The Patient with a New Positive Blood Culture: Contaminant vs. True Bacteremia

Preventing Unnecessary Prolonged Antibiotics and Workup for Contaminated Samples

dr Neeraj Manikath , claude.ai

Abstract

Blood culture contamination represents a critical clinical dilemma in modern medicine, occurring in 0.6-6% of all blood cultures drawn. The misinterpretation of contaminants as true pathogens leads to unnecessary antibiotic exposure, promoting antimicrobial resistance, increasing healthcare costs by an estimated $8,000-10,000 per contaminated culture, and exposing patients to iatrogenic complications including Clostridioides difficile infection. This review provides evidence-based guidance for distinguishing blood culture contaminants from true bacteremia, emphasizing the critical importance of clinical correlation, the two-set rule, and appropriate communication with microbiology laboratories. We present practical algorithms and clinical pearls to guide postgraduate physicians in making informed decisions when confronted with positive blood cultures.

Keywords: Blood culture contamination, bacteremia, coagulase-negative staphylococci, antibiotic stewardship, clinical correlation

Introduction

The notification of a positive blood culture often triggers a cascade of clinical decisions: initiation or modification of antimicrobial therapy, ordering of additional diagnostic studies, and potential consultation with subspecialists. However, not all positive blood cultures represent true infection. The ability to distinguish blood culture contaminants from genuine pathogens is a fundamental skill that directly impacts patient outcomes, healthcare costs, and antimicrobial stewardship efforts.

Blood culture contamination occurs when skin flora or environmental organisms are inadvertently introduced into culture bottles during the collection process. These "false-positive" results create diagnostic uncertainty and may lead to unnecessary interventions. Studies have demonstrated that blood culture contamination rates vary widely across institutions, with rates above 3% considered suboptimal by quality improvement standards.

Clinical Pearl #1: The single most important question when confronted with a positive blood culture is not "What organism grew?" but rather "Does this organism make sense in the clinical context of my patient?"

Why Distinguishing Contaminants from True Bacteremia is Essential

The Hidden Costs of Treating Contaminants

The ramifications of misinterpreting contaminants as true pathogens extend far beyond the individual patient:

1. Antimicrobial Resistance: Unnecessary antibiotic exposure drives selective pressure for resistant organisms, contributing to the global antimicrobial resistance crisis. Each day of unnecessary vancomycin therapy, for instance, increases the risk of selecting for vancomycin-resistant enterococci (VRE) colonization.

2. Clostridioides difficile Infection: Broad-spectrum antibiotic exposure, particularly with fluoroquinolones, cephalosporins, and clindamycin, is the primary risk factor for C. difficile infection. Studies show that each additional day of antibiotic therapy increases C. difficile risk by approximately 3%.

3. Economic Burden: The direct costs associated with treating contaminated blood cultures include additional hospital days, intravenous antibiotics, laboratory tests (including unnecessary echocardiograms for "endocarditis rule-out"), and subspecialty consultations. Indirect costs include prolonged hospital stays, lost productivity, and treatment of antibiotic-related complications.

4. Adverse Drug Events: All antibiotics carry risks of allergic reactions, organ toxicity (nephrotoxicity, hepatotoxicity), drug-drug interactions, and secondary infections. Vancomycin, commonly used for suspected coagulase-negative staphylococcal bacteremia, requires therapeutic drug monitoring and carries risks of nephrotoxicity and infusion reactions.

5. Diagnostic Distractors: Pursuing contaminants as true pathogens may divert attention from the actual source of a patient's illness, delaying appropriate diagnosis and treatment.

Clinical Pearl #2: Before starting or continuing antibiotics for a positive blood culture, always ask: "What is the pre-test probability that this organism represents true infection in my patient?"

The Usual Suspects: Common Blood Culture Contaminants

Understanding which organisms are commonly contaminants versus pathogens is crucial for interpretation. However, it is essential to recognize that nearly any organism can cause true bacteremia under the right circumstances, particularly in immunocompromised hosts or patients with indwelling devices.

Coagulase-Negative Staphylococci (CoNS)

CoNS, particularly Staphylococcus epidermidis, represent the most common blood culture contaminants, accounting for 70-80% of contaminated cultures. These organisms are ubiquitous skin commensals but are also legitimate pathogens in specific clinical scenarios.

When CoNS are likely contaminants:

Growth in only one of multiple blood culture sets
No indwelling prosthetic material (cardiac devices, prosthetic valves, vascular grafts, orthopedic hardware)
No intravascular catheters
Absent or mild clinical signs of infection
Patient improving clinically without specific anti-CoNS therapy

When CoNS may represent true bacteremia:

Growth in multiple blood culture sets (≥2 sets)
Presence of intravascular devices (especially within 48 hours of culture draw)
Prosthetic cardiac valves or other implanted devices
Neonatal or very young pediatric patients
Neutropenic or severely immunocompromised patients
Clinical signs of sepsis with no alternative explanation
Consistent species identification across multiple cultures

Oyster #1: Not all CoNS are created equal. S. lugdunensis, while technically coagulase-negative, behaves more like S. aureus and can cause aggressive native valve endocarditis. The microbiology lab can identify this species if requested, and it should never be dismissed as a contaminant.

Bacillus Species (Non-anthracis)

Bacillus species other than B. anthracis are environmental organisms and common blood culture contaminants, typically introduced from inadequate skin antisepsis. However, they can cause serious infections, particularly in specific patient populations.

When Bacillus spp. are likely contaminants:

Growth in a single blood culture bottle
No immunocompromise or intravenous drug use
No central venous catheter
Patient clinically well or improving
No evidence of deep-seated infection

When Bacillus spp. may be significant:

Intravenous drug users (particularly B. cereus endocarditis)
Immunocompromised hosts (especially neutropenic patients)
Presence of central venous catheters
Traumatic injuries with soil contamination
Post-neurosurgical patients (rare CNS infections)
Growth in multiple blood culture sets

Hack #1: If you see "Bacillus species" in a blood culture from an intravenous drug user or neutropenic patient, don't reflexively dismiss it. Call the microbiology lab and ask them to identify it to the species level. B. cereus can cause devastating endocarditis and requires specific therapy.

Corynebacterium Species (Diphtheroids)

Corynebacterium species are skin commensals that commonly contaminate blood cultures. However, certain species, particularly C. jeikeium, C. urealyticum, and C. striatum, are increasingly recognized as opportunistic pathogens.

When Corynebacterium spp. are likely contaminants:

Single positive culture
No prosthetic devices
Immunocompetent host
Clinical improvement without directed therapy
Mixed growth with other skin flora

When Corynebacterium spp. deserve attention:

Multiple positive blood culture sets
Immunocompromised patients (especially oncology patients)
Prosthetic cardiac valves or neurosurgical shunts
Long-term central venous catheters (especially in oncology/dialysis patients)
Recent cardiac surgery
Persistent or worsening clinical deterioration

Clinical Pearl #3: Corynebacterium isolates from oncology patients with long-term central lines should not be reflexively dismissed, especially if the patient has been on quinolone prophylaxis (which selects for diphtheroids).

Propionibacterium (Cutibacterium) acnes

This slow-growing anaerobic organism is a normal skin commensal but an important pathogen in specific contexts.

High-risk scenarios for true P. acnes bacteremia:

Shoulder surgery patients with prosthetic hardware
Prosthetic valve endocarditis (especially in young patients with previously "culture-negative endocarditis")
Neurosurgical shunt infections
Growth in multiple blood culture sets (may take 5-14 days to grow)

Clinical Pearl #4: If blood cultures from a patient with a prosthetic shoulder implant grow "skin flora" after 5-7 days, don't dismiss it. P. acnes is notoriously slow-growing and causes insidious prosthetic joint infections. Request extended incubation and anaerobic culture if clinical suspicion is high.

Micrococcus Species

These are almost always contaminants but have been reported in rare cases of endocarditis, particularly in immunocompromised hosts or those with prosthetic valves.

Viridans Group Streptococci

This group occupies a middle ground. While oral flora, they are common causes of subacute bacterial endocarditis, particularly in patients with valvular abnormalities or recent dental procedures.

Favor true bacteremia if:

Multiple positive blood culture sets
Known valvular heart disease or prosthetic valve
Recent dental procedure or periodontal disease
Clinical syndrome consistent with endocarditis
No concurrent growth of other oral flora

Oyster #2: A "mixed oral flora" blood culture (with multiple oral organisms including viridans streptococci) is almost always a contaminant. However, a single species of viridans streptococci growing in multiple culture sets from a patient with a heart murmur is endocarditis until proven otherwise.

The "Two-Set Rule": The Gold Standard for Interpretation

The single most valuable piece of information in determining whether a positive blood culture represents true bacteremia is whether the organism grew in multiple blood culture sets drawn from separate venipuncture sites.

Defining Terms

Blood culture set: Typically consists of one aerobic and one anaerobic bottle
Adequate blood culture collection: At least two sets (four bottles total) drawn from separate venipunctures, preferably separated by time and anatomical location
Fill volume: Each bottle should contain 8-10 mL of blood (adults) for optimal sensitivity

The Evidence Behind the Two-Set Rule

Multiple studies have validated that true bacteremia typically produces positive cultures in multiple sets:

Organisms growing in ≥2 sets have an 80-90% probability of representing true bacteremia
Organisms growing in only 1 of ≥2 sets have a 15-20% probability of representing true bacteremia (much lower for typical contaminants like CoNS)
Common contaminants (CoNS, Bacillus, Corynebacterium) growing in 1 set: <5% probability of true bacteremia
Recognized pathogens (S. aureus, E. coli, Streptococcus pneumoniae) growing in even 1 set: 95-100% probability of true bacteremia

Hack #2: When blood cultures are drawn, document in your note: "2 sets of blood cultures drawn from separate peripheral sites." This documentation helps you (and consultants) interpret results and demonstrates adherence to quality standards.

The Exception: Undeniable Pathogens

Certain organisms are virtually never contaminants and should be considered true pathogens even when isolated from a single blood culture bottle:

Staphylococcus aureus
Escherichia coli and other Enterobacteriaceae
Pseudomonas aeruginosa
Streptococcus pneumoniae
Streptococcus pyogenes (Group A Streptococcus)
Neisseria meningitidis
Listeria monocytogenes
Candida species
Salmonella species
Bacteroides fragilis and other obligate anaerobes

Clinical Pearl #5: If S. aureus grows in even a single blood culture bottle, treat it as real until proven otherwise. The consequences of missing S. aureus bacteremia (endocarditis, metastatic infections) far outweigh the risks of empiric therapy while gathering more information.

Time to Positivity: An Underutilized Clue

Modern continuous-monitoring blood culture systems report "time to positivity" (TTP)—the time from culture incubation to detection of growth. TTP can provide valuable clues:

True bacteremia: Usually TTP <15-20 hours for common pathogens
Contaminants: Often TTP >24-36 hours (lower bacterial load at inoculation)
Exception: Fastidious organisms (P. acnes, nutritionally variant streptococci) may have delayed TTP even when causing true infection

Hack #3: Check the time to positivity on your blood culture report. If CoNS grew in only one bottle after 48 hours, and your patient looks well, that's almost certainly a contaminant.

Clinical Correlation is King: Integrating the Clinical Picture

Laboratory results must always be interpreted within the clinical context. The following clinical parameters should guide interpretation:

Signs and Symptoms of Infection

Supportive of true bacteremia:

Fever (temperature ≥38.3°C) or hypothermia (<36°C)
Rigors or severe chills
Hemodynamic instability or shock
New or worsening organ dysfunction
Appropriate source identified (pneumonia, pyelonephritis, cellulitis, etc.)

Against true bacteremia:

Afebrile patient without hypothermia
Clinically improving or stable
No identifiable source of infection
Routine surveillance culture in asymptomatic patient

Laboratory Parameters

Supportive of true bacteremia:

Leukocytosis (WBC >12,000/μL) or leukopenia (<4,000/μL)
Bandemia (>10% immature forms)
Elevated inflammatory markers (ESR, CRP, procalcitonin)
Elevated lactate
Acute kidney injury or liver dysfunction

Against true bacteremia:

Normal white blood cell count and differential
Normal inflammatory markers
Absence of organ dysfunction

Oyster #3: Procalcitonin can be helpful but isn't perfect. An elevated procalcitonin (>0.5 ng/mL) supports bacterial infection, but it can be elevated in non-infectious conditions (burns, surgery, severe trauma). A normal procalcitonin (<0.25 ng/mL) makes bacterial bloodstream infection less likely but doesn't exclude it entirely, especially in early infection or localized disease.

Host Factors

Higher likelihood organism is significant:

Immunocompromised state (chemotherapy, neutropenia, transplant, HIV with CD4 <200)
Prosthetic material (cardiac valves, vascular grafts, orthopedic hardware, neurosurgical devices)
Intravascular catheters (especially long-term central lines)
Injection drug use
Recent surgery or instrumentation
Extremes of age (neonates, elderly >80 years)

Lower likelihood organism is significant:

Immunocompetent host
No prosthetic material or intravascular devices
Good baseline functional status
Clinically well or improving

Integrating the Clinical Decision Score

Several clinical decision rules have been proposed to integrate these factors. One practical approach is the following framework:

High probability of true bacteremia (≥80%):

Recognized pathogen in any culture OR
Common contaminant in ≥2 culture sets PLUS signs of sepsis

Moderate probability (20-80%):

Common contaminant in ≥2 culture sets without clear sepsis OR
Common contaminant in 1 set with prosthetic device and concerning clinical picture

Low probability (<20%):

Common contaminant in 1 set in immunocompetent patient without prosthetic devices or concerning clinical picture

Hack #4: Create a mental algorithm: "1 bottle + typical contaminant + looks well = probably not real." Document your thought process clearly in the medical record to justify decisions and help colleagues following the patient.

When to Repeat Blood Cultures and When to Stop Antibiotics

Indications to Repeat Blood Cultures

Always repeat when:

Initial positive culture shows recognized pathogen (S. aureus, Gram-negatives, Candida)
To document clearance of bacteremia (especially S. aureus bacteremia—repeat daily until negative)
Patient remains febrile or clinically deteriorating despite appropriate antibiotics
Suspicion for endocarditis (need multiple positive sets over time)
Immunocompromised host with any positive culture

Consider repeating when:

Single positive culture with common contaminant but high-risk patient (prosthetic material, severely immunocompromised)
Discordant clinical picture (patient looks septic but single bottle with CoNS)
To help clarify if initial culture was contaminant

Do not routinely repeat when:

Single positive culture with typical contaminant in low-risk, clinically well patient
Repeat cultures unlikely to change management

Clinical Pearl #6: The best time to prove a blood culture was a contaminant is before starting antibiotics. If you suspect contamination, repeat blood cultures immediately (before antibiotics if possible). If the repeat cultures are negative and the patient looks well, you've saved them days of unnecessary therapy.

Stopping Antibiotics: The Evidence-Based Approach

The decision to stop antibiotics initiated for a presumed contaminant requires courage but is essential for antimicrobial stewardship.

Stop antibiotics when ALL of the following are met:

Organism is a typical contaminant (CoNS, Bacillus, Corynebacterium, Micrococcus)
Organism grew in only 1 of ≥2 blood culture sets
Patient is clinically stable or improving
No prosthetic devices or other high-risk features
Repeat blood cultures (if drawn) are negative
Alternative explanation for clinical presentation identified or patient has improved without specific therapy

Observation period after stopping antibiotics:

Continue to monitor patient for 48-72 hours after stopping antibiotics
Check vital signs regularly
Monitor for clinical deterioration
Consider inflammatory markers if concern persists

Document clearly in the medical record:

Rationale for considering the organism a contaminant
Clinical and laboratory parameters supporting this conclusion
Plan for monitoring after discontinuation

Hack #5: When stopping antibiotics for a presumed contaminant, write a clear note explaining your reasoning and set an expectation with the team: "We will monitor clinically for 48 hours. If patient remains stable, this supports our interpretation that this was a contaminant." This prevents midnight panicked calls about 'stopping antibiotics with a positive culture.'

The Gray Zone: When Uncertainty Remains

Sometimes, despite best efforts, uncertainty persists. In these situations:

Short-course therapy approach:

Consider 3-7 days of targeted antimicrobial therapy (rather than 14 days)
Monitor clinical response closely
If patient improves rapidly, supports contaminant hypothesis
If patient remains ill, consider alternative diagnoses

Risk-benefit analysis:

For low-risk patients: Favor shorter/no therapy
For high-risk patients (prosthetic valves, severely immunocompromised): Favor completing therapy course
Involve infectious diseases consultation when available

Clinical Pearl #7: When in doubt, it's acceptable to treat with narrow-spectrum antibiotics for a short course while monitoring. "Empiric cefazolin for 5 days for possible CoNS bacteremia in a patient with a pacemaker" is reasonable. "Vancomycin for 14 days for likely contamination" is not.

Communicating with the Microbiology Lab: A Critical Partnership

Effective communication with the microbiology laboratory can dramatically improve interpretation of positive blood cultures. The laboratory staff are expert allies in clinical decision-making.

Information to Request

1. Species-level identification: Even for "likely contaminants," knowing the species can be invaluable:

S. lugdunensis vs. other CoNS
B. cereus vs. other Bacillus species
C. jeikeium vs. other Corynebacterium species

Clinical Pearl #8: Call the lab and ask: "Can you identify this to the species level?" Most labs can do this with MALDI-TOF mass spectrometry within hours. Species identification can change management dramatically.

2. Antimicrobial susceptibilities: Request full susceptibility testing even on suspected contaminants when:

Patient has prosthetic devices
Patient is immunocompromised
Uncertain if organism represents true pathogen
Organism grew in multiple culture sets

The susceptibility pattern can provide clues:

Methicillin-resistant CoNS with resistant pattern: More concerning for true pathogen
Pan-susceptible CoNS: Slightly favors contaminant (though not definitive)

3. Time to positivity (TTP): This is automatically reported by modern blood culture systems but may not be prominently displayed. Shorter TTP generally indicates higher bacterial load and favors true bacteremia.

4. Gram stain morphology and purity:

Pure culture of single organism type: Favors true bacteremia
Mixed growth with multiple morphologies: Strongly favors contamination
Request Gram stain description if multiple organisms reported

5. Comparative information across culture sets: Ask the lab: "Did the same organism grow in both sets?" Identical organisms (same species, similar TTP) in multiple sets strongly support true bacteremia.

Providing Clinical Context to the Laboratory

Communication should be bidirectional. Provide the laboratory with clinical context:

"Patient has prosthetic aortic valve—please identify CoNS to species level and perform susceptibilities"
"Patient with shoulder prosthesis—please use extended incubation for anaerobes and identify all isolates to species"
"Suspected contaminant, planning to discontinue antibiotics—can you confirm only one bottle positive?"

Hack #6: Save the microbiology lab phone number in your phone. Building a relationship with lab staff (especially the medical directors or senior technologists) pays enormous dividends. They can provide real-time guidance on interpretation and expedite testing.

Understanding Lab Reporting Practices

Different laboratories have different practices for reporting positive cultures:

Preliminary reports: Often available within 24 hours of detection, may only provide genus-level identification (e.g., "Gram-positive cocci in clusters—suggestive of Staphylococcus species")

Final reports: Include species identification and susceptibilities, typically available 48-96 hours after initial detection

Reflexive testing: Some labs perform species identification and susceptibilities automatically on all blood culture isolates; others require specific requests for suspected contaminants

Understand your institution's practices to anticipate information flow and know when to make specific requests.

Special Scenarios and Populations

Neonates and Young Children

Blood culture interpretation in pediatric patients, especially neonates, requires special consideration:

Even typical contaminants (CoNS) can be significant pathogens in neonates
Lower threshold for treating positive cultures
Volume of blood cultured is critical (need adequate volumes despite small patient size)
CoNS bacteremia in NICU patients with central lines may warrant treatment even if single positive culture

Oncology Patients

Patients with hematologic malignancies or receiving chemotherapy:

Lower threshold for treating positive cultures due to immunocompromise
Even typical contaminants may be significant in neutropenic patients
Central venous catheter-associated infections are common
Consider catheter salvage vs. removal based on organism and clinical course
C. jeikeium in this population deserves attention (often multi-drug resistant)

Patients on Hemodialysis

This population presents unique challenges:

High prevalence of vascular catheter-associated infections
CoNS and S. aureus are common pathogens
Consider catheter-related bloodstream infection when cultures positive
Need for appropriate antibiotic dosing based on dialysis schedule

Prosthetic Cardiac Valve Patients

Arguably the highest-stakes scenario:

ANY positive blood culture requires careful evaluation
Prosthetic valve endocarditis can be caused by "contaminants"
Lower threshold for echocardiography (preferably TEE)
Consider extended courses of therapy even for typical skin flora
Infectious diseases consultation strongly recommended

Oyster #4: A single bottle positive for CoNS in a patient with a prosthetic valve is not an automatic indication for 6 weeks of vancomycin and valve replacement, but it does warrant: (1) repeat blood cultures, (2) careful clinical assessment, (3) echocardiography (probably TEE), and (4) infectious diseases consultation. Document this thoughtful approach rather than reflexively starting prolonged therapy.

Practical Algorithms for Clinical Decision-Making

Algorithm 1: Initial Assessment

Positive Blood Culture Reported
↓
1. What organism? (Recognized pathogen vs. common contaminant)
2. How many sets positive? (≥2 sets vs. 1 set)
3. Clinical picture? (Signs of sepsis vs. clinically well)
4. Risk factors? (Prosthetic devices vs. none)
↓
HIGH PROBABILITY TRUE BACTEREMIA
(Pathogen in any set OR contaminant in ≥2 sets)
→ Treat appropriately, repeat cultures to document clearance
↓
MODERATE PROBABILITY
(Contaminant in ≥2 sets but no clear sepsis OR high-risk patient)
→ Infectious diseases consultation, consider short-course therapy
↓
LOW PROBABILITY (LIKELY CONTAMINANT)
(Contaminant in 1 set, clinically well, low-risk)
→ Consider repeat cultures, observation, discontinue antibiotics if appropriate

Algorithm 2: CoNS Management

CoNS Positive Blood Culture
↓
≥2 sets positive?
Yes → Likely true bacteremia
    → Prosthetic device? Yes → Treat (consider ID consultation)
    → No prosthetic device? → Consider 5-7 day course, investigate source
No (1 set only) → Likely contaminant
    → High-risk patient (prosthetic valve, severely immunocompromised)?
        Yes → Repeat cultures, careful observation, consider short-course therapy
        No → Repeat cultures, observe, likely discontinue antibiotics

Systems-Level Interventions to Reduce Contamination

While individual clinical decision-making is crucial, institutions should implement systems to reduce blood culture contamination rates:

1. Standardized collection protocols:

Dedicated phlebotomy teams (associated with lower contamination rates)
Blood culture collection kits with all necessary supplies
Strict adherence to skin antisepsis protocols (chlorhexidine preferred over povidone-iodine)
Adequate contact time for antiseptics (30-60 seconds)

2. Education and training:

Regular training for personnel who draw blood cultures
Visual aids and posters in clinical areas
Feedback to individuals with high contamination rates

3. Monitoring and feedback:

Track institutional contamination rates (goal <3%)
Unit-specific contamination data shared with clinical teams
Root cause analysis for contamination events

4. Antimicrobial stewardship integration:

Real-time review of positive blood cultures by stewardship teams
Guidance on discontinuation of antibiotics for likely contaminants
Educational interventions at time of positive culture notification

Summary and Key Take-Home Points

Blood culture contamination is common and costly—both in direct healthcare costs and in promoting antimicrobial resistance.
The "two-set rule" is your most valuable tool—true bacteremia typically grows in multiple culture sets; single-set positivity suggests contamination (especially for typical skin flora).
Clinical correlation is essential—laboratory results must be interpreted within the clinical context. A positive culture in a clinically well patient with no source of infection likely represents contamination.
Typical contaminants (CoNS, Bacillus, Corynebacterium, Micrococcus) can occasionally be true pathogens, especially in patients with prosthetic devices or severe immunocompromise—but single-set positivity in low-risk patients almost always represents contamination.
Recognized pathogens (S. aureus, Gram-negative rods, Candida, S. pneumoniae) should be considered true pathogens even when isolated from a single culture set.
Communication with the microbiology lab—request species identification and susceptibilities even for suspected contaminants; this information can be invaluable.
Stopping antibiotics for likely contaminants—requires clinical courage but is essential for antimicrobial stewardship. Document your reasoning clearly and monitor the patient closely.
When in doubt—repeat blood cultures, consider infectious diseases consultation, and use a risk-benefit approach tailored to the individual patient.

Final Pearl

The art of medicine lies not in reflexively treating every positive test result, but in integrating laboratory data with clinical judgment to determine when treatment is truly indicated. A positive blood culture is not a mandate for antibiotics—it's an invitation to think critically about your patient.

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This review is intended for educational purposes for postgraduate physicians in internal medicine. Clinical decisions should always be individualized to the specific patient scenario and institutional guidelines. When in doubt, consultation with infectious diseases specialists and close collaboration with microbiology laboratories is strongly encouraged.