Blood Culture Health Article

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.