Blood Culture

A blood culture is a laboratory test that checks whether bacteria, fungi, or other microorganisms are growing in a person's blood.

Normally, the bloodstream is sterile — no germs should be living in it. When a serious infection spreads into the blood, this is called bacteraemia (bacteria in the blood) or septicaemia (a bloodstream infection causing widespread illness, sometimes called blood poisoning). During a blood culture, a small amount of blood is drawn and placed into a special bottle containing nutrients that encourage any hidden microorganisms to grow and multiply. Laboratory staff then monitor the bottle, usually for up to five days, to see whether anything grows. If it does, they run further tests to identify exactly which germ is present and which antibiotics are likely to work against it.

A blood culture is ordered when a doctor suspects that an infection has entered the bloodstream. It is one of the most important tools for diagnosing sepsis (a life-threatening response to infection that can damage organs) and for guiding antibiotic treatment.

• Sepsis or suspected sepsis — a severe, whole-body response to infection
• High or persistent unexplained fever, especially when other causes have been ruled out
• Endocarditis (infection of the inner lining or valves of the heart)
• Meningitis (infection of the membranes surrounding the brain and spinal cord) when a blood source is suspected
• Pneumonia (lung infection) that is severe or not responding to treatment
• Osteomyelitis (bone infection) or septic arthritis (joint infection) when a bloodstream source is possible
• Infections related to catheters (thin tubes placed in veins) or other implanted medical devices
• Fever in patients whose immune system is weakened, for example after chemotherapy or in HIV
• Typhoid fever or other infections caused by Salmonella bacteria
• Fungal infections such as candidaemia (Candida fungus in the blood) in high-risk patients

Because a blood culture is a simple blood draw, there are very few situations where it cannot be done. However, the results may be less reliable if the patient has already been given antibiotics before the sample is taken, because the antibiotics can suppress the growth of bacteria in the bottle. Doctors usually try to collect the blood culture before starting antibiotic treatment whenever it is safe to wait.

A blood culture involves only a standard blood draw, so it carries very little risk. The following minor issues are recognised but are uncommon and short-lived.

• Brief pain or stinging at the needle site during or just after the draw
• Small bruise (haematoma) under the skin where the needle was inserted
• Lightheadedness or fainting in people who feel anxious around needles or blood
• Rarely, a small local infection at the puncture site if skin is not properly cleaned beforehand
• A 'contaminated culture' — skin bacteria accidentally introduced into the bottle, which can make the result harder to interpret; this is not harmful to the patient but may mean the test needs to be repeated

Blood cultures are often collected urgently, so there is usually no time for lengthy preparation. However, there are a few important points to be aware of before the sample is taken.

Fasting is not required. The patient can eat and drink normally beforehand. There is no need to pause regular medications for this test alone, but the team will note any antibiotics or antifungal medicines already being taken, because these can affect the result. If possible, the sample should be drawn before antibiotic treatment starts. Smoking and alcohol do not directly interfere with the test, but if the patient is unwell enough to need a blood culture, the medical team will give their own guidance on these.

Before or alongside the blood culture, the doctor will usually request additional tests to build a full picture of the infection. These commonly include:

• Full blood count (FBC) — to check white blood cells, which rise during infection
• C-reactive protein (CRP) — a protein that increases when there is inflammation or infection in the body
• Kidney and liver function tests
• Urine culture — to check whether the infection started in the urinary tract
• Chest X-ray — if lung infection is suspected
• Other cultures (for example, from sputum or wound swabs) depending on where the infection is thought to be

Here is what typically happens during the blood culture procedure itself:

• 1. The healthcare worker identifies the patient and explains the procedure.
• 2. A suitable vein, usually in the arm, is selected.
• 3. The skin over the vein is thoroughly cleaned with an antiseptic (germ-killing) solution and allowed to dry.
• 4. The tops of the culture bottles are also cleaned with antiseptic.
• 5. A needle is inserted into the vein and blood is drawn — usually enough to fill two or more bottles (an aerobic bottle that supports bacteria needing oxygen, and an anaerobic bottle for bacteria that grow without oxygen).
• 6. The needle is removed, and gentle pressure is applied to the puncture site.
• 7. The bottles are labelled and sent to the laboratory promptly.
• 8. In many cases, a second set of bottles is drawn from a different vein, either at the same time or shortly after, to improve the accuracy of the test.

Because a blood culture is only a blood draw, there is no physical recovery needed from the procedure itself. The aftercare focus is entirely on managing the underlying infection while waiting for and acting on the results.

• The puncture site may be slightly sore or bruised for a day or two; pressing a clean cloth on it for a few minutes after the draw usually stops any bleeding.
• If the patient is in hospital, the medical team will monitor for signs of worsening infection — such as rising fever, falling blood pressure, or changes in consciousness — while the culture incubates.
• Preliminary results can sometimes appear within 24 to 48 hours if bacteria grow quickly; full results, including which antibiotics work best (called a sensitivity or susceptibility report), usually take two to five days.
• Antibiotic treatment is often started before results are back, based on the doctor's best clinical judgement; once the culture identifies the specific microorganism, the treatment may be adjusted to target it more precisely.
• If the culture is negative (nothing grows) after the full incubation period, the doctor will reassess and consider other diagnoses.
• Patients treated at home will usually be asked to return or call if their fever persists, worsens, or new symptoms appear before results are available.
• Follow-up blood cultures may be ordered after treatment begins to confirm that the bacteria have been cleared from the bloodstream.
• Long-term lifestyle advice — such as managing any conditions that weaken the immune system — will be given by the infectious disease specialist based on the findings.