Blood cultures are done to look for organisms in cases of suspected infection in the blood (bacteraemia or septicaemia). The symptoms of a blood stream infection include blood include fever and chills, along with other systemic manifestations. The results help in selecting the appropriate antibiotics for treatment.
How are they done?
The blood is usually drawn under strict aseptic precautions, at the elbow. The local area is cleaned thoroughly with an antiseptic and allowed to dry. A tourniquet (elastic band) is applied around the upper arm, which causes veins below the tourniquet to fill with blood. The blood is collected in an airtight vial called a vacutainer or in a syringe. The tourniquet is removed during the procedure, to restore the circulation. After the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding. Bacteraemia (the presence of bacteria in blood) is usually intermittent, so a series of blood cultures may be done in highly suspect cases. The blood is transferred to a blood culture bottle, which contains a liquid known as broth. The culture bottle is kept at 37(C for some days.
The culture is examined for the presence of bacteria over several days. If organisms are present, further tests are done to identify the organism(s). A gram stain is done for a provisional report so that suitable antibiotics can be started. The final report includes the identity of the organism(s) along with their sensitivity to antibiotics.
What are the normal values?
The blood culture is negative or sterile i.e. there is no growth of bacteria in the culture medium.
What are positive cultures?
It means that bacteria are present in the blood. Common bacterial infections of the blood are due to typhoid or paratyphoid bacteria, bacteria causing pneumonia or those present in the gut. Bacterial endocarditis is a life threatening infection that requires blood cultures for a definitive diagnosis. The results of the blood culture test help in diagnosis and proper treatment of the infection.