Blood Culture Essentials

What Is a Blood Culture?  |   Why Are Blood Cultures Important?   |   When Should a Blood Culture Be Performed?   |   What Volume of Blood Should Be Collected?  | How Many Blood Culture Sets Should Be Collected? | Which Media to Use? | Timing of Blood Cultures | How to Collect Blood Cultures | How Many Days of Incubation Are Recommended? | Is It a Contaminant or a True Pathogen? 

Blood cultures should always be requested when a bloodstream infection or sepsis is suspected.¹⁵

The volume of blood that is obtained for each blood culture set is the most significant variable in recovering micro-organisms from patients with bloodstream infections. ^19,20

The optimal volume of blood to be obtained from infants and children is less well prescribed, however, available data indicate that the yield of pathogens also increases in direct proportion to the volume of blood cultured.^{17, 23}  The recommended volume of blood to collect should be based on the weight of the patient (see Table 1)²⁴, and an aerobic bottle should be used, unless an anaerobic infection is suspected.²⁵

Recent studies have shown that facultative anaerobes are detected earlier or solely in the anaerobic blood culture bottle in certain pediatric populations suggesting that they should continue to be used. For newborns, infants, and children, the volume of blood drawn should be no more than 1% of the patient’s total blood volume.¹

Specially formulated blood culture bottles are commercially available for use in children <2 years of age. They are specifically designed to maintain the usual blood-to-broth ratio (1:5 to 1:10) with smaller blood volumes, and have been shown to improve microbial recovery.¹ 

Since bacteria and fungi may not be constantly present in the bloodstream, the sensitivity of a single blood culture set is limited.

Using continuous-monitoring blood culture systems, a study investigated the cumulative sensitivity of blood cultures obtained sequentially over a 24-hour time period. It was observed that the cumulative yield of pathogens from three blood culture sets (2 bottles per set), with a blood volume of 20 mL in each set (10 mL per bottle), was 73.1% with the first set, 89.7% with the first two sets and 98.3% with the first three sets. However, to achieve a detection rate of >99% of bloodstream infections, as many as four blood culture sets may be needed.²⁶

A contaminant will usually be present in only one bottle of a set of blood culture bottles, in contrast to a true bloodstream infection, in which multiple blood culture bottles/sets will be positive. 

If two to three sets are taken and cultures are still negative after 24-48 hours incubation, and the patient is still potentially septic, two to three additional cultures may be collected, as indicated in the following diagram.¹⁷

Microorganisms causing bloodstream infections are highly varied (aerobes, anaerobes, fungi, fastidious microorganisms…) and, in addition to nutrient elements, may require specific growth factors and/or a special atmosphere.

In cases where the patient is receiving antimicrobial therapy, specialized media with antibiotic neutralization capabilities should be used. Antibiotic neutralization media have been shown to increase recovery and provide faster time to detection versus standard media.^27-30

If using a winged blood collection set, then the aerobic bottle should be filled first to prevent transfer of air in the device into the anaerobic bottle.

If using a needle and syringe, inoculate the anaerobic bottle first to avoid entry of air. If the amount of blood drawn is less than the recommended volume*, then approximately 10 mL of blood should be inoculated into the aerobic bottle first, since most cases of bacteremia are caused by aerobic and facultative bacteria. In addition, pathogenic yeasts and strict aerobes (e.g. Pseudomonas) are recovered almost exclusively from aerobic bottles. Any remaining blood should then be inoculated into the anaerobic bottle.¹

Studies have shown that the time interval between collecting two blood cultures is not considered to be a critical factor as the diagnostic yield remains the same.²¹

Guidelines recommend that the first two to three sets (2 bottles/set) of blood culture be obtained either over a brief time period (e.g. within 1 hour) or as a single sample taken at one time.^{1, 17, 21} The possible impact that the blood culture collection method used (e.g. single or multiple venipunctures, winged collection set or needle and syringe) may have on contamination rates should be considered.²¹

Drawing blood at spaced intervals, such as 1 to 2 hours apart, is only recommended to monitor continuous bacteremia/fungemia in patients with suspected infective endocarditis or other endovascular (i.e. catheter- related) infections.¹⁷

Two to three additional blood culture sets can be performed if the first two to three blood cultures are negative after 24-48 hours incubation in cases of severe infection or in order to increase detection sensitivity. This also depends on the microorganisms involved: while sensitivity is relatively good for organisms like Escherichia coli or Staphylococcus aureus, it is lower for Pseudomonas aeruginosa, streptococci or fungi.³²

1. Prior to use, examine the bottles for evidence of damage, deterioration or contamination. Do not use a bottle containing media which exhibits turbidity or excess gas pressure, as these are signs of possible contamination.
2. Check the expiry date printed on each bottle. Discard bottles that have expired.
3. Strictly follow the collection protocol in use in the healthcare setting, including standard precautions for handling blood at the bedside.
4. Blood culture bottles should be clearly and correctly labelled, including patient identification, date and collection time, puncture site (venipuncture or intravascular device). 
5. For adults, each blood culture set should include an aerobic and an anaerobic bottle.
6. For adults and older children, blood should be drawn from veins in the antecubital fossae.^{1, 21}
7. It is recommended to avoid drawing blood from a venous or arterial catheter, since these devices can be colonized with either bacteria or fungi.^{1, 21}
8. Carefully disinfect the skin prior to collection of the sample using an appropriate disinfectant, including chlorhexidine in 70% isopropyl alcohol, tincture of iodine, povidone-iodine in swab or applicator form.^{1, 21}
9. Transport the inoculated bottles at room temperature and the completed blood culture request to the clinical microbiology laboratory as quickly as possible, preferably within 2 hours per CLSI.¹ Delays beyond 2 hours could result in a delay in positive bottle detection or could cause a false negative result. If delays are expected, it is important to refer to the manufacturer’s Instructions For Use (IFU) for guidance.
10. All blood cultures should be documented in the patient’s notes, including date, time, collection site and indications.
    

A study by Bourbeau, et al. (JCM, 2005) demonstrated that 97.4% of clinically significant isolates were recovered within the first 3 days of incubation and 93.8% within 2 days of incubation.³⁵

Another study by Cockerill, et al. (CID, 2004) demonstrated that, when using a continuous-monitoring blood culture system, 99.5% of non-endocarditis bloodstream infections and 100% of endocarditis episodes were detected within 5 days of incubation.²² This data suggests that extended incubation periods previously recommended for detection of the fastidious micro-organisms* that sometimes cause endocarditis, are no longer necessary when using continuous-monitoring blood culture systems.¹⁷

With the exception of Bartonella spp., most rare or fastidious bacteria are still cultivable by traditional bacterial culture systems and can be recovered using blood culture protocols currently in use by most medical laboratories.¹

Contamination of blood cultures during the collection process can produce a significant level of false-positive results, which can have a negative impact on patient outcome.

A false positive is defined as growth of bacteria in the blood culture bottle that were not present in the patient’s bloodstream, and were most likely introduced during sample collection.

Contamination can come from a number of sources: the patient’s skin, the equipment used to take the sample, the hands of the person taking  the blood sample, or the environment.

Certain microorganisms such as coagulase-negative staphylococci, Bacillus spp, Cutibacterium spp., diphtheroids, Micrococcus spp. rarely cause severe bacterial infections or bloodstream infections. These are common skin contaminants, and a though they are capable of causing serious infection in the appropriate setting, their detection in a single blood culture set can reasonably be identified as a possible contaminant without clinical significance. However, it is important to consider that coagulase-negative staphylococci are the primary cause of both catheter- and prosthetic device-associated infections and may be clinically significant in up to 20% of cases.³⁷

The most difficult interpretation problem for the physician is  whether the organism recovered from a blood culture is a true pathogen causing bloodstream infection, or a contaminant. If it is a contaminant, the patient may be treated unnecessarily with antibiotics, leading to additional patient risks. Interpretation of true pathogen versus contaminant should be  based on whether the blood has been collected with a venipuncture or an intra-vascular device, and whether the same organism was recovered from multiple blood culture sets. This illustrates the crucial nature of having collection site information included with the blood culture request sent to the laboratory. 

In contrast to patients with infective endocarditis or other true positive bloodstream infections, patients whose blood cultures grow contaminants usually have only a single blood culture that is positive. This information is of great practical value for physicians, and underlines the importance of taking two to three blood culture sets from different anatomical sites.¹⁷

The American Society for Microbiology and CLSI recommend targeting contamination rates of 1% of the total of collected sets.^1,17