What is a blood culture?
A blood culture is a sample of blood, usually venous, from which a microbiology laboratory will try and grow bacteria or yeasts. Arterial blood can be cultured, though patients rarely have arterial blood taken as venous samples are easier to take.
Another important difference between adults and children is how much blood is put in the blood culture bottles. All blood culture bottles will take about 10mls of blood. This is important because in adults there is only up to about 1 colony forming unit (cfu) per ml of blood in bacteraemia. Therefore a 10 ml sample of adult blood will contain less than 10 bacteria. From experience I can tell you that it is impossible to take 10mls of blood from a baby, 1ml is more realistic. So why do blood cultures work for babies? The reason is that young children have a much higher bacteraemic load than adults, usually up to 100cfu/ml and therefore even a sample of only 1ml of blood is likely to have up to 10 times the number of bacteria in it compared to an adult’s 10ml sample.
Why take blood cultures?
Blood cultures are taken to try and grow bacteria from blood in order to help diagnose infections and guide appropriate antibiotic treatment. The identification of bacteria can help find the source of an infection and the antibiotic sensitivities can help guide the most appropriate treatments for the individual patient. If a blood culture is positive the result will include the name of any bacteria present and what antibiotics they are sensitive or resistant to.
So the patient with a positive blood culture has septicaemia, don’t they!?
Many people use the term septicaemia to mean the presence of bacteria in the bloodstream but this is incorrect. The correct term for the presence of bacteria in the bloodstream is bacteraemia; septicaemia is a combination of bacteraemia and sepsis (a clinical definition of infection and evidence of the systemic response to that infection see clinical scenarios sepsis). A patient can be bacteraemic without being septicaemic.
When is the best time to take blood cultures?
There is a myth that blood cultures should only be taken when a patient has a temperature of 38oC or more. I don’t know where this has come from but it doesn’t really make much sense. There are some clear reasons why this is not best practice:
- Hypothermia is as important as fever in diagnosing sepsis, if you only take blood cultures when a patients temperature goes above 38oC then patients who are septic and hypothermic will never have blood cultures taken
- Patients with intravascular infections e.g. infective endocarditis are usually bacteraemic all of the time. These patients often don’t spike a fever and would not get blood cultures taken if you wait for a temperature of 38oC. Blood cultures in these patients will be positive whenever they are taken.
- Elderly patients, especially those over 80 years old, often do not mount a fever in response to infection. I don’t know why this is but it is based on my own observations, please feel free to comment on whether this is your experience as well or not.
- Waiting for a high fever is not the best strategy. Patients are bacteraemic just before their temperature starts to rise; the fever is the body’s immune response to the bacteraemia. The rise in temperature (even by a few degrees) is the body’s way of trying to kill off the bacteria. Most bacteria have a narrow window of temperature in which they grow, as the body temperature reaches the peak of the fever the bacteria are starting to die off. Therefore the best time to take a blood culture is just before or as the temperature starts to rise. Perhaps this is a bit unrealistic, but it may make you think before waiting for a fever to take a blood culture.
Any patient with a suspected blood stream infection should have a blood culture taken, irrespective of their temperature.
How should blood cultures be taken?
Blood cultures should be taken as aseptically as possible, in order to prevent any contaminants from the skin being introduced in to the culture media. Most hospitals in the UK now use 2% Chlorhexidine to sterilise the skin although this is not used for very young children as there are concerns about the effect this chemical has on immature skin. In this case weaker concentrations are often used but their effectiveness is unclear.
Which blood sample first?
When taking blood cultures at the same time as other blood samples, blood cultures should always be taken first. The containers for other blood samples are not necessarily sterile and can contaminate the blood culture set which can give false positives. These are known as “pseudobacteraemias” because it appears that the patient has positive blood cultures when they don’t which can lead to unnecessary antibiotics and investigations.
Blue or Purple first?!
It is wise to take the aerobic bottle (blue) before the anaerobic (purple) as this is the bottle most likely to grow something significant. Therefore if you only manage to take one bottle make sure it is the aerobic bottle; patients move, needles slip, there can be many reasons for failing to get both bottles filled. The bloodstream is aerobic, it carries oxygen, and therefore it is more likely aerobic bacteria will be present as anaerobic bacteria do not like oxygen. Anaerobes are more likely found in deep tissue or pus where there is little oxygen. Taking the aerobic bottle first gives the best opportunity to grow bacteria...bonus if you fill the anaerobic bottle too! Some hospitals no longer take anaerobic bottles due to cost/benefit however missing a Clostridium septicum due to not taking an anaerobic bottle misses a diagnosis of caecal carcinoma.
Where should blood cultures be taken from?
Blood cultures can be taken from any blood vessel. The antecubital fossa is the traditional site, sampling from the cephalic or median cubital veins. There are rare circumstances when you may wish to deliberately sample from a different site, e.g. a lower limb when you are trying to diagnose an infected coarctation of the aorta. In this situation if you sample from an upper limb the culture may be negative as the blood has not passed the site of infection (the coarctation of the aorta), sampling from the lower limb ensures a better chance of getting a positive blood culture.
What about lines?
When trying to diagnose a line infection, it is useful to sample from the line itself. However when you do this there is a risk of over diagnosing line infections due to isolating bacteria that might only be colonising the line (see normal bacteria blog). Only take cultures from a line if you genuinely think the line is the source of infection, or when there is no other way to take a blood culture. Be careful how you interpret results of these cultures. One way of improving the validity of cultures taken through lines is to take a matched blood culture from a peripheral vein. A matched sample should be of equal volume, taken at the same time and incubated in the same way. These “paired” samples can be compared to help distinguish the source of the infection: line or systemic, this is known as “differential time to positivity”, for example:
- Arm/peripheral is NEGATIVE at 5 days but the Line is POSITIVE at 4 hours = the line is infected, as the peripheral is negative
- Arm/peripheral is POSITIVE at 10 hours and the Line is POSITIVE at 9 hours = systemic infection as both are positive within 2 hours of each other
- Arm/peripheral is POSITIVE at 13 hours and the Line is POSITIVE at 10 hours = the line is infected as the peripheral positive >2 hours after the line
Sending these glass blood cultures bottles!
Once taken, label with the patient’s details (name, DOB, time taken, site taken from, note if “paired”), and send them to the microbiology laboratory along with a complete request form including clinical details. Many blood culture bottles are glass, so do not send them in air tube systems - they break! Best practise is to hand-deliver them.
Don’t peel off our barcodes!
The barcode labels need to stay on the bottles as they are an essential part of how the microbiology laboratory processes the sample (it’s how the automated incubator recognises each bottle) removing or sticking them to the request form can make it impossible for the lab to process the sample.
Once in the Lab...
Once blood cultures arrive in the laboratory, what happens next will be in part 2 of this blog (next week). Why is this important? Because it influences when the results are communicated to the doctors looking after the patient and how it helps guide antibiotic management.