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Occult Bacteremia

by Geoffrey A. Weinberg, MD

Occult bacteremia is the presence of bacteria in the bloodstream of febrile young children who have no apparent foci of infection and look well. Diagnosis is by blood culture and exclusion of focal infection. Treatment is with antibiotics, either in the hospital or as outpatients; select children are treated pending blood culture results.

About 3% (range 2 to 10%) of children aged 1 to 36 mo with a febrile illness (temperature 39° C) and no localizing abnormalities have bacteremia, which is hence considered occult. Of these, about 5 to 10% develop focal bacterial infections (eg, septic arthritis, osteomyelitis, meningitis) or sepsis ( Sepsis and Septic Shock and see Neonatal Sepsis), which could be minimized by early identification and treatment of the bacteremia. The likelihood of progression to serious focal illness depends on the cause: 7 to 25% for Haemophilus influenzae type b (Hib) bacteremia but 4 to 6% for Streptococcus pneumoniae bacteremia. For further discussion of fever in infants and children, see Fever in Infants and Children.

Organisms

In the 1980s, 80% of occult bacteremia cases were caused by S. pneumoniae. The remainder was caused by Hib (10%), Neisseria meningitidis (5%), and others (predominantly Staphylococcus aureus and Salmonella sp). In the US, since the 1990s, routine Hib conjugate vaccination in infancy essentially eliminated Hib bacteremia. More recent routine use of the S. pneumoniae conjugate vaccine in infancy has reduced invasive pneumococcal disease in young children by > 66%, and increased use is expected to essentially eliminate the problem. Some meningococcal conjugate vaccines also have proved effective in this age group, so that in the future occult bacteremia may be largely preventable.

Symptoms and Signs

The major symptom is fever (temperature 38° C). By definition, children with apparent focal disease (eg, cough, dyspnea, and pulmonary crackles suggesting pneumonia; skin erythema suggesting cellulitis or septic arthritis) are excluded. A toxic appearance (eg, limpness and listlessness, lethargy, signs of poor perfusion, cyanosis, marked hypoventilation or hyperventilation) suggests sepsis or septic shock; bacteremia in such children is not classified as occult. However, early sepsis can be difficult to distinguish from occult bacteremia.

Diagnosis

  • Blood culture

Diagnosis requires blood culture. Ideally, two samples are taken (from separate sites, which helps minimize the problem of false positives due to skin contaminants), with results available within 24 h. CBC, urinalysis, and examination of the stool for leukocytes (if diarrhea is present) are done in select patients to identify specific infections and help stratify risk. Lumbar puncture for CSF analysis is done in toxic-appearing infants < 3 mo; some experts do lumbar puncture in all febrile infants < 28 days regardless of their appearance.

Recommendations regarding selection of febrile children for testing and choice of tests vary with age, temperature, and clinical appearance (see see Figure: Evaluation and management of the febrile infant aged 3 mo. and see Figure: Fever in children aged 3 to 36 mo.); the goal is to minimize testing without missing bacteremia. These algorithms are sensitive but relatively nonspecific. Thus, given the relatively low incidence of occult bacteremia in the population of febrile children, the algorithms have high negative predictive value but low positive predictive value (see Testing : Test Characteristics), making them much more effective in identifying children at low risk of infection who can be treated expectantly (bacteremia ruled-out) rather than in identifying children with true bacteremia.

CBC usually shows an elevated WBC count; however, only about 10% of children with WBC counts of > 15,000/μL are bacteremic, so specificity is low. Acute-phase reactants (eg, ESR, C-reactive protein) are used by some clinicians but add little information; however, in combination with elevated procalcitonin levels, acute-phase reactants may be more specific for serious illness. In children < 3 mo, band counts > 1500/μL and either low (< 5000/μL) or high (> 15,000/μL) WBC counts may indicate bacteremia.

Evaluation and management of the febrile infant aged 3 mo.

hpf = high-power field.

Fever in children aged 3 to 36 mo.

hpf = high-power field.

hpf = high-power field.

Treatment

  • Antibiotics (for those with positive cultures and for select patients pending culture results)

Children who receive antibiotics before bacteremia is confirmed by blood culture seem less likely to develop focal infections, although data are inconsistent. However, because of the low overall incidence of bacteremia, many children would receive unnecessary treatment if all who were tested were empirically treated. One common system for management before culture results (see Evaluation and management of the febrile infant aged 3 mo. and Fever in children aged 3 to 36 mo.) minimizes antibiotic use in most febrile infants and children who do not have serious bacterial infection and provides antibiotics promptly to the few who need them. Nevertheless, some authorities prefer to hospitalize all febrile infants < 1 to 2 mo of age and give parenteral antibiotics (eg, ceftriaxone) pending results of blood, urine, and CSF cultures.

All children are reexamined in 24 to 48 h. Those with persistent fever or positive blood or urine cultures have more cultures done and are hospitalized for evaluation of possible sepsis and parenteral antibiotic therapy. Children who are afebrile and well but have S. pneumoniae in their initial blood culture or an initial positive urine culture receive appropriate oral antibiotics (see elsewhere in The Manual ).

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