When an infection enters your bloodstream, it can quickly become a serious medical emergency. Blood culture tests are essential diagnostic tools that help doctors identify whether bacteria, fungi, or other pathogens have invaded your blood. Understanding how these tests work and what your results mean can help you take appropriate action when faced with a potential bloodstream infection.

This comprehensive guide covers everything you need to know about blood culture testing, from the procedure itself to interpreting your results.

What Is a Blood Culture Test?

A blood culture test is a laboratory procedure used to detect the presence of microorganisms – primarily bacteria and fungi – in your bloodstream. Under normal circumstances, blood is sterile, meaning it should not contain any living pathogens. When microorganisms enter the blood and begin multiplying, this condition is called bacteraemia (bacteria in the blood) or septicaemia (blood poisoning).

The test involves taking a sample of your blood and placing it in special bottles containing a nutrient-rich liquid called culture medium. This medium encourages any microorganisms present to grow and multiply, making them easier to detect and identify.

Why Would You Need a Blood Culture Test?

Your doctor may order a blood culture test if you show signs and symptoms of a bloodstream infection, including:

High fever or chills – Unexplained temperature spikes, especially with shaking chills

Rapid heart rate – Tachycardia that cannot be explained by other causes

Rapid breathing – Increased respiratory rate

Confusion or altered mental state – Particularly in elderly patients

Low blood pressure – Hypotension that may indicate septic shock

Extreme fatigue and weakness – General malaise that worsens rapidly

Nausea, vomiting, or diarrhoea – Gastrointestinal symptoms accompanying fever

Blood cultures are also commonly ordered for patients with existing infections that may spread to the bloodstream, such as urinary tract infections, pneumonia, skin infections, or infections following surgery.

How Is the Blood Culture Test Performed?

The blood culture procedure is relatively straightforward and similar to a standard blood test, though with some important additional steps to prevent contamination:

1. Site preparation:

The phlebotomist will thoroughly clean the skin at the collection site – usually the inside of your elbow or the back of your hand – using an antiseptic solution. This step is crucial because bacteria living on your skin could contaminate the sample and produce a false positive result.

2. Blood collection:

Using a sterile needle, the phlebotomist will draw blood directly into special culture bottles. Typically, two to three sets of blood cultures are collected from different sites or at different times. Each set usually includes two bottles – one designed to grow aerobic organisms (those requiring oxygen) and one for anaerobic organisms (those that grow without oxygen).

3. Multiple samples:

Taking multiple samples increases the likelihood of detecting an infection if one is present. It also helps distinguish between true infections and contamination – if the same organism grows in multiple samples, it is much more likely to be a genuine infection.

The entire procedure takes only a few minutes and causes minimal discomfort. For more information on what to expect during blood collection, read our guide on the procedure and after-effects of blood testing.

What Happens to Your Sample?

Once collected, your blood samples are transported to the laboratory and placed in an incubator. The warm environment (usually around 37°C – body temperature) encourages any microorganisms present to multiply.

Modern laboratories use automated blood culture systems that continuously monitor the bottles for signs of microbial growth. These systems can detect growth much faster than traditional methods, often within 24 to 48 hours for common bacteria.

If growth is detected, laboratory scientists will:

Perform a Gram stain to categorise the organism

Culture the organism on solid media to identify the specific species

Conduct sensitivity testing to determine which antibiotics will be effective

If no growth is detected after five to seven days, the culture is usually reported as negative.

Understanding Your Blood Culture Results

Negative result:

A negative blood culture means no microorganisms were detected in your blood sample. This is generally reassuring, though it does not completely rule out infection – some organisms are difficult to culture, and the timing of sample collection relative to antibiotic treatment can affect results.

Positive result:

A positive blood culture indicates that microorganisms were found growing in your blood. However, not all positive results indicate true infection:

True positive: The organism identified is a known pathogen and is consistent with your clinical presentation. Treatment is required.

Contaminant: The organism identified is typically found on skin (such as coagulase-negative staphylococci) and only grows in one of multiple samples. This usually does not require treatment.

Your doctor will interpret results in the context of your symptoms, medical history, and other test findings.

Common Organisms Found in Blood Cultures

The most frequently identified pathogens in blood cultures include:

Staphylococcus aureus – A common cause of skin infections that can spread to the blood

Escherichia coli (E. coli) – Often associated with urinary tract or abdominal infections

Streptococcus species – Including pneumococcus and group A streptococcus

Klebsiella – Another common cause of hospital-acquired infections

Pseudomonas aeruginosa – Particularly in immunocompromised patients

Candida species – Fungal infections, more common in hospitalised patients

The Importance of Early Detection

Bloodstream infections can progress rapidly to sepsis – a life-threatening condition where the body's response to infection causes widespread inflammation and organ damage. Early detection through blood culture testing allows doctors to:

Start appropriate antibiotic therapy promptly

Identify the source of infection

Adjust treatment based on sensitivity results

Monitor response to therapy

The C-reactive protein (CRP) test is often used alongside blood cultures to assess the level of inflammation in your body and monitor your response to treatment.

Blood Cultures and Other Blood Tests

Blood cultures are often ordered as part of a broader investigation that may include:

Full blood count – To assess white blood cell levels, which typically rise during infection

CRP and ESR – Inflammatory markers that indicate the body's response to infection

Liver function tests – To check for organ involvement

Kidney function tests – As sepsis can affect kidney function

Lactate levels – Elevated lactate may indicate tissue hypoxia in sepsis

How Long Do Results Take?

Preliminary results may be available within 24 to 48 hours if rapid bacterial growth is detected. However, final results, including organism identification and antibiotic sensitivity testing, typically take 48 to 72 hours. Some slow-growing organisms may take up to seven days to detect.

In urgent cases, doctors will often start broad-spectrum antibiotics while awaiting results, then adjust treatment once the specific organism and its sensitivities are known.

Preparing for a Blood Culture Test

Unlike some blood tests that require fasting, blood cultures do not require any special preparation. However, you should:

Inform your healthcare provider about any antibiotics you are currently taking

Mention any recent infections or medical procedures

Let the phlebotomist know if you have difficult veins or have had problems with blood draws before

Blood Culture Testing in London

At Private Blood Tests London, we offer comprehensive blood testing services at our CQC-registered clinic in South Kensington. While blood culture testing is typically performed in hospital settings due to the clinical urgency often associated with suspected bloodstream infections, we provide a wide range of routine and specialist blood tests that can help monitor your overall health and detect early signs of infection.

Explore our [full blood count testing](/full-blood-count-test), [health screening packages](/health-screening), and [all available tests](/all-blood-tests).

If you are experiencing symptoms that concern you, we recommend seeking prompt medical attention. For routine health monitoring and preventive screening, our experienced team is here to help.

Book your blood test appointment today and take a proactive approach to your health.

Everything You Need to Know Relating to a Blood Culture Test