One of the most important microbiological investigations we undertake is the blood culture. This is because blood does not normally have bacteria swimming around in it and therefore unlike from non-sterile sites, we can view any organism from a blood culture as potentially serious.
That said, as with any test we undertake, the opportunity for a sample to become contaminated is ever present. This is just as true for blood culture because the sample is taken from a vein and access to the vein is via a needle through the skin. Skin is not sterile but is home to many species of harmless bacteria, which we refer to as “normal skin flora”
Technology improvements over the years has progressed in the blood culture field and most labs use an automated incubator such as that in the photograph. Each individual bottle is logged and independently monitored by the attached computer. Whenever the machine detects a positive blood culture, an alarm sounds and the lights on the front of the machine change colour to alert the biomedical scientists of the new positive. The scientist will take the bottle off the machine, remove some of the liquid culture, place and dry on a microscope slide and perform a Gram stain. The results of the Gram stain are reported to the on-service consultant, who will make contact with the ward and/or clinical team to advise of the positive result.
The fluid from the bottle is also inoculated onto agar plates of different media and incubated overnight to further the identification and give an initial indication of the antibiotic sensitivity pattern. With blood cultures, we will often issue a series of laboratory results that build on what has gone the day before. The consultants work with the clinical teams to arrive at the significance of the result and advise on suitable antibiotic treatment. Positive blood cultures have been reported to a national scheme in Wales for many years to keep an eye on what organisms are causing blood stream infection both locally and nationally. These are reported on an annual basis and information for the Hywel Dda Health Board are available here. A more detailed explanation is available via our results pages.