Are gram variable rods in blood cultures always considered contaminants?

Gram Variable Rods in Blood Cultures: Clinical Significance

Gram variable rods in blood cultures are NOT always contaminants and require careful clinical correlation with specific criteria to distinguish true bacteremia from contamination. The determination depends on multiple factors including number of positive culture sets, time to positivity, patient risk factors, and clinical presentation.

Key Distinguishing Criteria

The most critical factors for determining clinical significance are:

• Number of positive blood culture sets - Multiple positive sets from separate venipunctures strongly indicate true bacteremia rather than contamination 1, 2
• Time to positivity - Earlier positivity (lower time to detection) suggests true infection rather than contamination 3
• Patient risk factors - Immunosuppression, prosthetic devices, intravascular catheters, and critical illness increase likelihood of true infection 2, 3
• Clinical presentation - Fever, hemodynamic instability, and signs of sepsis support true bacteremia 2, 4

Specific Organisms and Their Significance

Common Contaminants (But Not Always)

Laboratories should have abbreviated workup protocols for organisms typically considered skin contaminants, including diphtheroids, Bacillus species (other than B. anthracis), and coagulase-negative staphylococci, with contamination rates not exceeding 3%. 1 However, these organisms can cause true bacteremia in specific clinical contexts.

Gram Variable Rods Requiring Special Consideration

Certain gram variable rods have unique staining characteristics that can mislead clinicians:

• Rapidly growing mycobacteria (RGM) may appear as gram-positive rods and are more sensitive to decolorization procedures, potentially not staining at all with fluorochrome stains 1
• Brucella species can paradoxically stain as gram-positive rods or even gram-positive cocci in chains, despite being classically described as gram-negative rods 5
• Mycobacterium fortuitum can appear as gram-positive rods in blood cultures and should not be automatically dismissed as a contaminant, particularly in patients with risk factors 6

Clinical Algorithm for Management

Step 1: Initial Assessment When Gram Variable Rods Are Reported

Immediately obtain at least 2 additional blood culture sets from separate peripheral venipunctures before initiating antibiotics. 2, 4 This is the single most important action to distinguish contamination from true bacteremia.

Step 2: Risk Stratification

Assess for high-risk features that increase likelihood of true infection: 2, 3

• Prosthetic heart valves or other prosthetic devices
• Immunocompromised state (HIV, chemotherapy, transplant recipients)
• Intravascular catheters or recent invasive procedures
• Intravenous drug use
• Hemodynamic instability or septic shock
• Fever with no alternative source identified

Step 3: Interpretation Based on Culture Results

Single positive culture set: Likely contamination if patient is clinically stable without high-risk features; withhold antibiotics pending repeat cultures 2, 4

Multiple positive culture sets (≥2 from separate venipunctures): Strongly indicates true bacteremia requiring treatment, with probability of contamination being only 6-35% 4

Differential time to positivity: If blood drawn from a catheter becomes positive >2 hours before peripheral blood, this suggests catheter-related infection 2

Step 4: Treatment Decision

For hemodynamically unstable patients or those with high-risk features: Consider empiric therapy with vancomycin while awaiting repeat culture results and final identification 2, 4

For stable patients without high-risk features and single positive culture: Observe clinically and await repeat culture results before initiating therapy 2, 4

Common Pitfalls to Avoid

Do not automatically dismiss gram variable rods as contaminants based solely on organism morphology. 3, 6, 5 The clinical context and culture characteristics are more important than the Gram stain appearance alone.

Avoid drawing blood cultures from intravascular catheters when possible, as catheter-drawn cultures have significantly higher contamination rates (false positives) 1

Do not rely on Gram stain alone for mycobacteria detection, as RGM may be more sensitive to decolorization and may not stain with standard techniques 1

Ensure adequate blood volume is collected - minimum 20 mL per culture set in adults (10 mL per bottle) to optimize yield and reduce false negatives 1

Special Considerations for Specific Clinical Scenarios

In patients with granulomatous disease or cavitary lung lesions, gram-positive rods growing after 5 days may represent Mycobacterium fortuitum or other RGM, particularly in immunocompetent hosts with history of tap water exposure or intravenous drug use 6

In patients with fever and exposure history, atypical Gram stain morphology (gram-positive instead of expected gram-negative) should prompt consideration of Brucella species, which requires special handling and antimicrobial prophylaxis for exposed laboratory personnel 5

For patients on antimicrobial therapy, use blood culture media containing antibiotic-adsorbing substances (BacT/Alert FAN or BACTEC Plus/F) to increase recovery of significant pathogens 1