What is pyuria?

What is Pyuria?

Pyuria is the presence of white blood cells (leukocytes) in the urine, typically defined as ≥10 white blood cells per mm³ in uncentrifuged urine or ≥5 white blood cells per high-power field on microscopic examination of centrifuged urine. 1, 2

Definition and Diagnostic Thresholds

Pyuria can be quantified through several methods, each with specific cutoff values: 2

• Hemocytometer counting (uncentrifuged urine): ≥10 WBCs/mm³ is considered significant 1, 3, 4
• Microscopic examination (centrifuged urine): ≥5 WBCs per high-power field 1, 2
• Manual microscopy: ≥8 WBCs per high-power field reliably predicts positive urine culture 3
• Automated urinalysis: >2 WBCs per high-power field indicates significant pyuria 3
• Leukocyte esterase dipstick: Any positive result suggests pyuria 1, 2

The hemocytometer method using uncentrifuged urine is the most reproducible and accurate technique, as microscopic examination of centrifuged urine does not correlate well with actual leukocyte excretion rates. 4

Clinical Significance and Interpretation

Pyuria indicates an inflammatory response in the urinary tract and is the hallmark feature that distinguishes true urinary tract infection from asymptomatic bacteriuria or colonization. 2, 4

Key Clinical Points:

• Pyuria with symptoms: When combined with acute urinary symptoms (dysuria, frequency, urgency, fever, gross hematuria, or new/worsening incontinence), pyuria strongly suggests true UTI requiring treatment 1, 2

• Pyuria without symptoms: Asymptomatic bacteriuria with pyuria is common (prevalence 10-50% in long-term care facility residents) and should NOT be treated, as it represents colonization rather than infection 1, 2

• Absence of pyuria: A negative test for pyuria (negative leukocyte esterase AND no microscopic WBCs) has excellent negative predictive value approaching 100% for ruling out UTI in most populations 2, 4

Detection Methods and Diagnostic Performance

Leukocyte Esterase Testing:

• Sensitivity: 83% (range 67-94%) 2
• Specificity: 78% (range 64-92%) 2
• Combined with nitrite testing: Sensitivity increases to 93% with specificity of 96% 2

The leukocyte esterase test detects an enzyme released by white blood cells, producing a color change on dipstick testing reported as negative, trace, 1+, 2+, or 3+. 2

Microscopic Examination:

Direct visualization of WBCs in urine sediment remains the gold standard, particularly when using hemocytometer counts of uncentrifuged specimens. 4

Diagnostic Algorithm for Clinical Use

When evaluating pyuria, follow this approach: 1, 2

1. Assess for symptoms: Look specifically for dysuria, frequency, urgency, fever >38.3°C, gross hematuria, or new/worsening urinary incontinence 1

2. If symptomatic with pyuria: Obtain properly collected urine culture (catheterization in women if needed, midstream clean-catch in cooperative patients) and initiate treatment 1, 2

3. If asymptomatic with pyuria: Do NOT order cultures or treat, regardless of bacteriuria presence—this represents asymptomatic bacteriuria 1, 2

4. If pyuria absent (negative leukocyte esterase AND negative microscopy): UTI is effectively ruled out; consider alternative diagnoses 2, 4

Special Population Considerations

Long-Term Care Facility Residents:

• Pyuria has low predictive value due to high prevalence of asymptomatic bacteriuria (15-50%) 1
• Evaluate only with acute onset of specific urinary symptoms, not with nonspecific symptoms like confusion or functional decline alone 1
• Urinalysis and cultures should NOT be performed for asymptomatic residents 1

Catheterized Patients:

• Pyuria is nearly universal in patients with chronic indwelling catheters 1, 5
• Pyuria has only 37% sensitivity for catheter-associated UTI despite 90% specificity 5
• Strongest association is with gram-negative infections; much weaker for gram-positive cocci or yeasts 5
• Do NOT screen for or treat asymptomatic bacteriuria in catheterized patients 1

Pediatric Patients (2-24 months):

• Pyuria may be absent in up to 20% of febrile infants with culture-proven pyelonephritis 2
• Both urinalysis and culture should be obtained before antibiotics in febrile infants 1
• Threshold of ≥10 WBCs/mm³ on hemocytometer has 95% sensitivity for identifying positive cultures 6

Common Pitfalls to Avoid

Critical mistake: Treating bacteriuria without pyuria or symptoms leads to unnecessary antibiotic use, increased antimicrobial resistance, and potential adverse effects without clinical benefit. 1, 2

Other Important Caveats:

• Pyuria without bacteriuria: May indicate non-infectious inflammation, tuberculosis, interstitial nephritis, or other non-bacterial conditions 1, 2

• Low positive predictive value: Pyuria alone has poor diagnostic accuracy for predicting bacteriuria (only 25-54% depending on WBC count), with optimal cutoff at >25 WBCs/high-power field 7

• Contaminated specimens: High epithelial cell counts indicate contamination; repeat collection via catheterization if clinical suspicion remains high 2

• False negatives: Can occur with high urinary glucose, high specific gravity, or certain antibiotics affecting leukocyte esterase testing 2

Clinical Decision-Making Framework

The presence of pyuria alone is NEVER sufficient to diagnose or treat UTI—accompanying urinary symptoms are required to justify antimicrobial therapy. 1, 2

Use this decision tree: 1, 2

• Pyuria + specific urinary symptoms + positive culture = Treat as UTI
• Pyuria + asymptomatic = Do NOT treat (asymptomatic bacteriuria)
• No pyuria + symptoms = Consider alternative diagnoses
• No pyuria + asymptomatic = No further evaluation needed

This approach prevents overtreatment of asymptomatic bacteriuria while ensuring appropriate therapy for true infections, optimizing antimicrobial stewardship and patient outcomes. 1, 2, 7