Is a leukocyte (white blood cell) esterase positive dipstick always an indication of Urinary Tract Infection (UTI)?

Leukocyte Esterase Positive Dipstick is Not Always an Indication of UTI

A leukocyte esterase positive dipstick alone is not always an indication of a urinary tract infection (UTI) and requires additional clinical and laboratory findings for definitive diagnosis. 1

Understanding Leukocyte Esterase Test Performance

Leukocyte esterase (LE) testing has the following performance characteristics:

• Sensitivity: 83% (range 67-94%)
• Specificity: 78% (range 64-92%) 2, 1

This means that while the test is reasonably sensitive, it still produces both false positives and false negatives.

Key Causes of False Positive LE Results

Several conditions can cause a positive leukocyte esterase result without a true UTI:

1. Asymptomatic bacteriuria: Bacteriuria without pyuria often represents colonization rather than infection, particularly in school-aged girls 2
2. Vulvovaginitis: Especially common in prepubertal girls 3
3. Phimosis: Associated with false positive LE tests in boys 3
4. Contamination: Improper collection techniques can introduce leukocytes from external sources

Definitive UTI Diagnosis Requirements

According to current guidelines, definitive diagnosis of UTI requires:

• Positive urinalysis suggesting infection (pyuria and/or bacteriuria)
• AND ≥50,000 CFU/mL of a uropathogen from a properly collected specimen 1

A positive leukocyte esterase test alone is insufficient for diagnosis.

Diagnostic Algorithm for Interpreting Positive LE Test

1. Assess for symptoms:

   • Presence of dysuria, frequency, urgency, suprapubic pain strongly supports UTI diagnosis
   • Absence of symptoms should prompt consideration of alternative explanations

2. Evaluate additional urinalysis parameters:

   • Nitrite test: High specificity (98%) but low sensitivity (53%) 2, 1
   • Microscopy for pyuria: Higher sensitivity (95.6%) and specificity (60.9%) than LE alone 4
   • Combined tests (LE + nitrite): Sensitivity 93%, specificity 72% 2, 1

3. Confirm with urine culture:

   • Essential before initiating antimicrobial treatment
   • Particularly important in high-risk populations (pregnant women, children) 1

Special Population Considerations

Pregnant Women

• Asymptomatic bacteriuria carries significant risks (20-30 fold increased risk of pyelonephritis)
• Treatment is recommended even without symptoms 1

Children

• Both urinalysis and culture are necessary to confirm UTI diagnosis
• Asymptomatic bacteriuria should be distinguished from true UTI, as treatment may do more harm than good 2, 1

Common Pitfalls in LE Test Interpretation

1. Overreliance on a single test: The sensitivity of nitrite alone (23.31%) and leukocyte esterase alone (48.5%) is too low to rule out UTI 5

2. Failure to consider clinical context: Symptoms significantly increase the pre-test probability of UTI

3. Neglecting microscopic examination: Pyuria on microscopy has higher sensitivity (95.6%) than dipstick tests alone 4

4. Treating without culture confirmation: Leads to unnecessary antimicrobial use, promotes resistance, and increases adverse drug events 1

5. Ignoring alternative diagnoses: Particularly in prepubertal girls with vulvovaginitis and boys with phimosis 3

In conclusion, while leukocyte esterase testing is a valuable screening tool, it must be interpreted in the context of clinical presentation and confirmed with urine culture for definitive diagnosis of UTI.