What is leukocyte (white blood cell) esterase?

Leukocyte Esterase: A Diagnostic Marker for Pyuria

Leukocyte esterase is an enzyme released from white blood cells (leukocytes) that serves as a surrogate marker for pyuria (presence of white blood cells in urine) and is commonly used in urinary tract infection (UTI) screening. 1, 2

Biochemical Basis and Detection

• Leukocyte esterase is an enzyme specifically released from neutrophils (white blood cells) that is not present in serum, urine, or vaginal secretions in the absence of leukocytes 3
• The enzyme is detected through dipstick testing, which uses a chemical reaction that produces a color change when leukocyte esterase is present 1
• The test is typically reported as negative, trace, 1+, 2+, or 3+ based on the intensity of the color reaction 2

Diagnostic Performance

• Leukocyte esterase testing has moderate sensitivity (83%, range 67-94%) but limited specificity (78%, range 64-92%) for detecting UTIs 1
• When combined with nitrite testing, sensitivity increases to 93% (range 90-100%) with specificity of 72% (range 58-91%) 1, 2
• A negative leukocyte esterase test has excellent negative predictive value when combined with a negative nitrite test, effectively ruling out UTI in most cases 4
• The test performs best on fresh urine specimens (less than 1 hour after voiding at room temperature or 4 hours if refrigerated) 1

Clinical Applications

• Primary use is as a rapid screening test for UTIs, particularly in outpatient and emergency settings 5
• Can be used to screen for asymptomatic bacteriuria in high-risk populations 6
• Helps distinguish true UTIs from asymptomatic bacteriuria, as the absence of leukocyte esterase in bacteriuria suggests colonization rather than infection 1
• Used in screening for urethritis in males, including detection of chlamydial and gonococcal infections 1, 6
• Has applications beyond urine testing, including detection of leukocytes in other body fluids and fecal samples for inflammatory bowel disease monitoring 7

Interpretation in Clinical Context

• A positive leukocyte esterase test indicates the presence of pyuria but requires clinical correlation with symptoms and urine culture for definitive UTI diagnosis 2
• In febrile infants and young children, positive leukocyte esterase should prompt urine culture collection before initiating antimicrobial therapy 1
• In asymptomatic patients, isolated positive leukocyte esterase without symptoms does not warrant treatment 4
• For symptomatic patients with positive leukocyte esterase, urine culture should be obtained before starting antibiotics 2

Limitations and Considerations

• False-positive results can occur with contaminated specimens, certain oxidizing agents, and some medications 1
• False-negative results may occur with high urinary glucose, high specific gravity, or certain antibiotics 1
• The test has lower sensitivity in infants who void frequently, as shorter bladder dwell time may result in fewer detectable leukocytes 1
• Specimen collection method significantly impacts test accuracy, with catheterization or suprapubic aspiration preferred over bag specimens in young children 1

Recent Technological Advances

• Newer microfluidic paper-based devices can provide quantitative measurements of leukocyte esterase, potentially improving diagnostic accuracy 8
• Automated readers now allow for more standardized and quantitative interpretation of leukocyte esterase results compared to visual reading 7

Leukocyte esterase testing remains a valuable point-of-care test that can guide clinical decision-making and reduce unnecessary urine cultures when properly interpreted in the appropriate clinical context 5.