Blood bacterial culture is the gold standard for the clinical diagnosis of bacteremia, sepsis, septicemia, and septic shock; however, contamination of blood cultures is an unavoidable issue that poses significant challenges for both clinicians and laboratories. In light of this, how should laboratories evaluate and quantify blood culture contamination? I will provide an analysis:

 

Bacterial species:

Only skin‑resident and environmental microorganisms, as well as opportunistic pathogens that are considered nonpathogenic or only weakly pathogenic based on current knowledge, are regarded as potential contaminants. In accordance with the literature and the principles outlined above, we assessed the likelihood of contamination for the following bacteria: coagulase‑negative staphylococci (CoNS), Corynebacterium spp., Micrococcus spp., Bacillus spp. excluding Bacillus anthracis, Cutibacterium acnes, and viridans streptococci.

 

Bacteria that can be almost certainly identified as true positives are:

- Staphylococcus aureus, Streptococcus pneumoniae, Escherichia coli and other Enterobacteriaceae, Pseudomonas aeruginosa, and Candida species

 

Bacteria that can be almost certainly identified as true positives include:

- Streptococcus pyogenes, Streptococcus agalactiae, Listeria, Neisseria meningitidis and Neisseria gonorrhoeae, Haemophilus influenzae, Bacteroides fragilis group, Candida species, and Cryptococcus neoformans

 

Bacteria that are often associated with contamination or false positives, and may also be true positives, include:

- Coagulase-negative staphylococci (CoNS), micrococci, viridans streptococci, enterococci, and Clostridium perfringens

 

Bacteria that are often contaminants or false positives, and very rarely true positives, include:

- Corynebacterium, Bacillus species excluding Bacillus anthracis, and Cutibacterium acnes ^①

 

For bacteria that are considered true positives, the results may be reported directly to the clinical team; for organisms that are typically contaminants and rarely true pathogens, no identification or antimicrobial susceptibility testing is performed, and the results are not reported to the clinic; for species that may be contaminants or could also be pathogenic, contamination screening is conducted using the following procedures:

 

1. Number of positive culture tubes:

Potential contaminant organisms are often isolated from one or two bottles in a set of blood cultures; without performing a second, follow-up blood culture as a control, it is virtually impossible to distinguish between the suspected contaminant and the true pathogen.

Interpretation of results is based on the number of positive kits among those submitted for testing:

- One set includes one aerobic and one anaerobic bottle; collect ≥2 sets either simultaneously or sequentially.

- In at least two culture sets, growth in only one set is often due to contaminating microorganisms.

- 79% of laboratories endorse this indication.

 

CoNS positive predictive value (PPV)

- 1/1 (one set, one bottle, reported positive): Positive predictive value, 55%

- 1/2 (two sets per bottle, reported as positive): PPV, 20%

- 1/3 (three sets per bottle, reported as positive): PPV, 5%

 

PPV of the same bacterium cultured in the same or different sets

- 2/2 (two sets, two bottles for reporting Yang) Set: PPV, 98.0%

- 2/3 (three sets, two bottles reported as positive): PPV, 90.9%

- 3/3 (Three sets of three bottles for Yang promotion) Set: PPV, 100%

 

2. Have there been any other blood culture results or culture results from specimens obtained from other sites, either in the past or currently?

When only a single blood culture is available, and no other blood cultures or cultures from other body sites are performed, the result cannot be interpreted definitively. The result should be discussed with the attending clinician to determine its significance and to decide whether to proceed with antimicrobial susceptibility testing.

 

When isolates from two or more cultures belong to the same species, susceptibility testing is performed; if the isolates do not match, the organism is considered a contaminant and is not subjected to susceptibility testing. However, if the same organism is isolated from specimens obtained from other sites, it is identified as the pathogen and undergoes susceptibility testing.

 

3. Reporting Time:

Theoretical basis: In patients with bacteremia, the bacterial load in the blood (1–10 CFU/mL) should be significantly higher than the bacterial load at the source of contamination.

Numerous studies have shown that positive results observed 3–5 days after vaccination are predominantly attributable to contaminating organisms; in pediatrics, the probability of a positive PPV within 15 hours is 84%.

In terms of average growth time, true positives are significantly shorter than false positives; however, there is substantial overlap in individual cases, and relying solely on culture time does not allow for a reliable assessment.

 

4. Source of blood culture specimen: (venous blood or catheter blood)

The Impact of Venous Catheterization on Blood Sampling Results

- Three considerations for positivity: true positive, contamination, and catheter colonization (examples are shown in the table below).

 

5. Concordance with clinical findings:

Clinical indicators that, in conjunction with clinical data, support the physician’s determination of a true-positive result:

- Hyperthermia (> 40°C) or hypothermia (< 36°C)

- White blood cell count too high (> 20,000/μL) or too low (< 4,000/μL)

- Hypotension, etc.

- Clinical risk factors derived from a comprehensive assessment of available data can aid in distinguishing false positives (e.g., procalcitonin, high-sensitivity C-reactive protein, and the Galactomannan assay). - The presence of an identifiable source of infection, such as a central venous catheter, can help confirm true positivity.

 

A retrospective review of the medical records of relevant patients was conducted. Based on each patient’s medical history and clinical symptoms, the clinical significance of blood cultures was determined by integrating findings from white blood cell counts, body temperature, bacterial cultures from other sites, radiologic studies, pathology reports, the therapeutic response to antimicrobial therapy, and the progression of the disease. A blood culture was considered clinically significant when both of the first two criteria were met, or when one of those two criteria was present in combination with any two of the remaining criteria. Examples are provided in the table below.

 

Summarize and analyze the causes, and identify the issues:

The possible issue is as follows:

1) Blood was not drawn within the optimal time window;

2) Incorrect blood collection site;

3) Insufficient blood volume collected;

4) The number of blood culture sets and the allocation of blood culture bottles are not standardized;

5) Aseptic technique was not strictly followed;

6) Improper storage and transportation of blood culture bottles.

 

(① Reference: BloodCulture Contamination_Per Sisting Problems and Partial Progress_MINIREVIEW_2003)