What on a urine dipstick is indicative of a Urinary Tract Infection (UTI)?

Urinary Dipstick Indicators of Urinary Tract Infection (UTI)

Positive leukocyte esterase and/or nitrite on urinary dipstick are the most reliable indicators of UTI, with the combination providing the highest diagnostic accuracy (sensitivity 93%, specificity 72%).

Key Diagnostic Indicators on Urinary Dipstick

Leukocyte Esterase

• Sensitivity: 72-97% 1
• Specificity: 41-86% 1
• Indicates presence of white blood cells (pyuria)
• More sensitive but less specific than nitrite 2
• When used alone, has sensitivity of 48.5-83% 3, 2

Nitrite

• Sensitivity: 19-48% 1
• Specificity: 92-100% 1
• Indicates presence of nitrite-reducing bacteria
• More specific but less sensitive than leukocyte esterase 2
• When used alone, has sensitivity of 23.3-53% 3, 4

Combined Leukocyte Esterase and Nitrite

• Sensitivity: 46-100% 1
• Specificity: 42-98% 1
• Combined testing significantly improves diagnostic accuracy 2
• Negative predictive value of 95% for ruling out UTI 5

Microscopic Examination Correlation

• Pyuria (≥10 WBCs/high-power field) has sensitivity of 90-96% and specificity of 47-50% 1
• Presence of pyuria strongly supports UTI diagnosis 1, 2
• The sensitivity of microscopic examination increases with higher WBC counts:
  • 10 WBCs/μL: 100% sensitivity, 36% specificity 1
  • 100 WBCs/μL: 93% sensitivity, 71% specificity 1
  • 400 WBCs/μL: 77% sensitivity, 92% specificity 1

Interpretation Guidelines

1. Positive leukocyte esterase AND positive nitrite:

   • Highest probability of UTI
   • Proceed with empiric treatment while awaiting culture 2

2. Positive leukocyte esterase OR positive nitrite:

   • Moderate probability of UTI
   • Consider clinical symptoms and obtain urine culture 1

3. Negative leukocyte esterase AND negative nitrite:

   • Low probability of UTI (high negative predictive value)
   • Consider alternative diagnoses if symptomatic 2

Clinical Application

• In symptomatic patients, urine dipstick should be the initial diagnostic test 1
• Urine culture should be obtained when starting antibiotics for preliminary UTI diagnosis 1
• In febrile patients with negative dipstick but continued suspicion of UTI, obtain urine culture 1
• Asymptomatic bacteriuria should not be treated except in specific populations (e.g., pregnant women) 1, 2

Common Pitfalls

1. Overreliance on single parameters:

   • Neither leukocyte esterase nor nitrite alone is sufficient to rule out UTI 3
   • Combined testing significantly improves diagnostic accuracy 2

2. Improper specimen collection:

   • False negatives/positives can occur with improper collection techniques
   • Clean-catch midstream specimens are preferred 1, 2
   • For catheterized patients, change catheter before specimen collection 1

3. Delayed processing:

   • Specimens should be processed within 1-4 hours to preserve accuracy 2
   • False negatives can occur with prolonged storage at room temperature

4. Treating asymptomatic bacteriuria:

   • Avoid treating positive dipstick findings in asymptomatic patients 1
   • Exception: pregnant women and specific high-risk populations 2

By using urinary dipstick testing appropriately and understanding its limitations, clinicians can make informed decisions about UTI diagnosis and management, reducing unnecessary antibiotic use while ensuring appropriate treatment for true infections.