How is asymptomatic bacteriuria identified in urinalysis?

How to Identify Asymptomatic Bacteriuria in Urinalysis

Asymptomatic bacteriuria cannot be reliably identified by urinalysis alone—urine culture is required for definitive diagnosis, with specific quantitative bacterial thresholds that vary by sex and collection method. 1

Diagnostic Criteria Based on Urine Culture

The diagnosis of asymptomatic bacteriuria must be based on quantitative urine culture results, not urinalysis findings such as pyuria or dipstick tests. 1

For Women (Non-Catheterized)

• Two consecutive voided urine specimens are required 1
• Both specimens must show ≥10^5 CFU/mL of the same bacterial strain 1
• This two-specimen requirement exists because a single positive culture in women is only confirmed 80% of the time on repeat testing 1

For Men (Non-Catheterized)

• A single clean-catch voided specimen is sufficient 1
• Must show ≥10^5 CFU/mL of one bacterial species 1
• Single specimens in men have 98% reproducibility when repeated within one week 1, 2

For Catheterized Specimens (Either Sex)

• A single catheterized specimen is sufficient 1
• Must show ≥10^2 CFU/mL of one bacterial species 1
• Note the lower threshold (10^2 vs 10^5) for catheterized specimens 1

Why Urinalysis Alone Is Inadequate

Pyuria Is Not Diagnostic

• Pyuria (white blood cells in urine) accompanying bacteriuria is NOT an indication for treatment and should not be used to diagnose asymptomatic bacteriuria 1, 3
• Pyuria is commonly present with asymptomatic bacteriuria but does not distinguish it from symptomatic infection 1

Dipstick Tests Have Poor Performance

• Nitrite test: High specificity (99-100%) but very low sensitivity (37-60%), missing most cases 4, 5
• Leukocyte esterase test: Better sensitivity (70-100%) but poor specificity (64-65%) and low positive predictive value (16-28%), leading to many false positives 4, 6, 5
• Microscopic pus cell count: High sensitivity (100%) but poor specificity (64%), also yielding many false positives 4

Clinical Bottom Line on Screening Tests

• None of the rapid urinalysis screening tests (nitrite, leukocyte esterase, microscopy) have adequate sensitivity and specificity to replace urine culture 4, 6, 5
• These tests should not be used as standalone screening tools for asymptomatic bacteriuria 6, 5

Proper Specimen Collection

The urine specimen must be collected in a manner that minimizes contamination to ensure accurate diagnosis. 1

• Use clean-catch midstream technique for voided specimens 1
• Transport specimens to the laboratory promptly to prevent bacterial overgrowth 1
• Ensure proper storage if immediate processing is not possible 1

Common Pitfalls to Avoid

• Do not diagnose or treat based on pyuria alone, even when bacteria are present 1, 3
• Do not rely on dipstick urinalysis as a substitute for culture in populations requiring screening (e.g., pregnant women) 4, 6, 5
• Do not use a single specimen in women—two consecutive positive cultures of the same organism are required 1
• Do not confuse nonspecific symptoms (delirium, falls in elderly) with symptomatic UTI requiring treatment 3
• Remember that most asymptomatic bacteriuria should NOT be treated, except in pregnant women and patients undergoing urologic procedures with mucosal bleeding 1, 3