What should I find on a urinalysis (UA) if I am concerned for a urinary tract infection (UTI)?

Urinalysis Findings Indicative of Urinary Tract Infection

A positive urinalysis for UTI should show leukocyte esterase or nitrites on dipstick testing, or microscopic evidence of pyuria (≥10 WBCs/high power field) or bacteriuria. These findings, when combined with appropriate clinical symptoms, strongly suggest a urinary tract infection and warrant urine culture confirmation 1.

Key Urinalysis Components for UTI Diagnosis

Dipstick Testing

• Leukocyte esterase: Indicates the presence of white blood cells (WBCs)
  • Sensitivity: 83% (range 67-94%)
  • Specificity: 78% (range 64-92%) 1
• Nitrite test: Indicates the presence of bacteria that convert dietary nitrates to nitrites
  • Sensitivity: 53% (range 15-82%)
  • Specificity: 98% (range 90-100%) 1
  • Note: Requires bacteria to be present in bladder for approximately 4 hours to convert nitrates to nitrites 1

Microscopic Examination

• Pyuria: ≥10 WBCs per high-power field
  • Sensitivity: 73% (range 32-100%)
  • Specificity: 81% (range 45-98%) 1
• Bacteriuria: Presence of bacteria in unspun urine
  • Sensitivity: 81% (range 16-99%)
  • Specificity: 83% (range 11-100%) 1

Diagnostic Algorithm for UTI

1. Initial screening with urinalysis:

   • Perform dipstick testing for leukocyte esterase and nitrites
   • If available, perform microscopic examination for WBCs and bacteria
   • A positive result is defined as positive leukocyte esterase OR positive nitrite OR ≥10 WBCs/high power field OR presence of bacteria 1

2. Interpretation of urinalysis results:

   • Positive urinalysis: Proceed to urine culture for confirmation
   • Negative urinalysis: UTI is highly unlikely (<0.3% probability) 1
   • Combined sensitivity of leukocyte esterase test, nitrite test, or microscopy: 99.8% (range 99-100%) 1
   • Negative predictive value of urinalysis: 100% 2

3. Urine culture confirmation:

   • Required for definitive diagnosis of UTI
   • Specimen should be collected via clean catch, catheterization, or suprapubic aspiration
   • Positive culture defined as ≥50,000 CFU/mL of a uropathogen 1

Important Considerations

Specimen Collection

• Fresh urine specimen: Must be examined within 1 hour if kept at room temperature or within 4 hours if refrigerated 1
• Collection method:
  • Clean-catch mid-stream specimen is preferred when possible
  • Catheterization may be necessary for reliable specimens, especially in women who cannot provide clean-catch samples 1
  • Bag specimens are not reliable for culture due to high contamination rates 1

Common Pitfalls to Avoid

1. Relying solely on urinalysis for diagnosis: While a negative urinalysis effectively rules out UTI, a positive result requires culture confirmation 3

2. Misinterpreting asymptomatic bacteriuria: The presence of bacteria without symptoms does not constitute a UTI and should not be treated, especially in non-pregnant adults 1

3. Overlooking the timing of nitrite testing: False-negative nitrite results may occur if urine has not been in the bladder long enough (≥4 hours) for bacterial conversion of nitrates to nitrites 1

4. Failing to consider specimen quality: Contaminated specimens can lead to false-positive results; proper collection technique is essential 1

5. Testing without clinical indication: Urinalysis and urine cultures should not be performed for asymptomatic individuals 1

By following this systematic approach to urinalysis interpretation, you can accurately identify patients with suspected UTIs who require further evaluation and treatment.