine culture when:
  • Pyuria (≥10 WBCs/high-power field) or positive leukocyte esterase/nitrite test is present 2
  • Patient has symptoms consistent with UTI 2, 1
  • Suspected urosepsis (paired with blood cultures) 2

Common Pitfalls to Avoid

1. Overdiagnosis: Treating asymptomatic bacteriuria based solely on positive leukocyte esterase or culture without symptoms can lead to unnecessary antibiotic use and resistance 2, 1

2. Misinterpretation in elderly: Non-specific symptoms like confusion or functional decline are often attributed to UTI but may not be associated with it, despite positive urinalysis findings 2

3. Ignoring pre-test probability: The diagnostic value of leukocyte esterase varies based on patient population and clinical presentation 1

4. Relying on a single test: The combination of leukocyte esterase and nitrite testing provides better diagnostic accuracy than either test alone 1, 3

Special Considerations

• Elderly patients: High prevalence (10-50%) of asymptomatic bacteriuria means positive tests must be interpreted cautiously 2

• Catheterized patients: Nearly 100% have bacteriuria with long-term catheters; clinical symptoms are essential for diagnosis 2

• Neutropenic patients: Significant bacteriuria may occur without pyuria 2

• Suspected urosepsis: Obtain both urine and blood cultures regardless of urinalysis results 2

In conclusion, while leukocyte esterase is a valuable screening tool, it must be combined with clinical symptoms and other diagnostic findings to accurately diagnose UTI and avoid inappropriate antibiotic use.