Blood culture test explained: what it is, why it is done and what results mean

Normally the blood is sterile, meaning free from microorganisms. Lab Tests Online UK explains that when an infection starts in one part of the body and the immune system cannot keep it localised, it can spread into the bloodstream, causing bacteraemia (bacteria in the blood), fungaemia (fungi in the blood) or viraemia (viruses). A blood culture is the test that looks for the first two.

In the laboratory, the blood samples are placed in a warm environment for several days. Special nutrient media in the bottles encourage any bacteria or fungi to multiply, and automated instruments monitor the bottles continuously so that growth can be detected quickly. If an organism grows, the laboratory identifies it and tests which antibiotics or antifungal medicines will work best.

The test is therefore both a diagnostic and a guide to treatment. It tells the clinician whether there is an organism in the blood, what it is, and which medicines are likely to be effective. Bottles designed for bacteria cannot detect viruses, a point the source makes explicitly.

The main reason is suspected sepsis. The NHS describes sepsis as a serious reaction to an infection that can be life-threatening, that develops very quickly and needs urgent treatment in hospital. Lab Tests Online UK lists signs that prompt a blood culture: fever or chills, feeling sick, tiredness, confusion, rapid breathing, rapid heartbeat and decreased urine output, often developing during another illness such as a urinary tract infection, pneumonia or skin infection.

It also lists people at higher risk of bloodstream infection: those who have recently had surgery, who have an artificial heart valve or joint, who have a weakened immune system or take immunosuppressant medicines, who have an intravenous catheter or surgical drain, and newborns and young children. Blood cultures are also important for diagnosing endocarditis, infection of the heart lining or valves.

Timing matters. Lab Tests Online UK notes that antibiotics given before the cultures are taken can prevent bacteria from growing and give a false negative result, so it follows that samples are best taken before treatment starts where that is possible.

There is no special preparation for you. The person collecting the blood must take particular care over the skin. Lab Tests Online UK says the skin is thoroughly cleaned with an alcohol solution and allowed to dry, and that this reduces the chance of skin bacteria contaminating the sample and giving false results.

About 10 mL of blood is taken from adults, with smaller volumes for babies and children. It is put into two culture bottles, one for bacteria that need oxygen (aerobes) and one for those that do not (anaerobes). Usually two sets are collected from different veins or existing catheters. Sometimes more sets are taken at timed intervals: a third set is recommended if candidaemia is suspected, and if endocarditis is suspected the sample should be collected over 24 hours.

Taking several samples increases the chance of detecting bacteria, especially when they are present in small numbers or released into the bloodstream intermittently, and it helps to distinguish true infection from skin contamination. For you it feels like any other blood test, repeated once or twice.

A positive result means bacteria or fungi were found in the blood, which usually indicates a bloodstream infection that needs immediate treatment. Lab Tests Online UK adds that it can also be a false positive caused by skin contamination, and gives the reasoning clinicians use: if two or more sets are positive with the same bacteria it is likely to be true infection, whereas if one set is positive and one negative it could be infection or contamination. The type of bacteria and your clinical condition are weighed as well.

A negative result in both sets makes bacterial or fungal sepsis less likely, but the source says additional tests may be needed if symptoms persist. Reasons for continuing symptoms despite negative cultures include bacteria that are difficult to grow and need special media, viruses that cannot be detected by culture bottles designed for bacteria, and antibiotics given before the cultures were taken.

Because a result takes days, treatment cannot wait. Lab Tests Online UK says your doctor will likely start broad-spectrum antibiotics immediately, usually given intravenously, monitor you closely, order tests to find the source of infection and adjust treatment according to culture and susceptibility results. Follow-up cultures may be requested to confirm the infection is clearing.

A blood culture is rarely the only test. Lab Tests Online UK lists a full blood count, which checks white blood cell levels, C-reactive protein, which measures inflammation, procalcitonin, which helps identify bacterial infection, and cultures from other sites such as urine, sputum, cerebrospinal fluid or wounds to find the source. A flu test may be added in flu season, and antibiotic susceptibility testing is done on any bacteria that grow.

Treatment of sepsis itself is described by the NHS: it is treated in hospital, and may include antibiotics, which may start as a general antibiotic and then become more specific after the cause of the infection has been confirmed, oxygen to help with breathing, and fluids into a vein. The NHS says recovering can take weeks or months.

If you or someone you care for may have sepsis, the NHS says to call 999 or go to A&E for the symptoms it lists, such as very fast breathing, confusion or slurred speech, blue, pale or blotchy skin, a very high or low temperature or a rash that does not fade when pressed. Do not wait for a blood culture.