How is urinalysis used to diagnose urinary tract infections (UTIs)?

How to Utilize Urinalysis in Diagnosing UTI

Urinalysis should be used primarily to rule out UTI when negative, not to diagnose it when positive—the diagnosis of UTI must be based on clinical symptoms combined with urinalysis findings, never on urinalysis alone. 1

Core Diagnostic Principle

The positive predictive value of pyuria for diagnosing infection is exceedingly low, as it often indicates genitourinary inflammation from many noninfectious causes. 1 The key utility of urinalysis is its excellent negative predictive value: the absence of pyuria effectively rules out UTI in most patient populations. 1, 2

When to Order Urinalysis

• Order urinalysis when patients present with specific urinary symptoms: dysuria, frequency, urgency, fever, suprapubic pain, or gross hematuria 1, 2
• Do not order routine urinalysis for fever workup alone, especially in hospitalized or geriatric patients without urinary symptoms, as this leads to unnecessary testing and antimicrobial overuse 1
• Avoid screening asymptomatic individuals, as asymptomatic bacteriuria is common (10-50% prevalence in long-term care facilities) and should not be treated 1, 2

Interpreting Urinalysis Components

Leukocyte Esterase

• Sensitivity: 83% (range 67-94%); Specificity: 78% (range 64-92%) 1
• When combined with nitrite testing, sensitivity increases to 93% with specificity of 72% 1
• The absence of leukocyte esterase has excellent negative predictive value (82-91%) for ruling out UTI 1, 2
• The key advantage: leukocyte esterase distinguishes true UTI from asymptomatic bacteriuria, as it is typically absent in asymptomatic bacteriuria 1

Nitrite Test

• Sensitivity: 19-48% (poor); Specificity: 92-100% (excellent) 1
• Negative nitrite results have little value in ruling out UTI, particularly in infants and children who void frequently (requires ~4 hours bladder dwell time for conversion) 1
• When positive, nitrite is highly specific with few false-positives, making it useful for confirming infection 1
• Not all uropathogens reduce nitrate to nitrite, limiting sensitivity 1

Microscopic Pyuria

• Threshold for significance: ≥10 WBCs/high-power field in spun urine 1, 2
• Sensitivity: 73% (range 32-100%); Specificity: 81% (range 45-98%) 1
• Pyuria alone is insufficient for diagnosis—it must be accompanied by clinical symptoms 1, 2
• Enhanced urinalysis (counting chamber assessment plus Gram stain of uncentrifuged urine) has superior diagnostic performance when available 1

Microscopic Bacteriuria

• Sensitivity: 81% (range 16-99%); Specificity: 83% (range 11-100%) 1
• Presence of bacteria in fresh, Gram-stained uncentrifuged urine correlates with ≥10⁵ CFU/mL 1

Algorithmic Approach to Diagnosis

Step 1: Assess Clinical Symptoms

• If no urinary symptoms present: Do not order urinalysis or culture 1, 2
• If specific urinary symptoms present (dysuria, frequency, urgency, fever, hematuria): Proceed to Step 2 1, 2

Step 2: Obtain Proper Specimen

• Use catheterization or suprapubic aspiration in infants/young children 1
• Use midstream clean-catch in cooperative adults; consider in-and-out catheterization for women if contamination suspected 1, 2
• Bag-collected specimens cannot confirm UTI—positive results require catheterized confirmation 1
• Process specimen within 1 hour at room temperature or 4 hours if refrigerated 1

Step 3: Interpret Urinalysis Results

If leukocyte esterase AND nitrite both negative:

• UTI effectively ruled out in most populations (negative predictive value 78-98%) 1, 2
• No culture needed; consider alternative diagnoses 1

If leukocyte esterase OR nitrite positive:

• Combined with typical symptoms: Treat as uncomplicated cystitis in healthy nonpregnant patients without culture 1, 3
• Proceed to culture if: complicated UTI, recurrent UTI, suspected pyelonephritis, pregnancy, or treatment failure 1

Step 4: When to Order Urine Culture

• Complicated cases: Structural abnormalities, immunosuppression, catheters, systemic symptoms 1
• Suspected pyelonephritis: Fever, flank pain, costovertebral angle tenderness 1
• Recurrent UTIs: To guide targeted therapy 1
• Pregnancy: Culture is test of choice despite positive dipstick 4
• Simple uncomplicated cystitis in healthy nonpregnant patients: Routine cultures not necessary 1

Critical Pitfalls to Avoid

Overdiagnosis and Overtreatment

• Do not treat pyuria alone without symptoms—45% of patients with asymptomatic bacteriuria receive inappropriate antimicrobial treatment 1
• Do not treat based on urinalysis in patients with nonspecific symptoms (confusion, functional decline, falls) in elderly without specific urinary symptoms or fever 1, 2
• Asymptomatic bacteriuria prevalence: 0.7% in infants, 15-50% in long-term care residents—treatment causes more harm than good 1

Specimen Quality Issues

• High epithelial cell counts indicate contamination—obtain repeat specimen via catheterization if clinical suspicion remains high 2
• Vaginal discharge or irritation significantly decreases UTI probability (LR 0.2-0.3) 5

Special Population Considerations

Pediatric (2-24 months):

• Require both urinalysis suggesting infection (pyuria and/or bacteriuria) AND ≥50,000 CFU/mL on culture 1
• Leukocyte esterase sensitivity 94% in clinically suspected UTI 1

Elderly/Long-term care:

• Absence of pyuria can exclude bacteriuria (negative predictive value approaches 100%) 1
• Presence of pyuria has low predictive value due to high prevalence of asymptomatic bacteriuria 1
• Evaluate only with acute onset of specific urinary symptoms 1, 2

Catheterized patients:

• Do not screen for or treat asymptomatic bacteriuria 2
• Reserve testing for symptomatic patients with fever, hypotension, or specific urinary symptoms 1

Diagnostic Stewardship

Symptom-based testing is essential to prevent unnecessary urine culture testing and overtreatment of asymptomatic bacteriuria. 1 Educational interventions on diagnostic protocols provide 33% absolute risk reduction in inappropriate antimicrobial initiation. 1