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Updated 25 July 2012

Blood culture

Blood culture is a test done to detect micro-organisms which may be spreading infection through the blood. Patients needing this test are usually extremely ill.

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WHAT IS IT?

Blood culture is a test done to detect micro-organisms which may be spreading infection through the blood. Patients needing this test are usually extremely ill.

 

HOW IS IT DONE?

A sample of the patient’s blood is incubated using a special preparation which encourages the growth of any bacteria which may be present. Once enough organisms have been grown, a sample can be examined under the microscope for identification. The bacterium is then exposed to different antibiotics to test which ones it may be sensitive or resistant to.

SPECIAL PRECAUTIONS

This is not an ordinary blood sample, so certain precautions must be observed, especially in the actual sampling procedure.

This blood sample must be taken under sterile conditions. This means that

- The area around the patient must be clean

- The doctor/technician must scrub hands as he does for an operation and must wear a mask and sterile gloves

- Sterile drapes must be placed around the patient's arm, which is swabbed with alcohol or other antiseptic

- All syringes, needles and blood sample bottles must be sterile

- Blood (10-20ml)is drawn from the vein in the standard manner, and transferred to special sterile bottles (containing growth mediums) without the needle being touched

- The bottles are immediately placed in a special container and transferred to the laboratory incubators.

Several different sampling bottles are used, each with special nutrients encouraging the growth of different types of organisms: aerobic, anaerobic, and occasionally fungal. If there is a clinical suspicion of a particular organism, the lab will provide a special culture medium for the test.

Complete results, including antibiotic sensitivity, may take several days. Whilst waiting for results, the patient may be started on an antibiotic which is considered the most likely to work. If test results confirm the choice, treatment is continued. If the organism cultured proves insensitive to the antibiotic chosen, the correct one can then be selected.

WHEN IS THE TEST DONE?

Blood culture should be considered in all patients who are ill with a high fever and rigors, have signs of toxic shock, or whose history puts them at risk of blood-borne infection (for example, patients with abnormal or mechanical heart valves).

Some patients have obvious sources of sepsis, like an open, infected wound, and this may be swabbed to detect the organism in the wound. While it is likely that the same organism is in the blood and thus causing the illness, this is not necessarily so. Especially if the wound is treated but the patient remains ill, blood culture will be done.

RESULTS AND PITFALLS

Inadequate skin and site preparation and faulty technique can permit bacteria on the skin to be transferred along with the blood sample into the special bottles. These bacteria will therefore grow, and give a false result. This is usually recognised by the laboratory, because there are many different skin organisms: if the culture yields a variety of organisms, the result is probably due to contaminants.

If the test yields a "pure growth", that is, a heavy growth of only one type of organism, it is highly likely it is the one causing the patient’s condition. Treating with the appropriate antibiotic then has a good success rate.

A negative result – no growth after four days – does not necessarily mean there is no organism involved, especially if the patient’s condition does not improve. It could be a consequence of a patient being given antibiotics before the blood was sampled. If the number of organisms in the patient’s blood is low, the chances of growing it are also reduced. This can partly be overcome by using more blood per sample, and by repeating the test several times. Culture-negative endocarditis is a known entity, and must be considered when clinical finding are consistent with infective endocarditis.

Follow-up blood cultures may be needed (for instance, in patients with mechanical heart valves who have been treated for endocarditis). For this, three consecutive negative cultures are usually required before the patient is considered cured.

(Dr AG Hall, Health24, January 2008)

 
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