Bacteremia

What Is Bacteremia?

Human blood is supposed to be a sterile environment. Bacteremia is defined as the presence and detection of bacteria in blood. The body has several defense mechanisms against this, mainly barrier and immunologic defenses. There are several ways in which bacteria can enter the bloodstream. Bacteria can compromise the internal mucosal barrier of the body when the integrity of the gastrointestinal tract is compromised due to inflammation, chemotherapy, obstruction, or surgery. In addition to this, bacteremia can also be low grade and transient in settings such as chewing, brushing, or flossing. Bacteria can also enter the bloodstream when external barriers of the body are compromised, such as skin wounds or diabetic foot ulcerations. In addition, bacteremia may be a complication of a local infection such as pneumonia, osteomyelitis, cellulitis, or meningitis. Due to expansion of health care facilities, increased access to health care, and advanced diagnostic and therapeutic techniques, there are an increasing number of patients with health care exposures resulting in patients with long-term central venous catheters for hemodialysis, cardiac devices including pacemakers, prosthetic heart valves, and prosthetic joints. The presence of any of these devices increases the risk of bacteremia.

Bacteremia can overwhelm the body’s immune response resulting in significant adverse outcomes for the patient. Once bacteria gain access to the bloodstream, they may cause foci of infection in organs causing, for example, infective endocarditis, pyelonephritis, abscess formation, or osteomyelitis. Patients with bacteremia may present with sepsis, severe sepsis, or septic shock. Sepsis is a syndrome where the body’s immune system reacts to the circulating bacteria and chemicals released in response, resulting in an elevated body temperature, elevated heart rate, and increased respiratory rate with a probable or confirmed infection. In severe sepsis, there is development of organ failure such as mental status changes, decreased urine output, difficulty breathing, or abdominal pain. Septic shock occurs when there are signs and symptoms of severe sepsis with hypotension that does not respond to intravenous (IV) fluid resuscitation and requires vasopressor support.

Sepsis and septic shock can result in significant mortality and morbidity, along with significant financial implications on the health care system. Prompt recognition, diagnosis, and treatment with antibiotics of bacteremia are paramount to limit morbidity and mortality.

In Which Patients Should We Suspect Bacteremia?

A clinician should suspect bacteremia when a patient presents with certain signs and symptoms. These signs and symptoms are due to the intense immune response from circulating bacteria in the bloodstream. Patients can present with a temperature less than 36° C or greater than 38° C, rigors, elevated heart rate of more than 90 beats/min, white blood cell count of less than 4 μg/L or more than 12 μg/L, and elevated respiratory rate of more than 20 breaths/min. These signs and symptoms collectively are recognized as systemic inflammatory response syndrome (SIRS) ( Table 19.1 ).

If prompt identification and treatment are not made, this can progress to hemodynamic instability with a dangerous fall in a patient’s blood pressure leading to shock. Scoring systems have been proposed to prompt physicians to have a low threshold to suspect infection and bacteremia, like the Sequential (Sepsis-related) Organ Failure Assessment (SOFA) score, which includes:

• Respiratory rate ≥22 breaths/min

• Altered mentation

• Systolic blood pressure ≤100 mmHg

In summary, every patient who meets these clinical parameters with a suspected source of infection should prompt the physician to suspect an underlying bacteremia as the cause of a patient’s clinical decompensation. In a minority of patients, there will be no fever but rather hypothermia, as mentioned above, or euthermia.

What Is the Likelihood of Bacteremia in Different Clinical Settings?

• 1.

  The likelihood of bacteremia can vary and is based on three important factors: host factors, virulence of the organism, and site of infection. Table 19.2 lists host factors.

  TABLE 19.2

  Host Factors

  Immunocompromising conditions, including HIV/AIDS, systemic lupus erythematosus, and vasculitis

  Immunocompromising medications, including steroids, monoclonal antibodies, and chemotherapy drugs

  Diabetes mellitus

  Kidney disease, with or without hemodialysis dependence

  Chronic indwelling catheters or prosthetic devices

  Solid organ transplantation

  Hematologic transplantation

  Malignancy

• 2.

  Virulent organisms causing infections at alternative sites that include but are not limited to long-term catheters, artificial or damaged joints, and artificial or abnormal heart valves comprise the following:

  • ▪

    Methicillin-resistant Staphylococcus aureus (MRSA)

  • ▪

    Methicillin-sensitive S. aureus (MSSA)

  • ▪

    Pseudomonas aeruginosa

  • ▪

    Enterococcus faecalis

  • ▪

    Beta-hemolytic streptococcus

• 3.

  Source of infection

Large-scale retrospective reviews of patients with community-acquired pneumonia estimate an overall rate of bacteremia between 12% and 16%. It is important to note that the presence of clinical signs and objective data significantly increases the chance that the patient has bacteremia.

In patients who present with a urinary tract infection, studies estimate an overall rate of bacteremia between 10% and 15%. The most common organisms causing bacteremia are summarized in Table 19.3 .

The most common pathogens isolated from cultures in patients with cellulitis are group A beta-hemolytic streptococcus (Streptococcus pyogenes) and non–group A beta-hemolytic streptococci (groups B, C, F, and G). S. aureus is a rare cause of bacteremia due to cellulitis. Streptococcus species–related cellulitis usually presents with diffuse and sudden-onset redness without purulence.

In meningitis, pneumococcal and meningococcal meningitis are the most common causes, with the likelihood of bacteremia at approximately 20%. However, an important caveat to these percentages of bacteremia, regardless of what the studies estimate, is that all studies do mention that if the patient has signs of clinical instability, as documented in the SIRS and SOFA criteria, the chance of bacteremia increases significantly.

How Do We Establish a Diagnosis of Bacteremia?

Blood cultures should be collected from peripheral venous blood at two separate sites. If central venous catheters are present, blood should be draw through a catheter at the same time as peripheral venous blood cultures to determine whether the central catheter is a potential source of infection. Adequate sterile precautions (alcohol-based product to clean skin) ought to be taken while drawing blood cultures. Due to improved diagnostic techniques and laboratory procedures, skin colonizers like Staphylococcus epidermidis and Cutibacterium acnes often result in positive blood cultures. The presence of these organisms in blood cultures is most often due to contamination from incorrect sterile techniques. However, the growth of pathogenic bacteria such as Escherichia coli, Pseudomonas aeruginosa, Klebsiella species, and S. aureus are seldom considered to be contaminants.