The Laboratory Report series is written for those who ...



The Laboratory Report series is written for those who collect patient specimens. By providing this educational background on medical tests, it is intended that fewer errors in specimen collection will be made.

Drawing blood for a valid culture

The presence of bacteria in the blood is called bacteremia and can lead to a serious medical complication called sepsis. A bacteremic patient may be febrile, experience shaking, chills, or a high pulse rate; or have a low blood pressure. Drawing blood for culture is a very important way to identify both the invading bacteria and those antibiotics to which the organism is susceptible or resistant, thereby guiding appropriate therapy. Since bacteria differ in their tolerance to oxygen, blood cultures are usually drawn into a set of two bottles, one with and one without oxygen.

Since bacteria often enter the blood circulation on an intermittent basis or may be present in low numbers, two or three sets of blood cultures within a 24-hour period are usually necessary to ensure that the organism is captured. More than three sets are rarely helpful unless looking for a intravenous line as the source of bacteria. The optimal time to collect blood for culture is just as the patient's temperature begins to rise or as the patient begins to experience chills—these indicate that bacteria releasing pyrogens are likely to be present in the blood. Despite the patient's symptoms, the actual number of bacterial organisms in a milliliter of blood may be low, and therefore a relatively large volume of blood is required for a valid culture. On average, 10-20 mL of blood per set of cultures is needed for adults. Infant cultures must rely on 1-2 mL per set of cultures.

Many bacteria live on the skin and, therefore, the skin must be meticulously cleansed before venipuncture to avoid contamination. This is extremely important because organisms that are perfectly normal on the skin can sometimes gain access to the body and cause serious infection and bacteremia. Phlebotomy requires careful steps to avoid confusion about the accuracy of the blood culture results.

Proper venipuncture involves the following steps:

• Locate a suitable vein before disinfecting the skin.

• Clean the site with alcohol, starting in the center and, in a circular motion, moving outward.

• Clean the site next with an organic iodine solution, using the same circular pattern. (If the patient has an iodine sensitivity, two consecutive 10-second cleanings with alcohol are acceptable.)

• Let the site dry completely. It is during this drying period that the skin organisms are killed.

• Without touching the site, perform the venipuncture (some blood culture collection systems require that its stopper be disinfected with alcohol and allowed to dry before use).

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