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Influenza
infections cause significant morbidity and mortality in the United
States despite the availability of an effective vaccine. In fact,
influenza-related deaths have increased 83% from the 1976-1977 through
the 1997-1998 influenza seasons.1 Although an estimated
1 in 5 individuals in this country will be infected with influenza
each year, our ability to recognize the disease is still limited.2 One
factor may be the wide spectrum of clinical manifestations influenza
presents. In one study of health care workers with serologic evidence
of influenza infection, 59% of subjects did not report having had
influenza, and 28% did not recall having any upper respiratory tract
infection.3 On the other end of the spectrum, the mortality associated
with this infection, particularly among the very young and elderly,
is substantial.1,4
The clinical diagnosis of
influenza remains an imprecise science. The Monto and Ohmit studies
examined the clinical "predictors" of influenza infection
in adults and children, respectively, finding varying clinical presentations
in different age groups.5,6 Call et al performed a comprehensive
review of studies evaluating clinical findings of influenza.7 Poehling’s
investigation found that the bulk of influenza infections remain undiagnosed
in children.8
The interpretation of these
papers requires knowledge of key epidemiology terms, since they are
used to describe the predictors of influenza. Sensitivity refers
to the percentage of patients with influenza who will correctly be
diagnosed with influenza by the test, or in this instance, clinical
sign or symptom. Specificity is the percentage
of patients without influenza who will be correctly diagnosed as not
having influenza by the clinical finding. The positive
predictive value (PPV) is the probability a patient with
the sign or symptom will have influenza, whereas the negative
predictive value (NPV) is the probability a patient without
the sign or symptom will not have influenza. An important point is
that predictive values are highly dependent on disease prevalence.9 The
Centers for Disease Control and Prevention (CDC) tracks influenza cases
on a weekly basis, and the most current data can be found on their
website.10
Once the suspicion of influenza
infection arises, clinicians must then consider other important factors.
Should influenza testing be performed? Is this patient presenting within
48 hours of symptoms, such that antiviral therapy may be useful? Does
this patient have close contact with particularly vulnerable populations
such as those who are immunocompromised, or nursing home patients who
may require post-exposure prophylaxis? Does this patient have a travel
history that may have included areas where avian influenza has been
reported?
In conclusion, we remain
limited in our ability to diagnose influenza on clinical findings alone.
However, as the studies reviewed herein show, the presence or absence
of certain signs and symptoms in the appropriate populations can influence
clinical decision-making. The presence of both fever and cough
have a high positive predictive value (though low sensitivity) for
influenza in adults. A significant limitation in examining the
clinical diagnosis of influenza is that the inclusion criteria of many
studies include fever and/or acute respiratory illness. Thus, there
may be a proportion of influenza infections that go undiagnosed due
to atypical presentations. In practice, rapid diagnostic testing for
influenza and/or empiric therapy with appropriate antiviral agents
may be the best approach when influenza infection is suspected.
References
| 1. |
Thompson
WW, Shay DK, Weintraub E, et al. Mortality
associated with influenza and respiratory syncytial virus in the
United States. JAMA. 2003;289:179-186. |
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| 2. |
Harper
SA, Fukuda K, Uyeki TM, Cox NJ, Bridges CB. Prevention
and Control of Influenza: Recommendations of the Advisory Committee
on Immunization Practices (ACIP). MMWR. 2004;53(RR06):1-40. |
 |
| 3. |
Elder
AG, O'Donnell B, McCruden EA, Symington IS, Carman WF. Incidence
and recall of influenza in a cohort of Glasgow healthcare workers
during the 1993-4 epidemic: results of serum testing and questionnaire. BMJ. 1996;313:1241-1242. |
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| 4. |
Davis
MM, Taubert K, Benin AL, et al and the American Heart Association;
American College of Cardiology; American Association of Cardiovascular
and Pulmonary Rehabilitation; American Association of Critical
Care Nurses; American Association of Heart Failure Nurses; American
Diabetes Association; Association of Black Cardiologists, Inc;
Heart Failure Society of America; Preventive Cardiovascular Nurses
Association; American Academy of Nurse Practitioners; Centers
for Disease Control and Prevention and the Advisory Committee
on Immunization. Influenza
vaccination as secondary prevention for cardiovascular disease:
a science advisory from the American Heart Association/American
College of Cardiology. J Am Coll Cardiol. 2006;48(7):1498-1502. |
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| 5. |
Monto
AS, Gravenstein S, Elliot E, Colopy M, Schweinle J. Clinical
signs and symptoms predicting influenza infection. 2000. Arch
Intern Med: 160; 3243-3247. |
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| 6. |
Ohmit
SE, Monto AS. Symptomatic
predictors of influenza virus positivity in children during the
influenza season. Clin Infect Dis. 2006;43:564-568. |
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| 7. |
Call
SA, Vollenweider MA, Hornung CA, Simel DA, McKinney WP. Does
this patient have influenza? 2005. JAMA. 293(8);987-997. |
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| 8. |
Poehling
KA, Edwards KM, Weinberg GA, et al for the New Vaccine Surveillance
Network. The
underrecognized burden of influenza in young children. N
Engl J Med 2006;355:31-40. |
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Gordis
L. Epidemiology. 3rd edition. Elsevier Health Sciences: Philadelphia,
Pa; 2004. |
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| 10. |
Centers
for Disease Control and Prevention. Weekly Report: Influenza Summary
Update. Available at: www.cdc.gov/flu/weekly.
Accessed August 16, 2007. |
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