A blood culture is a laboratory test that helps determine if bacteria are present in the blood.

How is a blood culture drawn?

A sample of blood is obtained by a blood draw in a lab, clinic or hospital.  This blood draw can be done by a venous blood sample (the most common method), or by an arterial blood sample.  Less than 5 milliliters of blood (one teaspoon) is typically required in children, although the more blood that is used, the more likely the culture will identify bacteria.  A blood culture can be performed with a smaller amount of blood in infants if necessary.

How are medical tests interpreted?

Interpreting medical tests is an art and a science.  Doctors, laboratories and lab machines are not perfect.  Doctors must look at the "big" picture... which means they must look at the patient and perform a physical exam.  There is normal variation of all medical tests and a "normal" value in an adult may be "abnormal" in a child.  By looking at all available information and examining the patient, a doctor can decide the true significance of a lab value.

How is a blood culture performed?

A blood sample is placed in a bottle with a liquid rich in nutrients for bacteria.  Often 2 bottles are used, one with oxygen and one without.  This is done to detect bacteria that thrive in both environments with or without oxygen.  This liquid is placed in an "incubator" that heats the culture bottles to improve bacterial growth.  Samples of this fluid are placed on a petri dish on a regular basis.  If bacteria is present, visible colonies will grow on the dish.

What are "sensitivities"?

If a blood culture is positive, the bacteria on the culture can be tested to determine which antibiotic type is likely to work the best.  Some bacteria are resistant to certain antibiotics.  If an antibiotic is proven to kill the bacteria in the laboratory, the bacteria is said to be "sensitive" to that antibiotic.

 

Photo 1 - This 2005 photograph shows numbers of Legionella sp. colonies, which had been cultivated on an agar cultured plate, and illuminated using ultraviolet light.  At least 46 Legionella spp., and 70 serogroups have been identified. L. pneumophila, an ubiquitous aquatic bacterial organism, which thrives in warm environments, primarily at temperatures ranging from 32°- 45°C, causes over 90% of Legionnaires' disease (LD) in the United States. LD is the more severe form of legionellosis, and is characterized by pneumonia, commencing 2 - 10 days after exposure. Pontiac fever represents an acute-onset, manifesting as a flu-like, non-pneumonic illness, and occurs within a few hours, to two days after exposure. CDC James Gathany 2005. Used with permission.

Photo 2 - This Centers for Disease Control laboratorian was photographed while isolating Salmonella, Shigella or E. coli from a fecal specimen on an agar medium. The diagnosis of a foodborne illness is important in the control of outbreaks. While the food supply in the United States is one of the safest in the world, the CDC estimates that 76 million people get sick, more than 300,000 are hospitalized, and 5,000 Americans die each year from foodborne illness. CDC/ Dr. Kokko 1963. Used with permission.

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