What are bloodstream infections?

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A bloodstream infection involves the presence of large numbers of infectious microorganisms in circulating blood. The causative infectious agents are most often bacteria (leading to bacteremia) but may also be viruses, fungi, or parasites. Bacteria and other microorganisms multiplying in the bloodstream release harmful toxins, creating conditions sometimes referred to as blood poisoning. Infectious organisms in circulating blood may spread infection throughout the body (leading to sepsis), but not all bacteremia develops into widespread infection.
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Definition

A bloodstream infection involves the presence of large numbers of infectious microorganisms in circulating blood. The causative infectious agents are most often bacteria (leading to bacteremia) but may also be viruses, fungi, or parasites. Bacteria and other microorganisms multiplying in the bloodstream release harmful toxins, creating conditions sometimes referred to as blood poisoning. Infectious organisms in circulating blood may spread infection throughout the body (leading to sepsis), but not all bacteremia develops into widespread infection.

Initially, blood flow near an existing infected area may carry infective organisms into the bloodstream. Capillaries, the smallest blood vessels in the vascular system, may then transport microorganisms into organ tissue, sometimes affecting the function of body organs such as the heart, lungs, and kidneys (leading to systemic infection). Other biological factors can accelerate this process, including immune mediators, special infection-activated white blood cells, pro-inflammatory mediators, and coagulation factors.

Rapidly spreading bloodstream infection may overwhelm the body’s immune system, resulting in a systemic inflammatory response (sepsis) with abnormal blood coagulation and interruption of oxygen delivery to body tissues. Sepsis is a serious complication of bloodstream infection that can progress to organ failure, severe hypotension (septic shock), and death. The seriousness and progression of bloodstream infection depends on a person’s underlying health status and immune response.

Causes

Bloodstream infection may begin with bacteria migrating from an existing infection site, such as an open wound, a puncture wound, a burn, an ulcer, an infected prosthetic device, or a urinary tract infection. Other causes of bloodstream infection include infection of the skin (such as soft tissue infection or cellulitis), heart (endocarditis), lungs (such as pneumonia or a lung abscess), or kidneys (such as pyelonephritis). Even a tooth abscess or dental work may cause oral infection that enters the bloodstream.

Common external sources of infection are indwelling intravascular and urinary catheters that are used frequently in persons in acute and long-term care. Central venous catheters are the most frequent cause of hospital-acquired (nosocomial) bloodstream infection; hemodialysis catheters are also causes.

Causative bacteria that have been identified in bloodstream infection include Pseudomonas aeruginosa, Staphylococcus aureus, pneumococcal and streptococcal organisms, and Acinetobacter, Klebsiella, and Bacteroides species. A definite source of infection is found in only 30 to 50 percent of all cases.

Risk Factors

Surgery, injury, and placement of indwelling catheters present high risk for bloodstream infection. Risk is increased in persons with existing acute or chronic infection, chronic disease (such as diabetes), or chronic alcoholism, and with persons who use IV (intravenous) drugs. People with compromised immune response, including the elderly, young children, persons with human immunodeficiency virus (HIV) infection or other immune disorders, and those being treated with chemotherapy, corticosteroids, or immunosuppressive drugs following transplantation, are at greater risk for bloodstream infection.

Symptoms

Alternating chills and fever with prostration are early symptoms of bloodstream infection. Other symptoms vary depending on the primary infection site and overall health status. Progressive symptoms include mental confusion, changes in breathing and heart rate, and reduced urine output. Increased heart rate or low blood pressure may develop if the condition progresses rapidly toward sepsis.

Screening and Diagnosis

Early confirmation of bloodstream infection is crucial, and finding the infection source is important to treatment. A physical examination and the person’s symptoms and history may help to locate existing infections. Infection derived from an intravenous catheter is suspected in hospitalized patients when no open wounds, urinary tract infection, or other obvious sources exist, especially if the intravenous line has been in place for one week or more. A history of kidney infection, kidney stones, or prostate disease suggests urinary tract infection, as does decreased mental function or confusion in the elderly.

Blood culture is the primary diagnostic measure in confirming bacteremia. The presence of any type of infection, fever of unknown origin, recent surgery, or placement of indwelling catheters, calls for a blood culture. The doctor will order a complete blood count with white-cell differential and will likely order an elevated white-blood-cell count. If routine urinalysis points to urinary tract infection, the doctor will order a urine culture and a urine Gram’s stain.

Chest X rays may be taken to exclude lung tumor, pneumonia, or other lung infection. Diagnostic imaging such as ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI) may help to identify or exclude an infection source, especially intra-abdominal infection. Hospitalized persons who have abnormal vital signs and respiratory or cardiac symptoms will need to be evaluated for evidence of sepsis.

Treatment and Therapy

An aggressive approach is crucial when bloodstream infection is suspected or confirmed. The core treatment for infection is intravenous broad-spectrum antibiotics, administered to eliminate the causative organism and prevent progression to sepsis. Ideally, antibiotic therapy is begun within two hours of a physical examination. Culture results may suggest a change in the type of antibiotics used in treatment.

Removing an infected catheter can be a critical step. Intravenous fluids, including blood transfusion, may be given dependent on urinary output. A respirator or oxygen may be needed to assist with breathing difficulty. Surgery is sometimes needed to drain an abscess or other localized infection. Oral antibiotics will be given for some time after the infection has diminished. Rarely, if kidney infection results in subsequent kidney failure, hemodialysis may be required.

Prevention and Outcomes

Bloodstream infection cannot always be avoided, but seeking treatment early is important if fever or other signs of infection develop, particularly after injury or surgery, or in IV drug users. Hospital infection-control measures help prevent catheter-related bloodstream infection. These measures include the aseptic catheter insertion technique, the use of antibiotic-coated catheters, the use of chlorhexidine in central venous catheter protocols, and a reduction in the number of days catheters remain in place.

Bibliography

Barie, Philip S., and Steven M. Opal. “Infectious Complications Following Surgery and Trauma: Bloodstream Infection.” In Cohen and Powderly Infectious Diseases, edited by Jonathan Cohen, Steven M. Opal, and William G. Powderly. 3d ed. Philadelphia: Mosby/Elsevier, 2010. Addresses the risk of opportunistic infection, especially bloodstream infection, following surgery or injury.

Centers for Disease Control and Prevention. “Prevention and Control of Intravascular Catheter-Related Bloodstream Infections.” Available at http://www.cdc.gov/ncidod/dhap/dpac_iv.html. An online article on preventing bloodstream infections caused by misuse or poor use of intravascular catheters in health care settings.

Girard, T. D., and E. Wesley Ely. “Bacteremia and Sepsis in Older Adults.” Clinical Geriatric Medicine 23, no. 3 (2007): 633-647. Examines the causes, diagnosis, and treatment of bacteremia and the outlook for elderly persons.

Goede, Matthew R., and Craig M. Coopersmith. “Catheter-Related Bloodstream Infection.” Surgical Clinics of North America 89, no. 2 (2009): 463-474. A comprehensive look at catheter-related bloodstream infection and its treatment and prevention.

Que, Yok-Ai, and Philippe Moreillon. “Bloodstream Infections.” In Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, edited by Gerald L. Mandell, John F. Bennett, and Raphael Dolin. 7th ed. New York: Churchill Livingstone/Elsevier, 2010. A discussion of bloodstream infection caused by staphylococcal organisms and of treatment to prevent sepsis as a consequence of bloodstream infection.

Strand, Calvin L., and Jonas A. Shulman. Bloodstream Infections: Laboratory Detection and Clinical Considerations. Chicago: American Society of Clinical Pathologists, 1988. A technical monograph reviewing the concepts and factors important in the selection and development of optimal blood culture systems for the detection of blood infection.

Zucker-Franklin, D., et al. Atlas of Blood Cells: Function and Pathology. 3d ed. Philadelphia: Lea & Febiger, 2003. An excellent pictorial presentation of blood components in different stages of function and disease.

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