What does a urinalysis showing WBC (White Blood Cell) esterase positive and no nitrites indicate?

Interpreting WBC Esterase Plus 2 with No Nitrites in Urinalysis

A positive leukocyte esterase with negative nitrite test suggests a urinary tract infection (UTI) and should be considered clinically significant, especially when accompanied by symptoms, though urine culture is required for definitive diagnosis. 1

Understanding the Significance of These Findings

Urinalysis is a valuable initial diagnostic tool for UTI evaluation, but the interpretation requires understanding the meaning of each component:

Leukocyte Esterase (WBC Esterase)

• Indicates pyuria (presence of white blood cells in urine)
• Sensitivity: 83% (range 67-94%)
• Specificity: 78% (range 64-92%) 2
• A positive result (2+) suggests significant inflammation in the urinary tract

Nitrite Test

• Produced when bacteria convert dietary nitrates to nitrites
• Requires approximately 4 hours of bladder incubation time
• Highly specific: 98% (range 90-100%)
• Less sensitive: 53% (range 15-82%) 2, 1
• Negative nitrite does NOT rule out UTI, as:
  • Some pathogens don't produce nitrite (e.g., Enterococcus, Staphylococcus)
  • Frequent voiding prevents nitrate-to-nitrite conversion
  • Dilute urine or low dietary nitrate intake can cause false negatives 1

Clinical Implications

Diagnostic Considerations

1. Likely pathogens: Positive leukocyte esterase with negative nitrite may indicate:

   • Non-E. coli organisms (e.g., Enterococcus, Klebsiella) 3, 4
   • Early infection where bacteria haven't had time to convert nitrates to nitrites
   • Pyuria from other inflammatory conditions

2. Diagnostic accuracy:

   • The negative predictive value of a negative urinalysis is excellent (100%) 5
   • However, a positive leukocyte esterase alone has moderate specificity

Management Algorithm

1. For symptomatic patients:

   • Obtain urine culture before starting antibiotics 2
   • Consider empiric treatment while awaiting culture results if symptoms are significant
   • Choose antibiotics that cover both E. coli and non-E. coli organisms

2. For asymptomatic patients:

   • Consider the possibility of asymptomatic bacteriuria, which may not require treatment 2
   • Urine culture is essential to differentiate true infection from asymptomatic bacteriuria

3. For pediatric patients:

   • In febrile children 2 months to 2 years, a positive leukocyte esterase test can be used to make a preliminary diagnosis of UTI 2
   • Obtain urine culture when starting antibiotics 2
   • Consider that pyuria might be lacking in UTIs associated with Klebsiella and Enterococcus species 4

Important Caveats

1. Collection method matters:

   • Specimens must be fresh (within 1 hour at room temperature or 4 hours if refrigerated) 2
   • Collection method affects reliability (catheterization or suprapubic aspiration preferred over bag specimens) 2

2. Definitive diagnosis requires culture:

   • Urinalysis cannot substitute for urine culture to document UTI 2
   • The diagnosis of UTI requires both urinalysis results suggesting infection AND ≥50,000 CFU/mL of a uropathogen from a properly collected specimen 2

3. Special populations:

   • In diabetic patients, WBC sensitivity and specificity are 65.7% and 100%, respectively 6
   • In children, nitrite testing is less sensitive due to frequent voiding 1

Remember that while a positive leukocyte esterase with negative nitrite test suggests infection, urine culture remains the gold standard for definitive diagnosis and pathogen identification.