What is the significance of nitrite and leukocyte esterase (leukoestrase) in a urine test?

Significance of Nitrite and Leukocyte Esterase in Urine Testing

Nitrite and leukocyte esterase are rapid screening markers for urinary tract infection (UTI), with nitrite being highly specific (98%) but insensitive (53%), while leukocyte esterase is moderately sensitive (83%) but less specific (78%); when combined, they achieve 93% sensitivity and are most valuable for ruling out UTI when both are negative. 1

Understanding Each Test Component

Leukocyte Esterase Test

• Detects pyuria (white blood cells in urine) as a surrogate marker for inflammation or infection 1
• Sensitivity: 83% (range 67-94%) and specificity: 78% (range 64-92%) for detecting UTI 1
• Key advantage: Distinguishes true UTI from asymptomatic bacteriuria, as the test remains negative in asymptomatic colonization where pyuria is absent 1
• Clinical context matters: Sensitivity increases to 94% when used specifically in patients with clinically suspected UTI (symptomatic patients) 1

Nitrite Test

• Detects bacterial conversion of dietary nitrates to nitrites, which requires approximately 4 hours of bladder dwell time 1
• Sensitivity: 53% (range 15-82%) but specificity: 98% (range 90-100%) 1
• When positive, it is highly reliable with very few false-positives, making it a "rule-in" test 1
• Major limitation: Poor sensitivity, especially in infants and young children who void frequently and don't retain urine long enough for nitrate conversion 1
• Not all pathogens produce nitrite: Some urinary pathogens cannot reduce nitrate to nitrite, leading to false-negative results 1

Combined Testing Strategy

When Either Test is Positive

• Sensitivity increases to 93% (range 90-100%) with specificity of 72% (range 58-91%) 1
• This combination is excellent for ruling out UTI when both are negative 2

When Both Tests are Positive

• Specificity increases to 96% with maintained high sensitivity of 93%, strongly indicating true infection rather than colonization 1, 3
• In children with sickle cell disease specifically, this combination has particularly high diagnostic value for distinguishing infection from colonization 3

Clinical Application Algorithm

Step 1: Assess Clinical Context

• Determine if UTI symptoms are present: dysuria, frequency, urgency, fever, gross hematuria 2
• In older adults: Avoid testing for non-specific symptoms like confusion alone without urinary symptoms 2
• In infants/children: Consider fever as a key presenting symptom 1

Step 2: Interpret Results Based on Combination

• Both negative: Excellent negative predictive value; UTI is unlikely, no further testing needed in most cases 2
• Leukocyte esterase positive, nitrite negative: Moderate suspicion; proceed to urine culture before treatment 2, 4
• Nitrite positive (regardless of leukocyte esterase): High specificity for UTI; obtain culture and consider empiric treatment in symptomatic patients 1
• Both positive: Very high likelihood of UTI; obtain culture and initiate empiric treatment in symptomatic patients 1, 3

Step 3: Specimen Quality Considerations

• Specimen must be fresh: Within 1 hour at room temperature or 4 hours if refrigerated 1
• If contaminated (high epithelial cells): Consider catheterization for definitive specimen in patients with strong clinical suspicion 2

Critical Pitfalls to Avoid

Do Not Rely on Urinalysis Alone

• 10-50% of culture-proven UTIs can have false-negative urinalysis, particularly in children under 2 years 1
• Always obtain urine culture in febrile infants and children under 2 years before initiating treatment 1, 4
• Urinalysis cannot substitute for culture but guides decisions about empiric therapy while awaiting culture results 1

Distinguish Asymptomatic Bacteriuria from True UTI

• Asymptomatic bacteriuria is common (prevalence 15-50% in long-term care residents) and should not be treated 2
• The key distinguishing feature is pyuria: Asymptomatic bacteriuria typically lacks pyuria (negative leukocyte esterase) 1
• In asymptomatic patients, do not perform urinalysis or cultures routinely 2

Age-Specific Limitations

• Nitrite test has poor sensitivity in infants who void frequently and don't allow adequate bladder dwell time for nitrate conversion 1
• In young children, negative nitrite results have little value for ruling out UTI 1

False Results

• False-positive leukocyte esterase: Can occur with contaminated specimens, certain oxidizing agents, and some medications 2
• False-negative leukocyte esterase: May occur with high urinary glucose, high specific gravity, or certain antibiotics 2
• False-negative nitrite: Occurs with non-nitrate-reducing organisms, frequent voiding, or dilute urine 1

When to Proceed to Urine Culture

Obtain urine culture if: 2, 4

• Pyuria ≥10 WBCs/high-power field on microscopy
• Positive leukocyte esterase with UTI symptoms
• Positive nitrite (regardless of symptoms in most cases)
• Febrile infant or child under 2 years with any positive urinalysis finding
• Strong clinical suspicion despite negative screening tests