What does a urinalysis (UA) result with leukocyte esterase activity indicate?

Understanding Urinalysis Results with Leukocyte Esterase

A urinalysis (UA) result showing "leuk est A500" indicates the presence of leukocyte esterase in the urine, which suggests pyuria (white blood cells in urine) and is a significant marker for urinary tract infection (UTI). 1

What is Leukocyte Esterase?

• Leukocyte esterase is an enzyme released by white blood cells (leukocytes) in the urine and is detected through dipstick testing that produces a color change when the enzyme is present 1
• Results are typically reported as negative, trace, 1+, 2+, or 3+ based on the intensity of the color reaction, with "A500" likely representing a specific laboratory's reporting format for a positive result 1
• The presence of leukocyte esterase indicates pyuria (white blood cells in the urine), which suggests inflammation in the urinary tract 2

Diagnostic Significance

• Leukocyte esterase has moderate sensitivity (83%, range 67-94%) but limited specificity (78%, range 64-92%) for detecting UTIs 1
• When combined with nitrite testing, the sensitivity increases to 93% (range 90-100%) and specificity to 96%, significantly improving diagnostic accuracy 1, 3
• A positive leukocyte esterase test combined with symptoms such as dysuria, frequency, urgency, fever, or gross hematuria strongly suggests a UTI 1
• The absence of pyuria (negative leukocyte esterase and no microscopic WBCs) has excellent negative predictive value for ruling out UTI 1

Clinical Interpretation

• A positive leukocyte esterase test requires clinical correlation with symptoms and should prompt further evaluation with microscopic examination and potentially urine culture 2
• Leukocyte esterase alone is insufficient as a standalone diagnostic test for UTI due to its moderate specificity 2, 4
• The test should be interpreted in the context of patient symptoms, as asymptomatic bacteriuria with pyuria is common, especially in older adults 1
• In one study, leukocyte esterase testing demonstrated 100% sensitivity with 76% specificity in predicting significant bacteriuria, making it valuable as a screening test 5

Common Pitfalls and Limitations

• 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 due to shorter bladder dwell time 1
• A negative UA result in the presence of strong suspicion of UTI may suggest non-E. coli organisms, as demonstrated in a study where 59% of negative UA cases with positive cultures were due to non-E. coli organisms 6

Recommended Next Steps

• If leukocyte esterase is positive AND symptoms suggest UTI, a urine culture should be obtained before initiating antimicrobial therapy 1, 2
• Consider adding nitrite testing, as the combination of leukocyte esterase and nitrite testing significantly improves diagnostic accuracy 1, 2
• Microscopic examination for WBCs is recommended when leukocyte esterase is positive to confirm pyuria 1
• In asymptomatic individuals, routine urinalysis and urine cultures should not be performed 1
• Non-specific symptoms like confusion or functional decline alone should not trigger UTI treatment without specific urinary symptoms, especially in older adults 1, 2