Blood Culture

Blood cultures are used to detect the presence of bacteria or yeasts in the blood which may have spread from another site in the body, to identify the microorganisms present and to guide treatment. Cultures are incubated in a warm environment for several days to allow any microorganisms to multiply. In most laboratories automated instruments continuously monitor the samples for growth, allowing those with bacteria or yeasts in them to be detected more rapidly.

When a blood culture is positive, the specific microorganism causing the infection is identified and antibiotic susceptibility testing is performed to tell the doctor which antibiotics are most likely to be effective for treatment.

A doctor or healthcare professional may request blood cultures if a person has symptoms of sepsis. Someone with sepsis may have:

• Chills, fever
• Nausea
• Tiredness
• Confusion
• Rapid breathing
• Rapid heartbeat
• Decreased urine output

There is a higher risk of these symptoms following a recent infection, a surgical procedure, an artificial heart valve replacement or immunosuppressive therapy. Blood cultures are taken more frequently in newborns and young children who may have an infection but may not have the typical signs and symptoms of sepsis.

More severe infections may involve inflammation throughout the body and the formation of many tiny blood clots in the smallest blood vessels, giving rise to additional symptoms. One or more organs may be damaged and there may be a dangerous drop in blood pressure.

If the blood culture is positive, it may mean that there is a bacterial or fungal infection in the bloodstream that needs to be treated immediately. Sepsis can be life- threatening, especially in patients whose immune system is not working properly. The doctor may start treatment with a broad-spectrum antibiotic, often given intravenously, while waiting for the test results and will adjust the treatment depending on the antibiotic susceptibility results.

A positive result could also be a false positive caused by skin contamination. If two or more blood culture sets are positive with the same bacteria, it is more likely that the bacteria found in the culture are causing the infection. If one set is positive and one set is negative, it could be either an infection or contamination. The doctor will need to evaluate the clinical state of the patient and the type of bacteria found.

If both blood culture sets are negative, the probability of sepsis caused by bacteria or yeasts is low. However, if symptoms persist, for example a fever that does not go away, additional tests may be required. Reasons that symptoms may not resolve even though blood culture results are negative include:

• Some microorganisms are difficult to grow in culture. Additional blood cultures using special nutrient media may be done to try to grow and identify the pathogen
• Viruses cannot be detected using blood culture bottles designed to grow bacteria. If the doctor suspects that a viral infection may be the cause of the person’s symptoms then other laboratory tests would need to be performed. The tests would depend on the clinical signs and the type of virus the doctor suspects is causing the infection.

Results from other tests that may be done in conjunction with blood cultures can indicate sepsis even though blood cultures are negative. These include:

• Full Blood Count: An increased white blood cell (WBC) count may indicate infection
• Complement: Levels of C3 may be increased
• A urine, sputum, CSF or wound culture may be positive, indicating a possible source of infection that may have spread to the blood
• C-reactive protein and procalcitonin concentrations can increase in response to infection

Symptoms of sepsis such as fever, chills, muscle pains, and exhaustion may also be seen with influenza (the flu). If you are ill during the flu season, your doctor may do an influenza test to rule out this viral respiratory infection. Both the flu and sepsis can be especially serious in the very young, elderly, and patients with poor immune systems (immunocompromised). It is important to tell the difference between bacteraemia/fungaemia and viraemia because while they both need to be treated promptly, the treatments are different (antibacterial or antifungal versus antiviral).

It is important to remove all of the bacteria that are causing the problem. For some infections several weeks of treatment are necessary. This is especially important if you have endocarditis, which requires long-term therapy to cure.