What is the management for a urinary tract infection (UTI) with a urine culture positive for diphtheroids, not Corynebacterium?

Management of Diphtheroids in Urine Culture

Diphtheroids isolated from urine cultures typically represent contamination or colonization rather than true infection and should not be treated with antibiotics in asymptomatic patients. 1

Understanding Diphtheroids vs. Corynebacterium

Diphtheroids are skin flora (often Corynebacterium species or similar gram-positive rods) that commonly contaminate urine specimens, particularly those collected via bag collection or midstream catch. 1 The key distinction is whether the patient has:

• Symptoms of UTI (dysuria, frequency, urgency, fever, suprapubic pain, flank pain)
• Proper specimen collection (catheterization or suprapubic aspiration, not bag collection)
• Supporting urinalysis findings (pyuria, bacteriuria, positive leukocyte esterase or nitrites)

Clinical Decision Algorithm

Step 1: Assess for Symptoms

• If asymptomatic: Do not treat, regardless of colony count. 1 Treatment of asymptomatic bacteriuria increases antimicrobial resistance risk and provides no clinical benefit. 1, 2
• If symptomatic: Proceed to Step 2

Step 2: Evaluate Specimen Collection Method

• Bag collection or midstream catch: Likely contamination. 1 Consider recollection via catheterization or suprapubic aspiration if clinical suspicion remains high. 1
• Catheterization or suprapubic aspiration: More reliable for true infection. 1 Proceed to Step 3

Step 3: Review Urinalysis Results

• Negative urinalysis (no pyuria, no bacteriuria, negative leukocyte esterase/nitrites): UTI unlikely (<0.3% probability). 1 Diphtheroids likely represent contamination or colonization. Do not treat. 1
• Positive urinalysis (pyuria, bacteriuria, or positive leukocyte esterase/nitrites): Consider true infection. Proceed to Step 4

Step 4: Assess Colony Count and Clinical Context

• <50,000 CFU/mL: Generally considered contamination unless obtained by suprapubic aspiration. 1 Do not treat asymptomatic patients. 1
• ≥50,000 CFU/mL with symptoms and positive urinalysis: May represent true infection with non-diphtheriae Corynebacterium species (C. urealyticum, C. striatum, C. glucuronolyticum). 3, 4

When to Treat: True Corynebacterium UTI

True UTI with non-diphtheriae Corynebacterium species occurs primarily in:

• Patients with urologic procedures or instrumentation (100% in one series) 3
• Immunosuppressed patients (54% in one series) 3
• Elderly patients (>65 years: 65% in one series) 3
• Patients with previous antibiotic use (90% in one series) 3
• Patients with bladder abnormalities or calculi 4

Treatment Recommendations for Confirmed Corynebacterium UTI

First-line therapy:

• Vancomycin (all isolates 100% susceptible) 4
• Linezolid (all isolates 100% susceptible) 4

Important caveats:

• Non-diphtheriae Corynebacterium species show high rates of multidrug resistance (91.1% MDR rate). 4
• Resistance is common to fluoroquinolones, β-lactams, macrolides, and tetracyclines. 4
• Prolonged culture incubation (>48 hours) may be required for detection, as cultures may initially appear sterile. 3
• Adjust therapy based on susceptibility results once available. 1, 5

Treatment Duration

• 7-14 days for uncomplicated UTI 5
• 10-14 days for complicated UTI (structural abnormalities, immunosuppression) 5

Common Pitfalls to Avoid

1. Treating asymptomatic bacteriuria: This is the most common error, occurring in 45% of cases where treatment is inappropriate. 2 Female sex, pyuria, nitrite positivity, and gram-negative organisms increase inappropriate treatment rates. 2

2. Overinterpreting bag-collected specimens: These have high contamination rates with skin flora including diphtheroids. 1 Only catheterized or suprapubic specimens are suitable for culture. 1

3. Assuming all diphtheroids are contaminants: While most are, true Corynebacterium UTI exists in high-risk patients with proper specimen collection and supporting clinical/laboratory findings. 3, 4

4. Using empiric therapy without culture guidance: Given 91.1% MDR rates in non-diphtheriae Corynebacterium, empiric therapy with common oral agents (nitrofurantoin, TMP-SMX, fluoroquinolones) will likely fail. 4 Culture-guided therapy is essential.

5. Inadequate culture incubation time: Corynebacterium species may require prolonged incubation and can initially appear as "no growth." 3 Alert the laboratory if clinical suspicion is high.

Special Populations

Patients with indwelling catheters:

• All patients with long-term catheters develop bacteriuria (100%). 1
• Treatment of asymptomatic catheter-associated bacteriuria does not prevent symptomatic UTI or reduce mortality. 1
• Only treat if symptomatic (fever, suprapubic tenderness, costovertebral angle tenderness, altered mental status in elderly). 1

Pregnant patients:

• This is one of the few exceptions where asymptomatic bacteriuria should be treated, but diphtheroids are still likely contaminants requiring proper specimen collection for confirmation. 1