Leukocyte Esterase: A Diagnostic Marker for Urinary Tract Infections
Leukocyte esterase is an enzyme present in white blood cells (leukocytes) that serves as a surrogate marker for pyuria (presence of white blood cells in urine) and is commonly used in urinalysis to help diagnose urinary tract infections. 1, 2
Biochemical Basis and Detection
- Leukocyte esterase is detected through dipstick testing, which produces a color change when the enzyme is present, with results typically reported as negative, trace, 1+, 2+, or 3+ based on the intensity of the color reaction 2
- The test is part of standard urinalysis and provides rapid results without requiring microscopy 1
- The conversion of dietary nitrates to nitrites by bacteria requires approximately 4 hours in the bladder, which is why nitrite testing is often performed alongside leukocyte esterase testing 1
Diagnostic Performance
- Leukocyte esterase testing has moderate sensitivity (83%, range 67-94%) but limited specificity (78%, range 64-92%) for detecting UTIs 1, 2
- When combined with nitrite testing, sensitivity increases to 93% (range 90-100%) with specificity of 72% (range 58-91%), making the combination more reliable than either test alone 1, 2
- The absence of pyuria (negative leukocyte esterase and no microscopic WBCs) has excellent negative predictive value for ruling out UTI 2
Clinical Applications
Urinary Tract Infections
- Leukocyte esterase is a key component in diagnosing UTIs, especially when combined with symptoms such as dysuria, frequency, urgency, fever, or gross hematuria 2
- The American Academy of Pediatrics recommends using leukocyte esterase to help distinguish true UTIs from asymptomatic bacteriuria 1, 2
- A positive leukocyte esterase test indicates the presence of pyuria but requires clinical correlation with symptoms and urine culture for definitive UTI diagnosis 2, 3
Other Applications
- The Centers for Disease Control and Prevention recommends using leukocyte esterase in screening for urethritis in males, including detection of chlamydial and gonococcal infections 1, 2
- Leukocyte esterase activity has been studied as a diagnostic tool in other body fluids, including amniotic fluid for detecting chorioamnionitis 4 and synovial fluid for diagnosing septic arthritis 5
- It has also been explored as an alternative biomarker for inflammatory bowel disease in fecal samples 6
Interpretation in Clinical Context
- In urinalysis, a positive leukocyte esterase test should prompt further evaluation with microscopic examination for WBCs 1, 2
- If pyuria (≥10 WBCs/high-power field or positive leukocyte esterase) is present AND symptoms suggest UTI, a urine culture should be obtained 1, 2
- In asymptomatic individuals, urinalysis and urine cultures should not be performed routinely 1, 2
- In febrile infants and young children, positive leukocyte esterase should prompt urine culture collection before initiating antimicrobial therapy 1, 2
Limitations and Considerations
- False-positive results can occur with contaminated specimens, certain oxidizing agents, and some medications 2
- False-negative results may occur with high urinary glucose, high specific gravity, or certain antibiotics 2
- The test has lower sensitivity in infants who void frequently, as shorter bladder dwell time may result in fewer detectable leukocytes 1, 2
- Non-specific symptoms like confusion or functional decline alone should not trigger UTI treatment without specific urinary symptoms in older adults 1, 2
Recommended Diagnostic Algorithm
- Assess for symptoms consistent with UTI (dysuria, frequency, urgency, fever, gross hematuria) 2
- Perform urinalysis including leukocyte esterase and nitrite testing 1
- If leukocyte esterase is positive, proceed with microscopic examination for WBCs 1, 2
- If pyuria is confirmed (≥10 WBCs/high-power field or positive leukocyte esterase) AND symptoms suggest UTI, obtain a urine culture 1, 2
- Interpret results in clinical context, recognizing that asymptomatic bacteriuria with pyuria is common in certain populations 2