Is a positive leukoesterase (LE) test the same as pyuria?

Positive Leukocyte Esterase vs. Pyuria

Positive leukocyte esterase is not the same as pyuria, but rather serves as a surrogate marker for pyuria on dipstick testing. While they are closely related, they represent different detection methods for white blood cells in urine.

Definitions and Relationship

• Pyuria: Defined as the presence of white blood cells (WBCs) in urine, typically ≥5-10 WBCs per high-power field (HPF) on microscopic examination 1, 2
• Leukocyte Esterase (LE): An enzyme released from white blood cells that is detected by dipstick testing 1

The leukocyte esterase test on urine dipsticks serves as an indirect method to detect the presence of white blood cells in urine without requiring microscopy 1.

Diagnostic Performance

Leukocyte Esterase Test:

• Sensitivity: 83% (range 67-94%)
• Specificity: 78% (range 64-92%) 2
• Acts as a surrogate marker for pyuria 1

Microscopic Examination for Pyuria:

• More direct visualization of WBCs
• Generally considered more accurate than dipstick testing alone
• Sensitivity: 73% (range 32-100%)
• Specificity: 81% (range 45-98%) 1

Key Differences

1. Detection Method:

   • Pyuria is directly observed WBCs on microscopy
   • Leukocyte esterase detects enzymes released by WBCs

2. Accuracy:

   • Microscopic examination for pyuria is generally more accurate
   • Leukocyte esterase can produce false positives and false negatives

3. Clinical Significance:

   • Pyuria without bacteriuria may indicate sterile inflammation or partially treated UTI
   • Positive leukocyte esterase without pyuria can occur with sample degradation

Clinical Implications

• A positive leukocyte esterase test warrants further investigation with microscopy and/or urine culture when UTI is suspected 2
• The absence of leukocyte esterase in patients with asymptomatic bacteriuria is actually advantageous as it helps distinguish true UTI from asymptomatic bacteriuria 1
• Combined testing (leukocyte esterase plus nitrite) improves diagnostic accuracy:
  • Sensitivity: 93-99.8%
  • Specificity: 70-72% 1, 2

Important Caveats

• Pyuria may be absent in up to 20% of children with culture-confirmed UTIs 1
• Certain pathogens (Klebsiella spp. and Enterococcus spp.) are less likely to cause pyuria compared to E. coli 3
• Sterile pyuria (pyuria without positive culture) is common in patients with infections outside the urinary tract 4
• The degree of pyuria or leukocyte esterase positivity correlates with the likelihood of a positive urine culture 5

Best Practice

For optimal diagnostic accuracy in suspected UTI:

1. Use both leukocyte esterase and nitrite tests on dipstick
2. Confirm with microscopic examination for pyuria when available
3. Obtain urine culture before starting antibiotics, especially in high-risk populations 2

Remember that neither test alone is sufficient to rule out UTI, particularly in young children where up to 10% of UTIs may have negative urinalysis results 1.