A blood culture is a medical laboratory test used to detect bacteria or fungi in a person's blood. Under normal conditions, the blood does not contain microorganisms: their presence can indicate a bloodstream infection such as bacteremia or fungemia, which in severe cases may result in sepsis. By culturing the blood, microbes can be identified and tested for resistance to antimicrobial drugs, which allows clinicians to provide an effective treatment.
To perform the test, blood is drawn into bottles containing a liquid formula that enhances microbial growth, called a culture medium. Usually, two containers are collected during one draw, one of which is designed for aerobic organisms that require oxygen, and one of which is for anaerobic organisms, that do not. These two containers are referred to as a set of blood cultures. Two sets of blood cultures are sometimes collected from two different blood draw sites. If an organism only appears in one of the two sets, it is more likely to represent contamination with skin flora than a true bloodstream infection. False negative results can occur if the sample is collected after the person has received antimicrobial drugs or if the bottles are not filled with the recommended amount of blood. Some organisms do not grow well in blood cultures and require special techniques for detection.
The containers are placed in an incubator for several days to allow the organisms to multiply. If microbial growth is detected, a Gram stain is conducted from the culture bottle to confirm that organisms are present and provide preliminary information about their identity. The blood is then subcultured, meaning it is streaked onto an agar plate to isolate microbial colonies for full identification and antimicrobial susceptibility testing. Because it is essential that bloodstream infections are diagnosed and treated quickly, rapid testing methods have been developed using technologies like polymerase chain reaction and MALDI-TOF MS.
Procedures for culturing the blood were published as early as the mid-19th century, but these techniques were labour-intensive and bore little resemblance to contemporary methods. Detection of microbial growth involved visual examination of the culture bottles until automated blood culture systems, which monitor gases produced by microbial metabolism, were introduced in the 1970s. In developed countries, manual blood culture methods have largely been made obsolete by automated systems.
Medical uses Blood is normally sterile.Carroll, KC et al. (2015). p. 755. The presence of bacteria in the blood is termed bacteremia, and the presence of fungi is called fungemia.Turgeon, ML (2016). p. 510. Minor damage to the skin or mucous membranes, which can occur in situations like toothbrushing or defecation,Procop, GW & Koneman, EW (2017). p. 188.Pitt, SJ (2018). p. 26. can introduce bacteria into the bloodstream, but this bacteremia is normally transient and is rarely detected in cultures because the immune system and reticuloendothelial system quickly sequester and destroy the organisms.Mahon, CR et al. (2018). p. 866.Carroll, KC et al. (2015) pp. 755–6. Bacteria can enter the blood from infections such as cellulitis, UTIs and pneumonia;Mahon, CR et al. (2018). p. 867. and infections within the vascular system, such as bacterial endocarditis or infections associated with intravenous lines, may result in a constant bacteremia. Fungemia occurs most commonly in people with poorly functioning immune systems. If bacteria or fungi are not cleared from the bloodstream, they can spread to other organs and tissues, or evoke an immune response that leads to a systemic inflammatory condition called sepsis, which can be life-threatening.Bennett, JE et al. (2019). p. 990.
When sepsis is suspected, it is necessary to draw blood cultures to identify the causative agent and provide targeted antimicrobial therapy. People who are hospitalized and have a fever, a low body temperature, a high white blood cell count or a low count of granulocytes (a category of white blood cells) commonly have cultures drawn to detect a possible bloodstream infection. Blood cultures are used to detect bloodstream infections in febrile neutropenia, a common complication of chemotherapy in which fever occurs alongside a severely low count of neutrophils (white blood cells that defend against bacterial and fungal pathogens).Walls, R et al. (2017). p. 1497. Bacteremia is common in some types of infections, such as meningitis, septic arthritis and epidural abscesses, so blood cultures are indicated in these conditions. In infections less strongly associated with bacteremia, blood culture may still be indicated if the individual is at high risk of acquiring an intravascular infection or if cultures cannot be promptly obtained from the main site of infection (for example, a urine culture in pyelonephritis or a sputum culture in severe community-acquired pneumonia). Blood culture can identify an underlying microbial cause in cases of endocarditis and fever of unknown origin.
The pathogens most frequently identified in blood cultures include Staphylococcus aureus, Escherichia coli and other members of the family Enterobacteriaceae, Enterococcus species, Pseudomonas aeruginosa and Candida albicans.Ford, M. (2019). p. 95.McMullen, AR, Wilen, CB, & Burnham, CAD. Chapter 9 in Dunne, WM & Burnham, CAD eds. (2018). sec. "Bacteria". Coagulase-negative staphylococci (CNS) are also commonly encountered, although it is often unclear whether these organisms, which constitute part of the normal skin flora,Mahon, CR et al. (2018). p. 863. are true pathogens or merely contaminants. In blood cultures taken from newborn babies and children, CNS can indicate significant infections. The epidemiology of bloodstream infections varies with time and place; for instance, Gram-positive organisms overtook Gram-negative organisms as the predominant cause of bacteremia in the United States during the 1980s and 1990s,Mahon, CR et al. (2018). pp. 865–6. and rates of fungemia have greatly increased in association with a growing population of people receiving immunosuppressive treatments such as chemotherapy.McMullen, AR, Wilen, CB, & Burnham, CAD. Chapter 9 in Dunne, WM & Burnham, CAD eds. (2018). sec. "Fungal Bloodstream Infections". Gram-negative sepsis is more common in Central and South America, Eastern Europe, and Asia than in North America and Western Europe; and in Africa, Salmonella enterica is a leading cause of bacteremia.Bennett, JE et al. (2019). p. 996.
Procedure
Collection Blood cultures are typically drawn through venipuncture. Collecting the sample from an intravenous line is not recommended, as this is associated with higher contamination rates, although cultures may be collected from both venipuncture and an intravenous line to diagnose catheter-associated infections. Prior to the blood draw, the top of each collection bottle is disinfected using an alcohol swab to prevent contamination. The skin around the puncture site is then cleaned and left to dry; some protocols recommend disinfection with an alcohol-based antiseptic followed by either chlorhexidine or an iodine-based preparation, while others consider using only an alcohol-containing antiseptic to be sufficient. If blood must be drawn for other tests at the same time as a blood culture, the culture bottles are drawn first to minimize the risk of contamination.Pagana, KD et al. (2014). p. xiii. Because antimicrobial therapy can cause false negative results by inhibiting the growth of microbes, it is recommended that blood cultures are drawn before antimicrobial drugs are given, although this may be impractical in people who are critically ill.
A typical blood culture collection involves drawing blood into two bottles, which together form one "culture" or "set". One bottle is designed to enhance the growth of aerobic organisms, and the other is designed to grow anaerobic organisms. In children, infection with anaerobic bacteria is uncommon, so a single aerobic bottle may be collected to minimize the amount of blood required.Pitt, SJ (2018) p. 34. It is recommended that at least two sets are collected from two separate venipuncture locations. This helps to distinguish infection from contamination, as contaminants are less likely to appear in more than one set than true pathogens. Additionally, the collection of larger volumes of blood increases the likelihood that microorganisms will be detected if present.Mahon, CR et al. (2018). p. 870.
Blood culture bottles contain a growth medium, which encourages microorganisms to multiply, and an anticoagulant that prevents blood from clotting. Sodium polyanethol sulfonate (SPS) is the most commonly used anticoagulantAtkinson-Dunn, R. & Dunne, WM. Chapter 2 in Dunne, WM & Burnham, CAD eds. (2018). sec. "Introduction". because it does not interfere with the growth of most organisms. The exact composition of the growth medium varies, but aerobic bottles use a broth that is enriched with nutrients, such as brain-heart infusion or trypticase soy broth,Procop, GW & Koneman, EW (2017). p. 194. and anaerobic bottles typically contain a reducing agent such as thioglycollate. The empty space in an anaerobic bottle is filled with a gas mixture that does not contain oxygen.Ford, M (2019). p. 85.
Many commercially manufactured bottles contain a resin that absorbs antibiotics to reduce their action on the microorganisms in the sample. Bottles intended for paediatric use are designed to accommodate lower blood volumes and have additives that enhance the growth of pathogens more commonly found in children. Other specialized bottles may be used to detect fungi and mycobacteria. In low and middle income countries, pre-formulated culture bottles can be prohibitively expensive, and it may be necessary to prepare the bottles manually. It can be difficult to access the proper supplies and facilities, and in some regions, it may not be possible to perform blood cultures at all.Dondorp, AM et al. (2019). pp. 172–3.
It is important that the bottles are neither underfilled nor overfilled: underfilling can lead to false negative results as fewer organisms are present in the sample, while overfilling can inhibit microbial growth because the ratio of growth medium to blood is comparatively lower. A 1:10 to 1:5 ratio of blood to culture medium is suggested to optimize microbial growth.Tibbetts, RJ & Robinson-Dunn, B. Chapter 10 in Dunne, WM & Burnham, CAD eds. (2018). sec. "Introduction". For routine blood cultures in adults, the Clinical and Laboratory Standards Institute (CLSI) recommends the collection of two sets of bottles from two different draws, with 20–30 mL of blood drawn in each set. In children, the amount of blood to be drawn is often based on the child's age or weight.Revell, P & Doern, C. Chapter 8 in Dunne, WM & Burnham, CAD eds. (2018). sec. "Specimen Collection". If endocarditis is suspected, a total of six bottles may be collected.Bennett, JE et al. (2019). p. 202.
Culturing