With the development of controlled clinical evaluations of blood culture systems and media, there is now a sound scientific basis for the basic precepts of blood culture. A variety of factors affect accurate blood culturing: skin antisepsis, number and timing of cultures, correct volume for blood culture, the culture media and any additives, the length and atmosphere of incubation, and the interpretation of positive blood culture results.
Microbiological blood culture is used to locate infections in the blood. Because blood normally runs in a sterile environment, it is possible to analyze it for infection with blood culture analyzer systems. If a patient appears to have symptoms of a systemic infection, the blood culture analysis can verify the presence of the infection and pinpoint the microorganism(s) causing the infection. Infections that can be identified include puerperal fever, pneumonia, neonatal epiglottitis, sepsis, pneumonia, and more. While a blood culture can verify infection, a negative test result does not entirely eliminate the possibility of infections.
Being able to accurately detect bacteremia and fungemia has traditionally been one of the most critical jobs of clinical microbiology laboratories. When infection is detected, it means that host defenses have failed to contain the primary site of the infection or that the infectious process has not been stopped. More recently, clinical investigations and studies have established a much stronger scientific basis for the blood culture test. On the heels of this scientific development, manufacturers of blood culture analyzer systems have taken their products to a new level, most especially in the use of automated systems. One of the most important advances in the design of blood culture analyzing systems is in continuous-monitoring blood culture systems, which allow for ongoing analysis during the course of the infection.
Venipuncture, which is the taking of a blood sample from a vein as opposed to an artery, is the preferred method of obtaining blood for blood culture. Clinical studies have determined that venipuncture results are more reliable, and venous blood cultures have higher diagnostic yields than arterial blood cultures. However, studies that compare contamination rates for blood gathered with intravascular devices as opposed to venipuncture, have conflicting information. Some studies show that contamination rates increase when intravenous catheters are used to obtain blood culture samples. Therefore, if blood is obtained from intravenous lines, it should be compared alongside blood peripheral venipuncture sample results. Inaccurate blood analysis due to sample gathering issues is costly both in terms of hospital bills and, more importantly, lives.
The two-needle technique has often been put forward as a way to gather a pure sample, but in reality this method does not provide a contamination rate that is that much lower than the single-needle technique. Because handling any needle brings with it risk of injury, the use of two needles for reducing contamination rates is only slight and not worth the added possibility of injury. Unless further data contradicts these findings, single needle technique will likely continue to be the standard in obtaining a blood sample.
With advances in blood analyzers, it is recommended that just two blood cultures be taken. Ordering more than two cultures in a 24-hour period is not necessary, unless the doctor wants to confirm continuous bacteremia with an endovascular infection, like endocarditis. As such, it's a good idea to require the approval of the lab director when more than two samples of blood are taken during a 24-hour period.