What is the likelihood that a blood culture showing gram-positive cocci in clusters is a contaminant in a patient already discharged from the Emergency Department (ED)?

From the Guidelines

Blood cultures showing gram-positive cocci in clusters have a high likelihood (approximately 70-80%) of being contaminants, particularly when isolated from a single culture set in a patient discharged from the ED. These findings commonly represent coagulase-negative staphylococci (like Staphylococcus epidermidis) from skin contamination during collection, as highlighted in the guidelines for the prevention of intravascular catheter-related infections 1. However, this cannot be dismissed automatically as these organisms can cause true infections, especially in patients with indwelling devices or immunocompromised states. The clinical context is crucial - if the patient was discharged, they likely lacked signs of serious infection at the time of evaluation. Management typically involves:

• Contacting the patient for reassessment of symptoms
• Obtaining repeat blood cultures if the patient appears ill
• Considering empiric antibiotic therapy (such as cefazolin 2g IV q8h or vancomycin 15-20mg/kg IV q12h if MRSA is suspected) only if the patient has developed fever, hypotension, or other signs of systemic infection. The final determination should balance the low pre-test probability of true bacteremia in an ambulatory patient against the risk of untreated infection, with definitive identification and susceptibility testing of the organism guiding ultimate treatment decisions, as emphasized in the american society for microbiology evidence-based laboratory medicine practice guidelines to reduce blood culture contamination rates: a systematic review and meta-analysis 1. It's also important to consider the guidelines for evaluation of new fever in critically ill adult patients, which suggest that drawing three to four blood cultures with appropriate volume from separate sites of access within the first 24 hrs of the onset of fever is the most effective way to discern whether an organism found in blood culture represents a true pathogen or a contaminant 1.

From the Research

Likelihood of Contamination

• The likelihood that a blood culture showing gram-positive cocci in clusters is a contaminant in a patient already discharged from the Emergency Department (ED) is significant, as studies have shown that coagulase-negative staphylococci, which are commonly found on the skin, can contaminate blood cultures 2.
• A study found that the overall contamination rate for potential contaminants in blood cultures, including gram-positive cocci, was 83.9% 2.
• Another study found that coagulase-negative staphylococci were the most commonly isolated organisms (73%) in single-positive blood cultures growing gram-positive cocci 3.

Factors Influencing Contamination

• The use of repeat blood cultures can help differentiate true bacteremia from pseudobacteremia, as the contamination rate declined from 95% to 87% with this strategy 2.
• The presence of an 'oozing sign' surrounding clustered gram-positive cocci on Gram staining may be useful for the rapid prediction of Staphylococcus aureus in blood culture bottles 4.
• The management of gram-positive coccal bacteremia, including catheter management and antibiotic choices, can be complicated by limited vascular access options and the increasing prevalence of drug-resistant bacteria 5.

Diagnostic Challenges

• Differentiating true bacteremia from pseudobacteremia can be challenging, especially in patients with a single positive blood culture growing gram-positive cocci 3.
• The use of rapid polymerase chain reaction-based organism identification can provide supportive information for differentiating contamination from infection, but its impact on vancomycin-prescribing patterns and patient outcomes may be limited 3.
• The accuracy of blood culture Gram staining for the detection of Staphylococcus aureus can be improved by the use of the 'oozing sign' surrounding clustered gram-positive cocci 4.