What Leukocyte Esterase Means on a Urine Test
Leukocyte esterase on a urine test indicates the presence of white blood cells (pyuria) in the urine, which signals inflammation in the urinary tract and strongly suggests—but does not definitively confirm—a urinary tract infection when accompanied by clinical symptoms. 1
Biochemical Basis
Leukocyte esterase is an enzyme released specifically from neutrophils (white blood cells) that is not normally present in urine or influenced by bacteria, medications, or variable urine compositions. 2, 3 The dipstick test detects this enzyme through a chemical reaction that produces a color change, with results typically reported as negative, trace, 1+, 2+, or 3+ based on intensity. 1
Diagnostic Performance
The test has moderate sensitivity (83%, range 67-94%) and limited specificity (78%, range 64-92%) when used alone. 4 This means:
- Sensitivity of 83%: The test will miss approximately 17% of true UTIs (false negatives). 4
- Specificity of 78%: About 22% of positive results occur without actual infection (false positives). 4
When combined with nitrite testing, diagnostic accuracy improves dramatically to 93% sensitivity and 96% specificity. 1, 5 The combination of both positive leukocyte esterase AND positive nitrite strongly indicates active infection rather than colonization. 5
Clinical Interpretation Algorithm
Step 1: Assess for Symptoms
Determine if UTI-associated symptoms are present: 1
- Dysuria (painful urination)
- Urinary frequency or urgency
- Fever
- Gross hematuria
- Suprapubic pain
Step 2: Interpret the Result in Context
If leukocyte esterase is POSITIVE:
- With symptoms present: Strongly suggests UTI; obtain urine culture before starting antibiotics. 1, 6
- Without symptoms: May represent asymptomatic bacteriuria (colonization), especially in older adults where prevalence is 15-50% in long-term care facilities. 1 Do not treat asymptomatic bacteriuria. 6
If leukocyte esterase is NEGATIVE:
- Combined with negative nitrite: Excellent negative predictive value (96-99.8%) effectively rules out UTI in most cases. 4, 1
- The absence of pyuria distinguishes true UTI from asymptomatic bacteriuria, which is an advantage of the test. 4
Step 3: Confirm with Microscopy and Culture
When leukocyte esterase is positive, microscopic examination for WBCs (≥10 WBCs/high-power field) should be performed. 1 If pyuria is confirmed AND symptoms are present, obtain urine culture for definitive diagnosis. 1
Critical Distinctions
True UTI vs. Asymptomatic Bacteriuria
The key distinguishing feature is the presence of pyuria (positive leukocyte esterase). 4 Asymptomatic bacteriuria often shows bacteria on culture without pyuria, and antimicrobial treatment may do more harm than good in these cases. 4
E. coli vs. Non-E. coli Infections
Positive leukocyte esterase has a likelihood ratio of 2.5 for predicting E. coli versus non-E. coli UTI. 7 In contrast, 30% of culture-positive UTIs with negative urinalysis (including negative leukocyte esterase) are caused by non-E. coli organisms, with 59% of these being non-E. coli pathogens. 4, 7
Special Population Considerations
Febrile Infants and Children (2-24 months)
Positive leukocyte esterase should prompt urine culture collection via catheterization or suprapubic aspiration before initiating antimicrobial therapy. 1, 6 The test has 94% sensitivity in clinically suspected UTI in this population. 4
Older Adults
Non-specific symptoms like confusion or functional decline alone should not trigger UTI treatment without specific urinary symptoms, even with positive leukocyte esterase. 1 Asymptomatic bacteriuria with pyuria is extremely common and should not be treated. 6
Pregnant Women
Leukocyte esterase testing is 100% sensitive and 100% specific for detecting UTIs during pregnancy. 3
Common Pitfalls and How to Avoid Them
False-Positive Results occur with: 1
- Contaminated specimens (especially bag specimens in infants with 85% false-positive rate) 4
- Certain oxidizing agents
- Some medications
False-Negative Results occur with: 1
- High urinary glucose
- High specific gravity
- Certain antibiotics
- Frequent voiding in infants (shorter bladder dwell time reduces detectable leukocytes) 4
Critical Error to Avoid: Never rely on bag-collected urine specimens for positive results. A positive bagged urinalysis should prompt repeat collection via catheterization or suprapubic aspiration. 4 The specificity of bagged urine is only 70%, yielding an 85% false-positive rate. 4
When Culture is Mandatory
Urinalysis cannot substitute for urine culture to document UTI presence. 4 Culture is required when:
- Leukocyte esterase is positive with clinical symptoms 1
- Treatment decisions need confirmation 4
- Antibiotic sensitivities are needed 7
The specimen must be fresh (within 1 hour at room temperature or 4 hours refrigerated) to ensure accuracy. 4