On This Page: Role of Laboratory Diagnosis
Appropriate treatment of patients with respiratory
illness depends on accurate and timely diagnosis.
Early diagnosis of influenza can reduce the inappropriate
use of antibiotics and provide the option of using
antiviral therapy. However, because certain bacterial
infections can produce symptoms similar to influenza,
bacterial infections should be considered and appropriately
treated, if suspected. In addition, bacterial infections
can occur as a complication of influenza.
Influenza surveillance information and diagnostic
testing can aid clinical judgment and help guide
treatment decisions. The accuracy of clinical diagnosis
of influenza on the basis of symptoms alone is
limited because symptoms from illness caused by
other pathogens can overlap considerably with influenza.
Influenza surveillance by state and local health
departments and CDC can provide information regarding
the presence of influenza viruses in the community.
Surveillance can also identify the predominant
circulating types, subtypes, and strains of influenza.
Diagnostic tests available for influenza include
viral culture, serology, rapid antigen testing,
polymerase chain reaction (PCR) and immunofluorescence.
Sensitivity and specificity of any test for influenza
might vary by the laboratory that performs the
test, the type of test used, and the type of specimen
tested. Among respiratory specimens for viral isolation
or rapid detection, nasopharyngeal specimens are
typically more effective than throat swab specimens.
As with any diagnostic test, results should be
evaluated in the context of other clinical information
available to health-care providers.
Commercial rapid diagnostic tests are available
that can be used by laboratories in outpatient
settings to detect influenza viruses within 30
minutes. These rapid tests differ in the types
of influenza viruses they can detect and whether
they can distinguish between influenza types. Different
tests can detect 1) only influenza A viruses; 2)
both influenza A and B viruses, but not distinguish
between the two types; or 3) both influenza A and
B and distinguish between the two. The types of
specimens acceptable for use (i.e., throat swab,
nasal wash, or nasal swab) also vary by test. The
specificity and, in particular, the sensitivity
of rapid tests are lower than for viral culture
and vary by test. Because of the lower sensitivity
of the rapid tests, physicians should consider
confirming negative tests with viral culture or
other means. Further, when interpreting results
of a rapid influenza test, physicians should consider
the positive and negative predictive values of
the test in the context of the level of influenza
activity in their community. Package inserts and
the laboratory performing the test should be consulted
for more details regarding use of rapid diagnostic
tests. Additional information concerning diagnostic
testing is located at http://www.cdc.gov/flu/professionals/labdiagnosis.htm.
Despite the availability of rapid diagnostic tests,
collecting clinical specimens for viral culture
is critical, because only culture isolates can
provide specific information regarding circulating
influenza subtypes and strains. This information
is needed to compare current circulating influenza
strains with vaccine strains, to guide decisions
regarding influenza treatment and chemoprophylaxis,
and to formulate vaccine for the coming year. Virus
isolates also are needed to monitor the emergence
of antiviral resistance and the emergence of novel
influenza A subtypes that might pose a pandemic
threat.
Laboratory Diagnostic Procedures
for Influenza1
Diagnosis of influenza
Influenza illness can include any or all of these
symptoms: fever, muscle aches, headache, lack of
energy, dry cough, sore throat, and possibly runny
nose. The fever and body aches can last 3-5 days
and the cough and lack of energy may last for 2
or more weeks. Influenza can be difficult to diagnose
based on clinical symptoms alone because the initial
symptoms of influenza can be similar those caused
by other infectious agents including, but not limited
to, Mycoplasma pneumoniae, adenovirus,
respiratory syncytial virus, rhinovirus, parainfluenza
viruses, and Legionella spp.
A number of tests can help in the diagnosis of
influenza (see table). But, tests do not need to be done on all patients.
For individual patients, tests are most useful
when they are likely to give a doctor results that
will help with diagnosis and treatment decisions.
During a respiratory illness outbreak, however,
testing for influenza can be very helpful in determining
if influenza is the cause of the outbreak.
Appropriate samples for influenza testing can include
a nasopharyngeal or throat swab, nasal wash, or
nasal aspirates, depending on which type of test
is used. (See table.) Samples
should be collected within the first 4 days of
illness. Rapid influenza tests provide results
within 24 hours; viral culture provides results
in 3-10 days. Most of the rapid tests that can
be done in a physician's office are approximately >70%
sensitive for detecting influenza and approximately >90%
specific. Thus, as many as 30% of samples that
would be positive for influenza by viral culture
may give a negative rapid test result. And, some
rapid test results may indicate influenza when
a person is not infected with influenza.
Serum samples also can be tested for influenza
antibody to diagnose recent infections. Two samples
should be collected per person: one sample within
the first week of illness and a second sample 2-4
weeks later. If antibody levels increase from the
first to the second sample, influenza infection
likely occurred. Because of the length of time
needed for a diagnosis of influenza by serologic
testing, other diagnostic testing should be used
if a more rapid diagnosis is needed.
During outbreaks of respiratory illness when influenza
is suspected, some samples should be tested by
both rapid tests and by viral culture. The collection
of some samples for viral culture is essential
for determining the influenza subtypes and strains
causing illness, and for surveillance of new strains
that may need to be included in the next year's
influenza vaccine. During outbreaks of influenza-like
illness, viral culture also can help identify other
causes of illness when influenza is not the cause.
Influenza Diagnostic Table
Procedure |
Influenza
Types Detected |
Acceptable Specimens |
Time
for Results |
Rapid
result available |
Viral culture |
A and B |
NP swab 2 , throat
swab, nasal wash, bronchial wash, nasal
aspirate, sputum |
5-10
days 3 |
No |
Immunofluorescence DFA
Antibody Staining |
A and B |
NP swab 2 , nasal
wash, bronchial wash, nasal aspirate,
sputum |
2-4 hours |
No |
RT-PCR 5 |
A and B |
NP swab 2 , throat
swab, nasal wash, bronchial wash, nasal
aspirate, sputum |
1-2 days |
No |
Serology |
A and B |
paired acute and convalescent
serum samples 6 |
>2 weeks |
No |
Enzyme Immuno Assay
(EIA) |
A and B |
NP swab 2 , throat
swab, nasal wash, bronchial wash |
2 hours |
No |
Rapid Diagnostic
Tests |
Directigen Flu A 7
(Becton-Dickinson) |
A |
NP swab 2 , throat
swab, nasal wash, nasal aspirate |
<30 minutes |
Yes |
Directigen Flu A+B 7,
9
(Becton-Dickinson) |
A and B |
NP swab 2 , throat
swab, nasal wash, nasal aspirate |
<30 minutes |
Yes |
FLU OIA 7
(Thermo Electron) |
A and B 4 |
NP swab 2 , throat
swab, nasal aspirate, sputum |
<30 minutes |
Yes |
FLU OIA A/B 7, 9
(Thermo Electron) |
A and B |
NP swab 2 , throat swab,
nasal aspirate, sputum |
<30 minutes |
Yes |
XPECT Flu A/B 7, 9
(Remel) |
A and B |
Nasal wash, NP swab 2, throat
swab |
<30 minutes |
Yes |
NOW Flu A Test 7,
9
NOW Flu B Test 7, 9
(Binax) |
A
B |
Nasal wash, NP swab 2
Nasal wash, NP swab 2 |
<30 minutes
<30 minutes |
Yes
Yes |
QuickVue Influenza Test8
(Quidel) |
A and B 4 |
NP swab 2 , nasal wash,
nasal aspirate |
<30 minutes |
Yes |
QuickVue Influenza
A+B Test8
(Quidel) |
A and B 9 |
NP swab 2 , nasal
wash,
nasal aspirate |
<30 minutes |
Yes |
SAS Influenza A Test 7,
9
SAS Influenza B Test 7, 9 |
A
B |
NP wash 2 , NP
aspirate 2
NP wash 2 , NP aspirate 2 |
<30 minutes
<30 minutes |
Yes
Yes |
ZstatFlu 8
(ZymeTx)
|
A and B 4 |
throat swab |
<30 minutes |
Yes |
- List may not include all test kits approved
by the U.S. Food and Drug Administration
- NP = nasopharyngeal
- Shell vial culture, if available, may reduce
time for results to 2 days
- Does not distinguish between influenza A and
B virus infections
- RT-PCR = reverse transcriptase polymerase chain
reaction
- A fourfold or greater rise in antibody titer
from the acute- (collected within the 1st week
of illness) to the convalescent-phase (collected
2-4 weeks after the acute sample) sample is
indicative of recent infection.
- Moderately complex test – requires specific
laboratory certification.
- CLIA-waived test. Can be used in any office
setting. Requires a certificate of waiver or
higher laboratory certification
- Distinguishes between influenza A and B virus
infections
Disclaimer: Use of trade names or commercial sources
is for identification only and does not imply endorsement
by the Centers for Disease Control and Prevention
or the Department of Health and Human Services.
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