Diagnosis of Asymptomatic Bacteriuria
Asymptomatic bacteriuria is diagnosed by urine culture, not urinalysis. 1, 2
Why Culture is Required
Urinalysis alone cannot diagnose asymptomatic bacteriuria because it only detects pyuria (white blood cells) or nitrites, which are markers of inflammation or bacterial presence but do not provide the quantitative bacterial counts required for diagnosis. 1, 2
Urine culture is the gold standard because asymptomatic bacteriuria is defined by specific quantitative thresholds that can only be determined through culture: ≥10^5 CFU/mL for voided specimens and ≥10^2 CFU/mL for catheterized specimens. 1, 2
The diagnosis requires documentation of bacterial colony counts that meet specific criteria, which urinalysis cannot provide. 1
Specific Diagnostic Criteria by Population
For women:
- Two consecutive voided urine specimens are required, both showing ≥10^5 CFU/mL of the same bacterial strain. 1, 2
- The first positive culture is confirmed by the second specimen only 80% of the time, which is why two specimens are necessary. 1
For men:
- A single clean-catch voided specimen showing ≥10^5 CFU/mL of one bacterial species is sufficient. 1, 2, 3
- This single specimen has 98% reproducibility when repeated within one week. 1, 3
For catheterized specimens (any sex):
- A single catheterized specimen showing ≥10^2 CFU/mL of one bacterial species is sufficient. 1, 2
- The lower threshold reflects reduced contamination risk with catheterized collection. 1
Why Urinalysis is Insufficient
Pyuria (white blood cells in urine) does not indicate need for treatment and is commonly present with asymptomatic bacteriuria. 1, 2
Nitrite tests have poor sensitivity in populations that empty their bladders frequently (especially infants and young children), and not all urinary pathogens convert nitrate to nitrite. 1
Leukocyte esterase tests can be misleading because the absence of leukocyte esterase in asymptomatic bacteriuria is actually an advantage—it helps distinguish true UTI from asymptomatic bacteriuria. 1
Urinalysis findings like bacteriuria on microscopy or positive dipstick results require confirmation with quantitative culture before making a diagnosis. 1
Critical Pitfalls to Avoid
Never diagnose or treat based on urinalysis alone, even if bacteria are visible on microscopy or dipstick is positive. 1, 2
Never treat based on pyuria alone—pyuria accompanying asymptomatic bacteriuria is not an indication for antimicrobial treatment. 1, 2
Do not confuse asymptomatic bacteriuria with symptomatic UTI in febrile patients—the key distinguishing feature is the presence of pyuria, which suggests true infection rather than colonization. 1
Ensure proper specimen collection to minimize contamination: use clean-catch midstream technique for voided specimens and transport to the laboratory promptly or refrigerate if delayed. 1, 2
Clinical Context
Asymptomatic bacteriuria is extremely common in hospitalized patients (49% of positive urine cultures represent asymptomatic bacteriuria), yet 64% of these patients inappropriately receive antimicrobial therapy. 4
Treatment of asymptomatic bacteriuria is only indicated in two populations: pregnant women and patients undergoing urologic procedures with anticipated mucosal bleeding. 2, 5
In all other populations, treatment does not reduce morbidity or mortality and may cause harm through antimicrobial resistance and adverse drug effects. 6, 7