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Definition

A blood culture is a lab test designed to detect the presence of bacteria, yeast, or fungi in the bloodstream. A routine blood culture involves injecting a sample of the patient's blood into two bottles of sterile nutrient broth (one for aerobes and one for anaerobes), incubating the bottles at 35°C, and monitoring the bottles for growth over a period of five days. For positive cultures, it also involves identifying any organism that grows and performing antibiotic sensitivity tests to determine which antibiotics will be effective in treating the infection.

Purpose

Physicians normally order this test for patients with symptoms of bacteremia. Symptoms can include fever, chills, mental confusion, anxiety, rapid heartbeat, hyperventilation, blood clotting problems, and shock. These symptoms are especially significant if the patient already has another illness or infection, is hospitalized, or has trouble fighting infections because of a weak immune system. Because bacteremia can be a serious clinical condition that, untreated, can lead to death, a blood culture should be performed as soon as an infection is suspected. Early detection will give the patient the best chance for effective treatment and survival.

Blood cultures are sometimes used to determine the causes of infections in other parts of the body because these infections often spread to the blood. For example, bacterial pneumonia (an infection of the lung) and infectious endocarditis (an infection of the inner layer of the heart, including the heart valves) are known to leak bacteria into the bloodstream. Other sources might be boils, urinary tract infections, and oral bacteria spread during mouth trauma (such as injury or dental treatment).

Precautions

Patients who have bleeding disorders or are taking blood thinners might have trouble with bleeding following a venipuncture. Before having a blood sample drawn, such patients should tell the phlebotomist about their condition.

Description

There are many variables involved in performing a blood culture. Before ordering a blood culture, the physician must make the following decisions based on a knowledge of infections and the patient's clinical condition and medical history.

• type of blood culture that will best target the suspected microorganism
• number of blood cultures to request
• how often the blood cultures should be performed

Some factors influencing these decisions are the patient's symptoms or previous culture results, and whether or not the patient has had recent antibiotic therapy.

Types, numbers, and timing of blood cultures

Several groups of microorganisms can cause blood infections. These groups include bacteria (both aerobes and anaerobes), yeast, fungi, viruses, and mycobacteria. Routine blood culture medium will normally grow both aerobic and anaerobic bacteria, yeast, and most fungi. Viruses, mycobacteria, and certain other fungi require special media or special collection techniques and a longer incubation period. For example, Histoplasma is a fungus that requires a six-week incubation period.

A single set of blood cultures, which consists of two bottles of growth medium (one for aerobes and one for anaerobes) is not recommended. Two to three sets are usually adequate. After a blood infection has been diagnosed, confirmed by culture, and treated, an additional blood culture might be performed to ensure that the infection is gone.

Timing can be an important factor in performing blood cultures. Most blood infections are intermittent bacteremias, which means the microorganisms enter the blood at various times. For such infections, blood drawn randomly might miss the microorganisms. Since the microorganisms enter the blood 30–90 minutes before the person's fever spikes, collecting the culture just after the fever spike offers the best probability of finding the microorganism. The second and third cultures can be collected at the same time, but from different areas of the body. The physician might want to have the collections spaced at 30-minute or one-hour intervals. In continuous bacteremias, such as infective endocarditis, microorganisms are always in the blood, so the timing of culture collection is less important. Blood cultures should always be collected before antibiotic treatment begins, if possible. However, some studies of the effectiveness of automated computer-assisted blood cultures in detecting microorganisms in the blood of newborns show that the newer technology with improved media is faster in detecting positive cultures even when antibiotic therapy had already been started.

Laboratory analysis

Bacteria are the most common microorganisms found in blood infections, so routine blood cultures target bacterial growth, although they also support the growth of many other microorganisms. Laboratory analysis of a bacterial blood culture differs slightly from that of a fungal culture, and significantly from that of a viral culture.

For a routine blood culture, 20 ml of blood is drawn from the patient (see Preparation), put directly into a paired set of blood culture bottles (aerobic and anaerobic), and delivered to the lab immediately for incubation.

For a blood culture to be successful, the laboratory must complete several processes:

• Provide an environment suitable for microbial growth.
• Detect growth when it occurs.
• Identify any microorganisms that grow.
• Test any isolated microorganisms against certain antibiotics to determine which antibiotic will be effective.

The broth in the blood culture bottle is the first step in providing an environment suitable for microbial growth. It contains all the required nutrients. A commonly used medium for blood culture is tryptic soy broth supplemented with amino acids and carbohydrates. The aerobic bottles have ideal conditions for growing aerobes, while the anaerobic bottles have ideal conditions for growing both strict anaerobes and facultative anaerobes. The second step is providing an ideal temperature for growth by placing the bottles in an incubator at body temperature (35°C).

To detect growth when it occurs, the laboratories can monitor the bottles by a daily manual (visual) method. Visual signs of growth include cloudiness or a color change in the broth, gas bubbles, or clumps of bacteria. Many laboratories use one of the newer continuous-monitoring blood culture (CMBC) systems. CMBC systems are considered important technical advances in blood cultures. The instruments automatically monitor the bottles containing the patient's blood for evidence of microorganisms, usually every 10 minutes. A common approach is to measure the production of carbon dioxide in the culture medium which causes a color change that is sensed by the instrument. Many data points are collected daily for each bottle, and input into a computer for analysis. Sophisticated mathematical calculations can determine when microorganisms have grown. When growth is detected, an alarm is triggered to alert the technologist. This, combined with more frequent blood tests, make it possible to detect microbial growth earlier. In addition, all CMBC system instruments have the detection system, incubator, and agitator in one unit.

To identify any microorganisms that grow, the laboratory does a Gram stain and a subculture. If there is no evidence of growth after five days, the laboratory usually performs a Gram stain and subculture before discarding a bottle and reporting a negative result.

For the subculture, a drop of blood is placed on a culture plate and spread over the surface of the plate. The plate is then placed in an incubator at 35°C. If a bacterium is isolated, the laboratory identifies it using biochemical tests and the Gram stain. The bacterium is also tested against many different antibiotics to see which antibiotics can effectively treat the infection. This process is called sensitivity (or susceptibility) testing.

All test results are reported to the physician as soon as possible. An early report, known as a preliminary report, is usually available after one day. This report indicates whether any bacteria have been found yet and, if so, the results of the Gram stain. The next preliminary report might include a description of the bacteria growing on the subculture. The laboratory notifies the physician immediately when an organism is found and as soon as sensitivity tests are complete. Sensitivity tests could be complete before the bacterium is completely identified. The final report, which might not be available for five to seven days, includes a complete identification and a list of the antibiotics to which the bacterium is sensitive.

Preparation

To prevent contamination from the patient's skin, the blood sample must be drawn using strict sterile technique. Before drawing the blood, the phlebotomist should disinfect the skin by swabbing it first with 70% alcohol, then with iodine in a circular motion, starting at the puncture site and moving outward. The iodine should be allowed to dry completely before the blood culture is drawn. During this time, the caps of the blood culture bottles should be removed and the rubber stoppers should be cleansed with 70% alcohol.

Aftercare

After drawing the blood sample, the phlebotomist should use alcohol to remove the iodine from the skin to prevent hypersensitivity. Then, to reduce bruising, pressure should be applied to the puncture site until the bleeding stops. If the patient is taking blood thinners or has a blood disorder that causes bleeding, special care should be taken to ensure that the bleeding has stopped completely before pressure is withdrawn.

Complications

After having blood drawn, the patient might feel dizzy or faint, and might have discomfort or bruising at the puncture site. Warm packs can relieve discomfort.

Results

A negative (normal) blood culture indicates that there are no microorganisms growing in the patient's bloodstream. However, a single negative set of blood cultures does not completely rule out a blood infection. Three sets of negative cultures are needed to rule out bacteremia. False negatives can occur for the following reasons:

• Antibiotic therapy was started before the blood was drawn.
• Time of blood collection was inappropriate.
• Environment was not right for growth.
• Fastidious bacteria did not grow.

A positive blood culture indicates that microorganisms are growing in the patient's bloodstream. Finding the same microorganism in more than one set of bottles helps to rule out the possibility of contamination from poor collection or handling techniques. The physician's skill in interpreting the results is essential in distinguishing a blood culture that is positive because of a true infection from one that is positive because of contamination.

In a true bacteremia, the patient's clinical condition is consistent with a blood infection caused by the microorganism that was isolated. The microorganism usually grows soon after the bottles are incubated, is usually found in more than one set of bottles, and is often the cause of an infection somewhere else in the patient's body.

When a culture is positive because of contamination, the patient's clinical condition usually is not consistent with an infection from the microorganism that was isolated. The microorganism is often one that is commonly found on the skin and that rarely causes infection. It is usually found in only one set of bottles after several days of incubation. Contaminated cultures frequently contain more than one microorganism.

Health care team roles

Several health care professionals work together to ensure a successful blood culture. The physician uses his training and expertise to decide when a blood culture should be ordered. A phlebotomist, or sometimes a nurse, collects the blood, and the clinical laboratory scientist, CLS(NCA)/medical technologist, MT(ASCP) monitors the cultures and performs appropriate tests when the cultures are positive.

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KEY TERMS

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Aerobe—An organism that grows in the presence of oxygen. If an aerobe cannot grow without oxygen, it is called a strict of obligate aerobe.

Anaerobe—An organism that grows in the absence of oxygen. If an anaerobe cannot grow when oxygen is present, it is called a strict or obligate anaerobe. An anaerobe that can also grow in the presence of oxygen is called a facultative anaerobe.

Bacteremia—Bacteria in the blood.

Continuous bacteremia—A kind of bacteremia in which bacteria are always in the blood.

Intermittent bacteremia—A kind of bacteremia in which the bacteria enter the blood at various time intervals.

Phlebotomist—A person who draws blood from a vein.

Venipuncture—The puncture of a vein to withdraw a blood sample.

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Beverly G. Miller MT (ASCP)