What makes a Urinalysis (U/A) positive for a Urinary Tract Infection (UTI)?

Criteria for a Positive Urinalysis for Urinary Tract Infection

A positive urinalysis for UTI requires either positive leukocyte esterase, positive nitrites, microscopy showing WBCs (pyuria), or microscopy showing bacteria. 1

Key Diagnostic Components

Urinalysis Parameters

• Leukocyte esterase test: 83% sensitivity, 78% specificity 2, 1
• Nitrite test: 53% sensitivity, 98% specificity - highly specific but less sensitive 2, 1
• Microscopy for WBCs (pyuria): 73% sensitivity, 81% specificity 2
• Microscopy for bacteria: 81% sensitivity, 83% specificity 2

Combination Approaches

• Leukocyte esterase OR nitrite positive: 93% sensitivity, 72% specificity - good screening approach 2, 1
• Leukocyte esterase AND nitrite test OR microscopy positive: 99.8% sensitivity, 70% specificity - highest sensitivity approach 2, 1

Definitive Diagnosis Criteria

• According to the American Academy of Pediatrics guidelines, a definitive UTI diagnosis requires both:
  1. Urinalysis results suggesting infection (pyuria and/or bacteriuria)
  2. Positive urine culture with ≥50,000 CFU/mL of a uropathogen 2, 1

Specimen Collection Considerations

• Fresh urine specimens must be processed within 1 hour at room temperature or 4 hours if refrigerated 2, 1
• For children, catheterization or suprapubic aspiration is preferred for definitive diagnosis 2
• Bagged specimens may have false positives; negative results may be useful but positive results require confirmation 2, 1

Common Pitfalls to Avoid

• Do not rule out UTI based solely on negative nitrite results, as this would miss many true infections 2, 1
• Do not diagnose UTI based on positive culture alone without evidence of pyuria, as this may represent asymptomatic bacteriuria rather than true infection 2, 1
• A negative urinalysis (negative for both leukocyte esterase and nitrites) makes UTI unlikely but does not completely rule it out 2, 3
• Pyuria is not present on initial urinalysis in approximately 20% of febrile infants with pyelonephritis 2

Interpretation in Different Populations

• In children, the sensitivity of urinalysis is approximately 82% and does not vary by age 3
• In pregnancy, positive dipstick testing is likely specific for asymptomatic bacteriuria, but urine culture remains the test of choice 4
• In elderly patients, nitrites are likely more sensitive and specific than other dipstick components 4
• Pyuria is commonly found in the absence of infection in older adults with lower urinary tract symptoms 4

Remember that while urinalysis is a valuable screening tool, urine culture remains the gold standard for definitive diagnosis of UTI 4.