Blood culture

A blood culture is a vital medical laboratory test used to detect microorganisms in a person's blood. Under normal conditions, human blood is sterile, meaning it contains no living things. If bacteria enter the blood, the condition is called bacteremia. If fungi enter the blood, it is called fungemia. These infections can be very serious because germs can spread to other organs or tissues. They may also trigger a massive immune response known as sepsis, which is a life-threatening systemic inflammatory condition.

By performing a culture, clinicians can identify the specific germ and test it for resistance to antimicrobial drugs. This allows doctors to provide targeted and effective treatment.

The process begins with a blood draw, usually performed through venipuncture. To prevent contamination from skin flora, the skin around the puncture site is cleaned with antiseptics like alcohol, chlorhexidine, or iodine. The blood is drawn into specialized bottles containing a culture medium. This medium is a liquid formula designed to enhance microbial growth. Most bottles also contain an anticoagulant, such as sodium polyanethol sulfonate (SPS), to prevent the blood from clotting. SPS is commonly used because it does not interfere with the growth of most organisms.

Blood culture sets are organized by the oxygen needs of the microbes. One bottle is designed for aerobic organisms, which require oxygen to grow. Another bottle is for anaerobic organisms, which do not need oxygen. In an anaerobic bottle, the empty space is filled with a gas mixture that lacks oxygen. These two bottles together constitute one "set." For adults, the Clinical and Laboratory Standards Institute (CLSI) recommends collecting two sets from two different venipuncture locations. This helps doctors distinguish a true infection from contamination. If a germ only appears in one set, it likely represents skin bacteria that accidentally entered the sample.

Once collected, the bottles are placed in an incubator at body temperature. This warmth encourages any present microorganisms to multiply. Most common bloodstream pathogens are detected within 48 hours, though bottles are often kept for up to five days. In manual systems, scientists look for visible signs of growth. These signs might include a film called a pellicle on the surface, bubbles from gas production, or turbidity, which is a cloudiness in the liquid.

If growth is detected, a Gram stain is conducted to provide preliminary information about the identity of the organisms.

After the initial detection, the lab performs a subculture. This involves streaking the blood onto an agar plate to isolate individual microbial colonies. These colonies allow for full identification and antimicrobial susceptibility testing. Because speed is essential for treating infections, modern science has developed rapid testing methods. These include polymerase chain reaction (PCR) and MALDI-TOF MS, a technology used to identify microbes quickly.

These tools help clinicians act much faster than older methods allowed.

The history of blood culturing shows how much technology has changed. Procedures for culturing blood were published as early as the mid-19th century. However, these early techniques were extremely labor-intensive. For a long time, detection required constant visual examination of the bottles. This changed in the 1970s when automated blood culture systems were introduced. These machines monitor the gases produced by microbial metabolism. In many developed countries, these automated systems have made manual methods largely obsolete.

Blood cultures are used in many different medical scenarios. They are often drawn for patients with a fever, a high white blood cell count, or a low count of granulocytes. They are also essential for detecting infections in people with febrile neutropenia, a complication of chemotherapy. Certain conditions, such as meningitis, septic arthritis, and endocarditis, are also strongly associated with the need for blood cultures. The pathogens found can vary by location. For example, in Africa, Salmonella enterica is a leading cause of bacteremia. In the United States, Gram-positive organisms became the predominant cause of bacteremia during the 1980s and 1990s.