Blood culture

Definition

A blood culture is a laboratory test to check for bacteria or other microorganisms in a blood sample. Most cultures check for bacteria.

A culture may be done using a sample of blood, tissue, stool, urine, or other fluid from the body. See also:

Alternative Names

Culture - blood

How the test is performed

A blood sample is needed. Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

It is very important that the blood sample does not become contaminated. The sample is sent to a laboratory, where it is placed in a special dish and watched to see if microorganisms grow. This is called a culture. Most cultures check for bacteria. If bacteria does grow, further tests will be done to identify the specific type.

A gram stain may also be done. A gram stain is a method of identifying microorganisms (bacteria) using a special series of stains (colors). For example, see skin lesion gram stain.

How to prepare for the test

No special preparation is needed for a blood culture. For information on preparing for a blood sample, see venipuncture.

How the test will feel

There is no pain associated with a blood culture. For information on how giving a blood sample feels, see venipuncture.

Why the test is performed

Your doctor may order this test if you have symptoms of a blood infection such as bacteremia or septicemia, a serious, life-threatening infection that gets worse very quickly.

The blood culture will help identify the type of bacteria causing the infection. This helps the doctor determine your best course of treatment.

Normal Values

A normal value means that no bacteria or other microorganisms grew in the laboratory dish.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What abnormal results mean

An abnormal (positive) result usually means that you have bacteria or other microorganisms in your blood. This is a sign of infection.

However, contamination of the blood sample can lead to a false-positive result, which means you do not have a true infection. Your health care provider can help determine the difference.

What the risks are

The blood culture is done in a lab. There are no risks to the patient. For information on risks related to giving a blood sample, see venipuncture.

Special considerations

A bacterial blood infection sometimes comes and goes, so a series of three blood cultures may be done to confirm results.

References

Shapiro NI, Zimmer GD, Barkin AZ. Sepsis syndromes. In: Marx, JA, ed. Rosen's Emergency Medicine: Concepts and Clinical Practice. 6th ed. Philadelphia, Pa: Mosby Elsevier; 2006: chap 136.

Croft AC, Woods GL. Specimen collection and handling for diagnosis of infectious diseases. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: Saunders Elsevier; 2006:chap 63.

Murray PR, Witebsky FG. The clinician and the microbiology laboratory. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 17.

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