Blood Cultures

Background

Blood cultures play a critical role in the diagnosis and management of bloodstream infections, including sepsis, endocarditis, and other life-threatening conditions caused by bacterial or fungal pathogens. It could also mean transient bacteremia (such as after flossing your teeth) or simply contamination during collection. By detecting and identifying microorganisms present in the blood, blood cultures enable clinicians to determine the appropriate antimicrobial therapy, ensuring targeted and effective treatment. Early and accurate diagnosis through blood cultures is essential in preventing complications, reducing mortality rates, and minimizing the emergence of antibiotic resistance by avoiding unnecessary or inappropriate antibiotic use. Furthermore, blood cultures help guide public health initiatives by identifying patterns of antimicrobial resistance and tracking the spread of infectious diseases within healthcare settings and the community.

Of course, this all depends on accurate results, and these depend on proper collection, handling, workup, and interpretation. This starts with the phlebotomist. Contamination during drawing is the most common cause of inaccurate results. Skin flora will grow in the blood culture bottles and give false positive results if the site is not properly cleaned prior to phlebotomy and labs should be tracking contamination rates.

Blood cultures: Sepsis, bacteremia, or just contamination?

Proper Collection of Blood Cultures

Site Selection and Aseptic Technique

An appropriate venipuncture site should be selected, typically the antecubital fossa or another accessible vein. Hand hygiene must be performed, and sterile gloves should be worn to prevent contamination. The venipuncture site should be cleaned thoroughly using an antiseptic agent, such as 70% isopropyl alcohol followed by 2% chlorhexidine or povidone-iodine, to remove skin flora that could compromise the results. The antiseptic should be allowed to dry completely before proceeding, as premature puncture can reduce its effectiveness.

Blood Collection

Blood should be drawn using a sterile syringe or a closed blood collection system, such as a vacutainer. The appropriate volume of blood must be collected to ensure optimal microbial detection, typically 8–10 mL per bottle for adults and 1–5 mL per bottle for pediatric patients. If multiple sets of blood cultures are required, blood should be either drawn from separate venipuncture sites or at least 15 minutes apart. This practice increases the likelihood of detecting true pathogens while also differentiating contaminants from actual bloodstream infections.

Bottle Inoculation

Once blood is collected, it should be immediately transferred into both aerobic and anaerobic culture bottles. If using a syringe, the aerobic bottle should be inoculated first to prevent air from entering the anaerobic bottle. However, if a vacutainer system is used, the anaerobic bottle should be filled first. If you realize you don't have enough blood for both bottles, just use an aerobic bottle, and common organisms will grow in that medium

Labeling and Documentation

Each blood culture bottle should be properly labeled with patient identifiers, the date and time of collection, and the venipuncture site used. It is also necessary to document details of the procedure, including the number of sets collected, the volume drawn, and any relevant clinical indications. Proper documentation ensures that results are correctly attributed to the patient and supports clinical decision-making.

Incubation

In most labs, the blood culture bottles will be checked into the microbiology laboratory and placed into an incubation system that reads the bottles for growth. If the bottles turn positive (aka demonstrate growth) they will alarm. If the automated system fails for any reason, the bottles can be manually sampled and plated to check for growth.

Blood Culture Workup

Gram Stain

This is quite simple. Report the gram stain of the organism. Remember, there is no need to quantify because they've been growing in culture and the number when it goes positive is meaningless. Presence or absence is what's important.

Workup

The plates you use will vary by laboratory, but generally blood agar, chocolate agar, and MacConkey agar are the basics. Adding at least one all-purpose anaerobic plate for positive anaerobic cultures is warranted. Most of the time, cultures will be pure, so the isolation of different isolates is less of an issue that with wound, respiratory, or urine cultures.

All organisms present in the blood culture should be worked up, including sensitivities if possible. Again, we do not want to report quantity because it is meaningless. All blood cultures will be growing into the fourth quadrant.

Real positive cultures will usually alarm within 2 days of collection, sometimes withing 24 hours. Very slow growing organisms can sometimes take longer. The culture should be finalized after 5 days of incubation.

Contamination Rates

Contamination rates are often tracked by hospitals in order to reduce waste and improve the quality of patient care. The CDC suggests doing that as follows.

Calculating the Blood Culture Contamination (BCC) Rate

The BCC rate is determined by identifying instances where a skin commensal organism grows in only one of multiple blood culture sets taken from a patient within a 24-hour period. Such isolated growth often indicates contamination. The calculation involves the following steps:

1. Define the Denominator: Count all blood culture sets collected from adult patients (aged 18 and over) within the evaluation period (typically monthly). A blood culture set usually comprises one aerobic and one anaerobic bottle, with at least two sets recommended per patient within 24 hours to ensure adequate blood volume for accurate detection.

2. Define the Numerator: Identify the number of blood culture sets that grew a skin commensal organism in only one set out of the multiple sets collected from the same patient within the 24-hour timeframe. Common skin commensals include coagulase-negative staphylococci, Corynebacterium species, and Propionibacterium species.

3. Calculate the BCC Rate: Divide the numerator by the denominator and multiply by 100 to obtain the contamination rate percentage.

For example, if 200 blood culture sets were collected in a month and 1 set showed growth of a skin commensal organism without matching growth in other sets from the same patient, the BCC rate would be:

(1 ÷ 200) × 100 = 0.5%

The Clinical and Laboratory Standards Institute (CLSI) recommends that the overall blood culture contamination rate should not exceed 3%. However, many facilities strive for rates below 1% by adhering to best practices in blood culture collection.