Blood Culture

You don’t need any special preparation for this test. However, the person collecting your blood must take special care to disinfect your skin properly. This reduces the chance of skin bacteria contaminating the sample and giving false results. 

Blood is collected from a vein in your arm using a needle. Here’s what to expect: 

• Your skin will be thoroughly cleaned with an alcohol solution and allowed to dry 
• About 10 mL of blood will be taken from adults (smaller volumes for babies and children) 
• Your blood will be put into two special culture bottles – one for bacteria that need oxygen (aerobes) and one for bacteria that don’t (anaerobes) 
• Usually two sets are collected from different veins or existing catheters 
• Sometimes additional sets are taken at timed intervals, for example collecting a third set is recommended if candidaemia is suspected and if endocarditis is suspected then sample should be collected over 24hr period. 

Important: Multiple samples increase the chance of detecting bacteria, especially when they’re present in small numbers or released into your bloodstream intermittently. Taking multiple samples also helps distinguish true infection from skin contamination. 

What the test measures 

Blood cultures detect and identify bacteria and fungi in your blood. 

Normally, your blood is sterile (free from microorganisms). Infections typically start at a specific site in your body. If your immune system and white blood cells can’t keep the infection localised, it may spread to your bloodstream, causing: 

• Bacteraemia: bacteria in the blood 
• Fungaemia: fungi in the blood 
• Viraemia: viruses in the blood 

When your body has an excessive response to bloodstream infection, causing organ damage and dysfunction, this is called sepsis. If sepsis also reduces blood flow through your organs, it’s called septic shock – a serious, overwhelming, and often fatal illness. 

In the laboratory, your blood samples are placed in a warm environment for several days. Special nutrient media in the bottles encourage any bacteria or fungi to multiply. Automated instruments continuously monitor the samples, allowing faster detection when microorganisms are present. 

If bacteria or fungi grow, the laboratory will identify the specific microorganism and test which antibiotics or antifungal medications will work best for treatment. 

What your results mean 

If your blood culture is positive: 

A positive result means bacteria or fungi were found in your blood. This usually indicates a bloodstream infection that needs immediate treatment. 

However, a positive result could also be a false positive caused by skin contamination. Your doctor will consider: 

• If two or more sets are positive with the same bacteria: likely a true infection 
• If one set is positive and one is negative: could be infection or contamination 
• Your clinical symptoms and the type of bacteria found 

Your doctor may start you on broad-spectrum antibiotics immediately while waiting for full results. Treatment will be adjusted once the specific bacteria and antibiotic susceptibility results are known. 

Sepsis can be life-threatening, especially if your immune system isn’t working properly. 

If your blood culture is negative: 

If both blood culture sets are negative, the probability of bacterial or fungal sepsis is low. 

However, if your symptoms persist (such as a fever that won’t go away), additional tests may be needed. Reasons for ongoing symptoms despite negative cultures include: 

• Some bacteria are difficult to grow in culture and may require special nutrient media 
• Viruses cannot be detected using blood culture bottles designed for bacteria 
• You may have received antibiotics before the cultures were taken 

Other tests that can help diagnose sepsis even with negative blood cultures: 

• Full blood count (increased white blood cells may indicate infection) 
• Complement levels (C3 may be increased) 
• Cultures from other sites (urine, sputum, CSF, or wounds) 
• C‑reactive protein and procalcitonin (can increase with infection) 
• Influenza test (if symptoms occur during flu season) 

If sepsis is suspected, your doctor will likely: 

• Start broad-spectrum antibiotics immediately (usually given intravenously) 
• Monitor you closely for changes in symptoms 
• Order additional tests to find the source of infection 
• Adjust antibiotics based on culture and susceptibility results 
• May request follow-up blood cultures to confirm the infection is clearing 

For endocarditis or infections with artificial valves or joints, you may need: 

• Several weeks of antibiotic treatment 
• Repeat blood cultures to ensure the infection has cleared 
• Further investigation or surgery in some cases 

Important: Even if you feel better, it’s crucial to complete the full course of antibiotics to ensure all bacteria are eliminated. 

Skin contamination: 

The biggest factor affecting results is contamination from skin bacteria. This is why proper skin disinfection is essential before blood collection, and why multiple samples are taken from different sites. 

Previous antibiotic use: 

If you’ve taken antibiotics before the blood culture, this can prevent bacteria from growing, giving a false negative result. 

Number of samples: 

The more blood sampled, the better the chance of detecting bacteria, especially when they’re present in small numbers. 

Timing: 

Bacteria may be released into your bloodstream intermittently. Multiple samples taken at different times increase the likelihood of detection. 

Blood cultures are often done alongside other tests: 

Full blood count: Checks white blood cell levels and signs of infection 

C‑reactive protein (CRP): Measures inflammation in your body 

Procalcitonin: Helps identify bacterial infections 

Complement tests: C3 levels can increase with infection 

Cultures from other sites: Urine, sputum, CSF, or wound cultures to find infection source 

Antibiotic susceptibility testing: Done on bacteria from positive cultures to guide treatment 

Influenza test: If symptoms occur during flu season, to rule out viral infection 

Sepsis is a life-threatening condition where your body’s response to infection causes organ damage. It’s a medical emergency requiring immediate treatment. 

For more information about sepsis, visit the NHS page on sepsis.