What indicates a urinary tract infection (UTI) on a urinalysis?

Diagnostic Criteria for Urinary Tract Infection on Urinalysis

A urinary tract infection (UTI) is diagnosed when both urinalysis shows evidence of infection (pyuria and/or bacteriuria) AND urine culture grows at least 50,000 CFU/mL of a uropathogen from a properly collected specimen. 1, 2

Key Urinalysis Components for UTI Diagnosis

Dipstick Tests

• Leukocyte esterase: Positive result indicates pyuria with 83% sensitivity and 78% specificity 1, 2
• Nitrite: Positive result strongly suggests presence of bacteria with 53% sensitivity but excellent 98% specificity 1, 2
• Combined approach: Positive result for either leukocyte esterase OR nitrite increases sensitivity to 93% 1, 2

Microscopic Examination

• Pyuria: Defined as ≥10 WBCs/high-power field on microscopic examination 1, 3
• Bacteriuria: Presence of bacteria in uncentrifuged urine specimen 1
• Enhanced urinalysis: Combining counting chamber assessment of pyuria with Gram staining of uncentrifuged urine provides greater sensitivity and specificity than standard urinalysis 1

Diagnostic Algorithm

1. Initial Screening: Perform urinalysis with dipstick testing for leukocyte esterase and nitrite 1, 3

2. Microscopic Examination: Assess for WBCs and bacteria 1, 3

3. Interpretation:

   • Most sensitive approach: Positive if leukocyte esterase test AND nitrite test OR microscopy positive (99.8% sensitivity) 1, 2
   • If urinalysis is completely negative (negative for both leukocyte esterase and nitrites), UTI is highly unlikely (<0.3%) 2, 3

4. Urine Culture: Only proceed to culture if urinalysis suggests infection 1, 3

   • Positive culture: ≥50,000 CFU/mL of a single uropathogen 1
   • Specimen collection: Catheterization or suprapubic aspiration preferred for definitive diagnosis 1

Special Considerations

Specimen Collection

• Mid-stream or clean-catch specimen is acceptable for cooperative patients 1
• For women, in-and-out catheterization may be necessary 1
• For patients with indwelling catheters, change catheter prior to specimen collection 1
• Process specimens promptly or refrigerate to prevent bacterial overgrowth 1

Interpretation Pitfalls

• Asymptomatic bacteriuria: Positive culture without pyuria; common in school-aged girls and older adults, should not be treated 1, 3
• Bag specimens: High contamination rates make positive results unreliable; negative results may be useful 1, 3
• Catheterized patients: Bacteriuria and pyuria are virtually universal and not necessarily indicative of symptomatic infection 3

Test Performance Characteristics

Test	Sensitivity (%)	Specificity (%)
Leukocyte esterase	83 (67-94)	78 (64-92)
Nitrite	53 (15-82)	98 (90-100)
Leukocyte esterase OR nitrite	93 (90-100)	72 (58-91)
Microscopy for WBCs	73 (32-100)	81 (45-98)
Microscopy for bacteria	81 (16-99)	83 (11-100)
Combined approach	99.8 (99-100)	70 (60-92)

1, 2