What does the presence of viridans streptococci in a urine culture indicate?

Viridans Streptococci in Urine Culture: Contamination vs. True Infection

Viridans streptococci in urine culture most commonly represents contamination from periurethral, genital, or oral flora rather than true urinary tract infection, particularly when isolated from clean-catch or bag specimens. 1

Understanding the Source

Viridans streptococci are normal inhabitants of the oral cavity, upper respiratory tract, and genital/periurethral areas. Their presence in urine typically occurs through:

• Contamination during collection - The most common mechanism, especially with clean-catch midstream specimens where periurethral flora contaminates the sample 1
• Poor collection technique - Inadequate cleansing before collection significantly increases contamination rates (23.9% without cleansing vs. 7.8% with proper cleansing) 1
• Bag collection in children - Bag specimens have contamination rates of 65-68%, making them highly unreliable for definitive diagnosis 2

When to Consider True Infection

While uncommon, viridans streptococci can occasionally cause genuine urinary tract infections. Consider true infection when:

• High colony counts from reliable specimens - Growth of ≥10³-10⁵ CFU/mL from catheterized specimens or any growth from suprapubic aspiration 2, 3
• Pure culture with pyuria - Isolation of a single organism with significant pyuria (≥10 WBCs/HPF) and bacteriuria on urinalysis 2
• Clinical correlation - Symptomatic patients (fever, dysuria, hematuria) with corresponding laboratory findings 4, 5
• Polymorphonuclear leukocyte phagocytosis - Visualization of bacteria being phagocytosed by white blood cells on microscopy strongly suggests true infection rather than contamination 6

Critical Interpretation Factors

Collection method is paramount for interpretation:

• Suprapubic aspiration - Any growth (≥10² CFU/mL) is significant; this method has the highest reliability 2, 3
• Catheterization - Requires ≥10³-10⁵ CFU/mL of a single organism with sensitivity of 95% and specificity of 99% 2
• Clean-catch midstream - Requires ≥10⁵ CFU/mL, but contamination rates remain 27% 1, 2
• Bag specimens - Have only 15% positive predictive value and should never be used for definitive diagnosis 2

Common Clinical Pitfalls

Do not treat based on bag specimens alone - The American Academy of Pediatrics recommends that positive cultures with mixed or unusual flora from bag collection must be confirmed by catheterization or suprapubic aspiration before initiating treatment 2

Beware of misidentification - Viridans streptococci are fastidious organisms often confused with other streptococcal species or dismissed as contaminants 4, 5, 7. Modern identification methods like MALDI-TOF mass spectrometry have improved recognition of these organisms as potential pathogens 5, 7

Check for multiple organisms - The presence of viridans streptococci alongside multiple other bacterial species strongly indicates contamination rather than infection 1, 2

Recommended Approach

When viridans streptococci appear in urine culture:

1. Assess collection method - If from bag or poorly collected clean-catch specimen, consider the result unreliable 2
2. Review urinalysis - Absence of pyuria suggests contamination or asymptomatic bacteriuria 2
3. Evaluate clinical context - Asymptomatic patients with low colony counts likely have contamination 1
4. Recollect if needed - Use catheterization for reliable confirmation if clinical suspicion for UTI remains high despite questionable initial results 2
5. Correlate all findings - Colony count, collection method, urinalysis results, and clinical symptoms must align before diagnosing true infection 2, 3

In symptomatic patients with significant colony counts from properly collected specimens and supporting urinalysis findings, viridans streptococci should be treated as a true pathogen 4, 5, 6, though this scenario is uncommon compared to contamination.