Is mixed skin flora in a urine sample indicative of contamination?

Mixed Skin Flora in Urine: Contamination Interpretation

Yes, mixed skin flora in a urine sample almost always indicates contamination from periurethral, vaginal, or perineal organisms rather than true urinary tract infection. 1, 2

Understanding Mixed Flora Results

Mixed flora refers to the presence of multiple bacterial species in urine culture, which most commonly represents contamination with commensal organisms from the collection site rather than genuine infection. 2 The Infectious Diseases Society of America emphasizes that urine is easily contaminated with commensal flora, and specimens obtained without proper technique routinely contain mixed flora, making determination of the true etiologic agent difficult. 1

Key Diagnostic Indicators of Contamination

• The presence of >10 white blood cells alongside a significant number of epithelial cells strongly suggests contamination rather than infection. 1, 2
• Specimens with multiple organisms (≥2 non-uropathogens or 1 uropathogen in low quantity) meet the definition of mixed flora contamination. 3
• Growth of three or more species of commensal bacteria definitively indicates contamination. 4

Collection Method Determines Contamination Risk

The method of urine collection dramatically impacts contamination rates:

• Bag collection: 65-68% contamination rate 2
• Clean-catch midstream: 27-33% contamination rate 2, 3
• Catheterization: 4.7% contamination rate with 95% sensitivity and 99% specificity 1, 2, 3
• Suprapubic aspiration: Lowest contamination, considered gold standard 1

Catheterization provides specimens 7.4 times less likely to show contamination compared to midstream samples. 3

Clinical Management Algorithm

When Mixed Flora is Reported:

1. Do not treat based on bag specimen or poorly collected clean-catch samples alone 2
2. If clinical suspicion for UTI remains high, recollect using catheterization or suprapubic aspiration 1, 2
3. For febrile infants <24 months, confirm any positive culture with mixed flora by catheterization before initiating treatment 2

Important Exception to Consider:

In patients with long-term indwelling catheters, polymicrobial bacteriuria may represent true mixed infection rather than contamination. 5 These patients develop biofilm formation on catheter surfaces, and symptomatic UTI with multiple organisms is a common outcome associated with increased risk for bloodstream infections and mortality. 1, 5

Correlation with Urinalysis Findings

The presence of pyuria (>10 WBCs/HPF) with bacteriuria increases likelihood of true infection despite mixed flora. 2 However:

• Leukocyte esterase has 84% sensitivity but lower specificity 2
• Nitrites have 99% specificity but lower sensitivity 2
• Absence of pyuria suggests contamination or asymptomatic bacteriuria rather than infection 2

Common Pitfalls to Avoid

• Squamous epithelial cells alone do not reliably predict contamination - studies show 94% of catheterized samples contain squamous cells without bacterial contamination 4
• Proper perineal cleansing reduces contamination from 23.9% to 7.8%, though the Infectious Diseases Society of America notes this remains controversial 1, 2
• Heavy mixed growth may mask a true pathogen, potentially requiring repeat testing 2
• Specimens from indwelling catheters in place >few hours frequently contain colonizing flora that does not represent infection 1