It’s the most sniffly time of the year – Influenza pearls!

Thank you to Swalpa for presenting a super interesting case of h flu pneumonia->bacteremia->endocarditis. This patient did not have influenza virus, but plenty of people at our 3 hospitals do! So this is a great opportunity to review influenza testing and treatment
Who to test
  • The CDC and IDSA are very consistent about who to test. See the flowchart below. You should test
    • anyone being admitted to the hospital with symptoms
    • anyone who you would consider treating
	Figure: Guide for considering influenza testing when influenza viruses are circulating in the community(regardless of influenza vaccination history)
What test do we use and how good is it?
There are two categories of influenza tests
  • Rapid antigen tests (from the nasopharynx)
    • pros – come back within minutes, ~90% specific, don’t require a laboratory (so can be done in an office)
    • Cons – not very sensitive (~50% in an average risk population)
  • Influenza PCR (from the nasopharynx, though you can also do this on BAL samples)
    • pros – highly sensitive (~95%) and specific
    • Cons – requires a true laboratory, takes 2-3 hours to come back
  • Respiratory viral panels use PCR to test other respiratory viruses in addition to influenza A & B
Who to treat?
  • Anyone sick enough to be hospitalized
  • People at high risk of influenza complications OR high-risk comorbidities (prepare yourself for a long list that  compromises basically all my clinic patients)
    • Children under 5
    • Adults over 65
    • Pregnant women and women up to 2 weeks post partum
    • nurse home residents
    • Native Americans and Alaska natives
    • people with the following comorbidities
      • Asthma
      • Neurological and neurodevelopmental conditions [including disorders of the brain, spinal cord, peripheral nerve, and muscle such as cerebral palsy, epilepsy (seizure disorders), stroke, intellectual disability (mental retardation), moderate to severe developmental delay, muscular dystrophy, or spinal cord injury].
      • Chronic lung disease (such as chronic obstructive pulmonary disease [COPD] and cystic fibrosis)
      •  Significant heart disease including congenital heart disease, congestive heart failure and coronary artery disease
      • Blood disorders (such as sickle cell disease)
      • Endocrine disorders including diabetes
      • Kidney disease
      • Liver disease
      • Metabolic disorders (such as inherited metabolic disorders and mitochondrial disorders)
      • Immune compromised people, including those with HIV, on steroids, or receiving chemotherapy.
      • People younger than 19 years of age who are receiving long-term aspirin therapy
      • BMI>40
Is it ever appropriate to treat despite a negative test?
  • Depends on the test you use.
    • If you use an antigen-based rapid test, they are quite insensitive so if you have a high clinical suspicion, you can consider empiric treatment.
    • PCR tests (we use these exclusively at ZSFG) are so sensitive that false negatives are extremely rare. Unless you have a very high pre-test probability (say a SNF resident where there is a known flu outbreak with fevers and respiratory failure) a negative test should obviate the need for influenza treatment.
Rachel, is there one, supremely important thing I should know?
  • Get your flu shot, wash your hands, and wear masks when you suspect a patient might have the flu. Flu vaccination of healthcare workers reduces patient mortality. Plus your happiness/healthiness is paramount. Love, your Jeopardy chief =)
Sources
Lisa Winston – our hospital epidemiologist, was enormously helpful in putting this together
More info from the CDC: https://www.cdc.gov/flu/
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