In an asymptomatic patient without urinary catheter or other risk factors, is methicillin‑resistant Staphylococcus epidermidis isolated from a urine culture considered a contaminant?

Is MRSE in Urine Culture a Contaminant?

In an asymptomatic patient without a urinary catheter or risk factors, methicillin-resistant Staphylococcus epidermidis (MRSE) isolated from urine culture should be considered a contaminant and not treated.

Diagnostic Criteria Required Before Treating Any Urine Culture Result

Both of the following must be present to justify antimicrobial therapy 1, 2:

• Pyuria: ≥10 white blood cells per high-power field on microscopy or positive leukocyte esterase 1, 2
• Acute urinary symptoms: dysuria, frequency, urgency, fever >38.3°C, gross hematuria, suprapubic pain, or costovertebral angle tenderness 1, 2

If either criterion is absent, do not treat—regardless of what organism grows on culture. 1

Why S. epidermidis Is Almost Always a Contaminant in Urine

• S. epidermidis is normal skin flora and the most common contaminant in urine specimens, particularly when collection technique is suboptimal (inadequate perineal cleansing, midstream catch without proper preparation) 3, 4

• Contamination rates for clean-catch specimens range from 7.8% to 27%, and the presence of epithelial cells alongside S. epidermidis strongly indicates peri-urethral contamination rather than bladder infection 3, 4

• Mixed flora (≥2 organisms including S. epidermidis) is contamination until proven otherwise and should never trigger treatment 3, 4

Rare Exceptions When S. epidermidis May Be a True Uropathogen

S. epidermidis can cause genuine urinary tract infection only in these high-risk scenarios 5, 6, 7, 8, 9:

• Indwelling urinary catheter or recent instrumentation (cystoscopy, urologic surgery) 5, 7, 9
• Implanted urologic devices (ureteral stents, nephrostomy tubes) 5, 7
• Profound immunosuppression (absolute neutrophil count <100 cells/µL, chemotherapy, solid-organ transplant) 8
• Structural urinary abnormalities (neurogenic bladder, vesicoureteral reflux, urinary stones) 6, 9

Even in these populations, treatment is warranted only when both pyuria and acute urinary symptoms are documented. 1, 2

Management Algorithm for MRSE in Urine Culture

Step 1: Assess for Urinary Symptoms

• If asymptomatic: Stop. Do not treat. Document that the isolate represents colonization or contamination. 1
• If symptomatic: Proceed to Step 2.

Step 2: Confirm Pyuria

• If pyuria absent (negative leukocyte esterase and <10 WBC/HPF): Stop. Do not treat. The negative predictive value for bacterial UTI is 82–91%. 2
• If pyuria present: Proceed to Step 3.

Step 3: Evaluate Risk Factors

• No catheter, no instrumentation, no immunosuppression, no structural abnormality: The MRSE is a contaminant. Obtain a properly collected repeat specimen (in-and-out catheterization for women; midstream clean-catch after thorough cleansing for men) to identify the true pathogen. 2, 3
• Catheter, recent instrumentation, profound immunosuppression, or structural abnormality present: MRSE may be a true pathogen. Proceed to Step 4.

Step 4: Initiate Empiric Therapy (Only for High-Risk Patients with Symptoms + Pyuria)

• Vancomycin is the drug of choice for methicillin-resistant S. epidermidis infections. 5, 7
• For serious infections (urosepsis, pyelonephritis), combine vancomycin with rifampin or gentamicin. 5
• Treatment duration: 7–14 days for complicated UTI; longer if bacteremia or device-related infection is present. 5, 7
• Remove or replace indwelling catheters or devices whenever feasible, as antimicrobial therapy alone often fails without source control. 5, 7

Critical Pitfalls to Avoid

• Never treat asymptomatic bacteriuria—even with MRSE—outside of pregnancy or pre-urologic procedures with anticipated mucosal bleeding. Treatment offers no benefit and promotes resistance. 1

• Do not assume all positive cultures represent infection. Asymptomatic bacteriuria occurs in 15–50% of older adults and long-term care residents. 1, 2

• Mixed flora (MRSE plus other organisms) is contamination. Recollect the specimen using proper technique before making treatment decisions. 3, 4

• Pyuria alone (without urinary symptoms) does not justify treatment, as it is present in most cases of asymptomatic bacteriuria. 1, 2

• Non-specific geriatric symptoms (confusion, falls, functional decline) without dysuria, fever, or other acute urinary symptoms do not warrant UTI treatment. 1, 2

Specimen Collection to Minimize Contamination

• Women: In-and-out catheterization is preferred when initial specimens show high epithelial cell counts or mixed flora. 2, 3
• Men: Midstream clean-catch after thorough perineal cleansing or use of a freshly applied clean condom catheter. 2, 3
• Process specimens within 1 hour at room temperature or refrigerate within 4 hours to prevent bacterial overgrowth and falsely elevated counts. 2, 3

Quality-of-Life and Antimicrobial Stewardship Impact

• Unnecessary antibiotic treatment for contaminants or asymptomatic bacteriuria increases antimicrobial resistance, exposes patients to adverse drug effects (including Clostridioides difficile infection), and provides no clinical benefit. 1, 2

• Educational interventions on proper diagnostic protocols achieve a 33% absolute risk reduction in inappropriate antimicrobial initiation. 2