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The reliability of rapid influenza diagnostic tests depends largely on the conditions under which they are used, and are entirely based on the experience with seasonal influenza.
• For detection of seasonal
influenza virus infection,
sensitivities of rapid diagnostic
tests are approximately 50-70% when
compared with viral culture or
RT-PCR, and specificities of rapid
diagnostic tests for influenza are
approximately 90-95%. Sensitivity
and specificity of these tests for
detection of the novel H1N1 flu
virus are unknown.
• False-positive (and true-negative)
results are more likely to occur
when influenza is uncommon in the
community, which is generally at the
beginning and end of an outbreak.
• False-negative (and true-positive)
results are more likely to occur
when influenza is common in the
community, which is typically at the
height of an outbreak.
• Test sensitivity may vary
depending on when in the course of
illness the specimen is collected.
Respiratory specimens for testing
should be collected in the first 4-5
days of illness when viral shedding
is greatest.
Given these limitations, the
decision of whether or not to test
patients with rapid influenza
diagnostic tests should be based
upon the patient’s presenting
symptoms, whether or not cases of
novel H1N1 have been confirmed in
the area, and/or the patient’s risk
for severe disease or other
complications.
• How to interpret a positive test
result:
A patient testing positive for
influenza B by rapid diagnostic test
likely has been infected with
seasonal influenza B virus that is
continuing to circulate or is a
false-positive result. Such a
patient is unlikely to have novel
H1N1 virus infection.
There are several possibilities when
a patient tests positive for
influenza A by rapid antigen test:
o The patient might have novel H1N1
virus infection
o The patient might have seasonal
influenza A virus infection or
o The patient might have a false
positive test result.
• Information provided by states and
local health authorities should be
consulted to determine whether
public health authorities are
advising that patients who test
positive on a rapid influenza
antigen test need additional
testing. In areas with many new
confirmed cases of novel H1N1 flu
infection and where community spread
of H1N1 is occurring, patients who
test positive on a rapid influenza
diagnostic test can be treated
empirically with antiviral
medications if clinically indicated.
In areas with no or few confirmed
cases of novel H1N1 flu, a
nasopharyngeal swab/aspirate or
nasal aspirate should be collected
and sent to the state public health
laboratory for RT-PCR to determine
if the patient has H1N1 infection,
seasonal influenza A virus
infection, or a false-positive test
result.
• How to interpret a negative
result:
Novel H1N1 flu virus infection
cannot be excluded when a patient
tests negative for influenza A by
rapid antigen test. If the patient
has an epidemiologic link to a
confirmed case (i.e. had close
contact with a confirmed case), or
has either traveled to or resides in
a community where there are one or
more confirmed novel H1N1 cases,
further testing and treatment should
be based upon clinical suspicion,
severity of illness, and risk for
complications. If there is no
epidemiologic link and the patient
has mild illness, further testing
and treatment are not recommended
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