Often, when a child with a fever is evaluated in a doctor's office or emergency room, a complete blood count or CBC is ordered as part of the workup. This can be reassuring to parents. After all, a blood test is an objective test, immune to the human errors we might attribute to the "art" of medicine. Think again. In most cases, the CBC is completely unnecessary in the evaluation of fever in children. In many cases it is downright harmful because of the regularity with which it is misinterpreted and leads to the wrong diagnosis and unnecessary treatment.
The history of the use of the CBC is inextricably intertwined with that of Occult Bacteremia, a condition whose mysterious name has nothing to do with the supernatural. “Bacteremia” refers to the presence of bacteria in the blood stream, whereas “occult” just means “hidden”. Occult Bacteremia is the presence of bacteria in the bloodstream in a well-appearing febrile child, usually between the ages of 3 months and 36 months, with no identifiable focus of infection.
The history of the use of the CBC is inextricably intertwined with that of Occult Bacteremia, a condition whose mysterious name has nothing to do with the supernatural. “Bacteremia” refers to the presence of bacteria in the blood stream, whereas “occult” just means “hidden”. Occult Bacteremia is the presence of bacteria in the bloodstream in a well-appearing febrile child, usually between the ages of 3 months and 36 months, with no identifiable focus of infection.
To
demonstrate bacteria in the blood stream one must utilize a blood culture: a
test whose result can be delayed several days as blood must be incubated in a
proper medium before the growth of bacteria can be detected. Understandably, doctors have wanted a quicker
test, as bacteremia can be associated, albeit rarely, with the development of
other serious infections such as meningitis.
In the search for a reliable quick test, the first order of business was
to look at the statistics on the rate of prevalence of Occult Bacteremia.
In
the 1970s, the rate of Occult Bacteremia in febrile children outside the
newborn period but less than three years of age ranged from 3% to 10% based on
several studies. One clear discriminator
in identifying children at particular risk for this condition was the height of
the temperature: children with a
temperature of less than 102.2 degrees Fahrenheit (39 degrees Celsius) were
found to almost never have Occult Bacteremia.
The results were very similar when the white blood cell count was
utilized with a WBC greater than 10,000 being considered abnormal. Using this cutoff however, an abnormal white
count vastly overestimated the likelihood of bacteremia: only 5.8% of children would actually be expected
to have positive blood cultures when the WBC was greater than 10,000. A WBC less than 10,000, on the other hand,
was highly predictive of the absence of bacteremia with an accuracy of
99.2%. This may come as no surprise
after our earlier examples, because this is another case of a disease with a
low prevalence.
What
if we tried to be more discriminating, by using the height of temperature in
conjunction with an elevated WBC? In
fact, let’s try using 15,000 instead of 10,000 as our cut-off for an abnormal
WBC. In the 1970s, a child under 2 years
of age with a temperature greater than 102.2 and a white blood cell count
greater than 15,000 had a 10% chance of having occult bacteremia.
Based
on this data, several recommended approaches to the highly febrile child with
no apparent source of infection were published.
The most influential was a consensus paper published in July of 1993 in
the journal, Pediatrics, the official
journal of the American Academy of Pediatrics.
This paper established guidelines which included the recommendation that
children 3 to 36 months of age with fever greater than 102.2 degrees Fahrenheit
and a WBC greater than 15,000 should have a blood culture and be treated with
antibiotics at least until the results of the blood cultures are available.
It
is important to understand that when recommendations such as these are
formulated they are not a direct consequence of the available data but involve
the making of judgment calls that take into account economic, political,
ethical and socio-cultural factors. The
data available at the time demonstrated that of 10,000 children with a
temperature greater than 102.2 ̊̊, 300 would be expected to have Occult
Bacteremia (using a rate of 3%). Of
these, only about 25 would go on to have an invasive infection. The panel thus concluded that it is
preferable to submit thousands of children to further blood testing and
unnecessary antibiotics than allow 25 from developing a serious infection. I am not disputing the decision; rather, I
want to point out that there is a trade-off.
Other countries, particularly those with fewer resources or at a time
where antibiotic resistance is of graver concern, may have easily reached
different conclusions.
The
net effect was to establish the WBC of 15,000 as an important signpost in the
evaluation of fever in children, but many doctors forgot the details of these
recommendations or completely misunderstood their limited scope. The guidelines applied only to children
between the age of 3 months and 36 months – older children are not at risk for
Occult Bacteremia. Also, they were
specifically geared for children who do not have an otherwise recognizable
source of infection. Many practices
routinely obtain CBCs to “rule out Occult Bacteremia” in situations that are
completely different from those outlined in the consensus paper wrongly
believing that they are adhering to these guidelines.
I
once attended a tall, stocky 14 year old who was dispatched to the hospital by
his Pediatrician; his mother frantic because she had just been told that the
child had “an infection in his blood”.
From the young man’s appearance I was immediately doubtful of the
accuracy of this assessment. It turns
out that the young man had started having an achy throat the night before, and
had started running a fever during the night.
Earlier that morning he had a headache and was having pain with
swallowing his breakfast.
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He
went to his pediatrician who diagnosed a Strep Throat infection based on a
positive rapid strep test. But then the
pediatrician also obtained a CBC which showed a White Blood Cell count of
17,000. He told the mother that the teen
had “Occult Bacteremia, a serious blood infection”, and referred him to the hospital. I carefully explained to the mother that her
son did not actually have a blood infection and, though I would end up angering
the referring pediatrician (who thereafter stopped sending me patients), I
wrote a prescription for Penicillin to treat his throat infection and
immediately sent the family home.
There
were so many foibles in this case that it is hard to know where to start. First off, Occult Bacteremia is a concern for
children under 36 months of life, not teenagers. Even if we invoked the 1993 guidelines in
this setting, this young man would not be suspected of having this
condition. Furthermore, even in 1993,
most children under 36 months of age did not actually have Occult Bacteremia;
only about 3% did. But what of the
elevated WBC of 17,000? Since the CBC is
not part of the routine evaluation of Strep Throat infections, this test is
simply indecipherable. What is the
expected WBC in a child with Strep Pharyngitis?
Who knows! I tried researching
this question but was unable to find any concrete answer. To my knowledge, there never has been a study
on the height of the WBC in children with strep throat infection because it is
an irrelevant test for this condition.
Unfortunately,
I’ve been involved in scores of similar instances where children were diagnosed
with having “an infection in the blood” on the sole basis of the WBC: an impossible assessment. I would therefore
like to take this opportunity to launch an appeal to all pediatricians: please stop telling parents that children
have a blood infection on the basis of an elevated white blood cell count!
In
the 1970s, two bacteria accounted for the majority of Occult Bacteremia:
Streptococcus Pneumonia (65% to 70% of cases) and Haemophilus Influenza B (10%
to 20% of cases). Much has changed since
the 1970s. In 1990 a Haemophilus
Influenza B vaccine was introduced, and in 2000 one for Streptococcus
Pneumoniae became part of the vaccination schedule for all infants. The result has been a steep drop in the
occurrence of this disease in the last twenty years. Whereas in the 1970s studies reported that 3%
to 10% of children with fever had occult bacteremia, more recent studies
indicate rates below 1%. The CBC is even
less reliable as a predictor of occult bacteremia than it was when the 1993
guidelines were issued. The current
consensus is that well appearing children with high fever do not require a CBC
or a blood culture. Many pediatricians,
however, seem to be stuck in the early 1990’s.
There
is one test that is indispensable in the evaluation of high fever in boys under
one year of age and in girls under 36 months of age: the evaluation of a urine
sample for evidence of a Urinary Tract Infection (UTI). Of the “hidden” infections, this is by far
the most common: one that can have serious repercussions if not identified and
treated early. But pediatricians prefer
to get a CBC than a urine sample. In the
office, obtaining a blood sample for a CBC is a simple procedure that requires
little skill: a needle is used to poke a finger and a few drops of blood are
obtained. On the other hand, to get an
adequate, uncontaminated urine sample in a child who is not yet toilet trained
requires introducing a tiny catheter through the urethra and into the
bladder. This is a procedure that
requires some degree of skill, patience and time. Many doctors avoid it altogether by referring
children to the Emergency Room if they believe that a urine specimen is
needed. But they happily poke holes in
children’s fingers.
This
reminds me of the joke of a gentleman who, late one night after having too much
to drink, is seen on all fours under a lamppost feeling his way around the
pavement. A Good Samaritan stops and
asks if he can be of assistance. The
inebriated man explains that he’s lost his keys. The kind stranger at once crouches down and
joins the search. After some ten minutes
of rummaging, the Samaritan asked the other man if he was sure he’d dropped the
keys in that area. “Oh no”, says the
carouser, “I dropped them in that dark alley across the street”.
“Then
why in good heavens are you looking for them here?”
“Well”, the tipsy man said, “the light’s much
better under the lamppost”.
Some
doctors would rather embark in a fruitless search with blood work than looking
in the right place, simply because obtaining the indicated test is
inconvenient. They’d rather avoid the
dark alleys.
