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Occult Bacteremia

By Geoffrey A. Weinberg, MD

Occult (hidden) bacteremia is the presence of bacteria in the bloodstream of a child who has a fever but who looks well and has no obvious source of infection.

  • Most commonly, occult bacteremia is caused by Streptococcus pneumoniae bacteria.

  • Typically, children have no symptoms other than fever.

  • The diagnosis is based on blood tests.

  • Routine childhood vaccination can prevent the infection.

  • Antibiotics can eliminate the infection.

(See also Bacteremia.)

Children younger than 3 years commonly develop fevers. Most of the time, they have other symptoms, such as a cough and runny nose, which help doctors diagnose the cause of the fever. Sometimes, however, children have fever and no other symptoms. That is, they have a fever with no apparent source or cause. In most of these children, the fever is caused by a viral infection that goes away without treatment. In some of these children, a urinary tract infection is present. In a few children who have fever with no obvious cause, the fever is caused by bacteria circulating in their bloodstream (bacteremia). Circulating bacteria are almost never present in older children or adults who have fever and no other symptoms.

In the past, children 3 to 36 months of age who had a temperature higher than or equal to 102.2° F (39° C) and no other symptoms were evaluated by doctors for occult bacteremia. Most occult bacteremia was caused by Streptococcus pneumoniae. A smaller percentage was caused by Haemophilus influenzae type b, and an even smaller percentage was caused by Neisseria meningitidis. In infants and young children, these bacteria circulating in the bloodstream could attack various organs, resulting in serious illnesses, such as pneumonia, meningitis, or sepsis. Because about 5 to 10% of children with occult bacteremia developed these serious illnesses, doctors did blood tests and blood cultures (growing bacteria in a laboratory) to identify the bacteria before such problems developed. If children had an elevated white blood cell count, which indicated a higher risk of bacterial infection, doctors gave the children antibiotics before the blood culture results were available.

Now, routine vaccination of infants with the Haemophilus influenzae type b conjugate vaccine and the Streptococcus pneumoniae conjugate vaccine has nearly eliminated occult bacteremia caused by these bacteria. Occult bacteremia can still occur in children who have received no vaccines or not enough doses and in children who have an impaired immune system. Newer conjugate vaccines against Neisseria meningitidis are being tested for use in young children. The use of these vaccines is expected to essentially eliminate occult bacteremia in children.


The major symptom of occult bacteremia is

  • Fever higher than or equal to 102.2° F (39° C)

Children who have other symptoms, such as cough, shortness of breath, little or no interest in anything (listlessness), or red or blue discoloration of the skin, are not considered to have occult bacteremia. They most likely have a specific bacterial infection.


  • Blood culture

  • Urine culture and urinalysis

  • Sometimes other blood and stool tests and a spinal tap

Because doctors cannot tell with certainty which children who have a fever have occult bacteremia, doctors need to identify any bacteria by doing a blood culture. Because the bacteria are too few or too small to see, doctors send samples of blood to the laboratory so the bacteria can be examined and grown ( cultured) for identification.

Infants or children of any age who have a fever and who appear seriously ill, regardless of whether they have received vaccinations, have samples of blood, urine, and spinal fluid taken for testing. The spinal fluid is removed during a spinal tap (lumbar puncture), which involves withdrawing a sample of spinal fluid with a small needle. All samples are sent to a laboratory for testing to look for signs of bacterial infection. In most cases, the infant or child is admitted to the hospital and given antibiotics. Children who have trouble breathing will likely have a chest x-ray as well.

Other blood tests and stool tests may also be done depending on the age of the child.

Rapid detection tests for viruses may also be done in some children. For these tests, a swab is used to take a sample from the nose or throat. The results usually are available within a few hours.

Infants under 3 months of age

When infants are under 3 months of age and have a fever, doctors cannot always tell just by looking at them whether they have bacteremia. In these infants, doctors usually do laboratory tests (complete blood count, urinalysis, and blood cultures). If the results of the blood and urine tests seem normal, some doctors have the parents or caretakers monitor the infant at home and then return to the doctor's office within 24 hours so the infant can be re-examined and the blood cultures checked. They do not prescribe antibiotics during this time. Other doctors prefer to admit these infants to the hospital and do further tests of the blood, urine, and spinal fluid. Most doctors consider infants less than 30 days old to be at high risk of bacteremia. Infants in this age group are typically admitted to the hospital and tests of the blood, urine, and spinal fluid are done.

Infants and children 3 months to 3 years of age

Infants and children in this age group who have a fever but who have complete routine vaccinations and who appear well are at very low risk of bacteremia. Because of this low risk, doctors may decide to monitor the children without doing blood tests. However, a urinalysis and a urine culture are typically done to look for a urinary tract infection as a cause of the fever. The parents or caretakers are asked to monitor the children's symptoms and follow up with the doctor (by visit or telephone) in 24 to 48 hours. Children who worsen or whose fever does not go away have blood tests and possibly chest x-rays or a spinal tap.


  • Antibiotics

Sometimes, before results of the culture are known, doctors give an antibiotic to children who have a fever and who appear seriously ill and who are at high risk of bacteremia. Usually, doctors give an injectable antibiotic such as ceftriaxone.

Children with positive culture results who do not appear very ill are given an injectable antibiotic or antibiotics by mouth at home. Children who have positive culture results and who show signs of serious illness are admitted to the hospital and given antibiotics by vein.

All children who were being observed at home are re-examined in 24 to 48 hours. Children who still have fever or who have positive blood or urine cultures and have not been given antibiotics already have more cultures done and are hospitalized. They are then evaluated for serious illness and are given antibiotics by vein.

Children 3 months to 3 years of age may be given antipyretic drugs (such as acetaminophen) for discomfort. These drugs may make children feel better by lowering their body temperature.

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