Blood culture
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.
"Blood cultures" redirects here. For the indie pop group, see Blood Cultures (band).Blood culture
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[edit]
Blood is normally sterile.[1] The presence of bacteria in the blood is termed bacteremia, and the presence of fungi is called fungemia.[2] Minor damage to the skin[3] or mucous membranes, which can occur in situations like toothbrushing or defecation,[4][5] 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.[3][6] Bacteria can enter the blood from infections such as cellulitis, UTIs and pneumonia;[7] and infections within the vascular system, such as bacterial endocarditis or infections associated with intravenous lines, may result in a constant bacteremia.[4] Fungemia occurs most commonly in people with poorly functioning immune systems.[2] If bacteria or fungi are not cleared from the bloodstream, they can spread to other organs and tissues,[3] or evoke an immune response that leads to a systemic inflammatory condition called sepsis, which can be life-threatening.[8][9]
When sepsis is suspected, it is necessary to draw blood cultures to identify the causative agent and provide targeted antimicrobial therapy.[10] 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.[11][12] 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).[13][14][15] 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).[16][17] Blood culture can identify an underlying microbial cause in cases of endocarditis[18] and fever of unknown origin.[11][19]
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.[20][21] Coagulase-negative staphylococci (CNS) are also commonly encountered, although it is often unclear whether these organisms, which constitute part of the normal skin flora,[22] are true pathogens or merely contaminants.[21] In blood cultures taken from newborn babies and children, CNS can indicate significant infections.[23] 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,[24] and rates of fungemia have greatly increased in association with a growing population of people receiving immunosuppressive treatments such as chemotherapy.[25] 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.[26]
Limitations[edit]
Blood cultures are subject to both false positive and false negative errors. In automated culture systems, identification of positive bottles is based on the detection of gases produced by cellular metabolism, so samples with high numbers of white blood cells may be reported as positive when no bacteria are present. Inspection of the growth curve produced by the instrument can help to distinguish between true and false positive cultures, but Gram staining and subculturing are still necessary for any sample that is flagged as positive.[61]
Blood cultures can become contaminated with microorganisms from the skin or the environment, which multiply inside the culture bottle, giving the false impression that those organisms are present in the blood.[11] Contamination of blood cultures can lead to unnecessary antibiotic treatment and longer hospital stays.[29] The frequency of contamination can be reduced by following established protocols for blood culture collection, but it cannot be eliminated;[83] for instance, bacteria can survive in deeper layers of the skin even after meticulous disinfection of the blood draw site.[29] The CLSI defines an acceptable contamination rate as no greater than 3% of all blood cultures.[11] The frequency of contamination varies widely between institutions and between different departments in the same hospital;[83] studies have found rates ranging from 0.8 to 12.5 percent.[29]
When faced with a positive blood culture result, clinicians must decide whether the finding represents contamination or genuine infection. Some organisms, such as S. aureus or Streptococcus pneumoniae, are usually considered to be pathogenic when detected in a blood culture, while others are more likely to represent contamination with skin flora; but even common skin organisms such as coagulase-negative staphylococci can cause bloodstream infections under certain conditions. When such organisms are present, interpretation of the culture result involves taking into account the person's clinical condition and whether or not multiple cultures are positive for the same organism.[29]
False negatives may be caused by drawing blood cultures after the person has received antibiotics or collecting an insufficient amount of blood. The volume of blood drawn is considered the most important variable in ensuring that pathogens are detected: the more blood that is collected, the more pathogens are recovered.[11] However, if the amount of blood collected far exceeds the recommended volume, bacterial growth may be inhibited by natural inhibitors present in the blood and an inadequate amount of growth medium in the bottle. Over-filling of blood culture bottles may also contribute to iatrogenic anemia.[28]
Not all pathogens are easily detected by conventional blood culture methods. Particularly fastidious organisms, such as Brucella and Mycobacterium species, may require prolonged incubation times or special culture media. Some organisms are exceedingly difficult to culture or do not grow in culture at all, so serology testing or molecular methods such as PCR are preferred if infection with these organisms is suspected.[45][84]