Update on 2009 pandemic influenza A (H1N1) virus

INFECTION CONTROL IN THE HOSPITAL AND IN THE COMMUNITY

Influenza is very contagious and is spread in droplets via sneezing and coughing (within a 3-foot radius), or via unwashed hands—thus the infection-control campaigns urging you to cover your cough and wash your hands.

As noted, for patients being admitted or transferred to the hospital, we need to have a low threshold for testing for influenza and for isolating patients suspected of having influenza. For patients with suspected or proven seasonal influenza, transmission precautions are those recommended by the CDC for droplet precautions (www.cdc.gov/ncidod/dhqp/gl_isolation_droplet.html). A face mask is deemed adequate to protect transmission when coming within 3 feet of an infected person. CDC guidelines for pandemic H1N1 recommends airborne-transmission-based precautions for health care workers who are in close contact with patients with proven or possible H1N1 (www.cdc.gov/ncidod/dhqp/gl_isolation_airborne.html). This recommendation implies the use of fit-tested N95 respirators and negative air pressure rooms (if available).

The recent Institute of Medicine report, Respiratory Protection for Healthcare Workers in the Workplace Against Novel H1N1 Influenza A (www.iom.edu/CMS/3740/71769/72967/72970.aspx) endorses the current CDC guidelines and recommends following these guidelines until we have evidence that other forms of protection or guidelines are equally or more effective.

Personally, I am against this requirement because it creates a terrible administrative burden with no proven benefit. Requiring a respirator means requiring fit-testing, and this will negatively affect our ability to deliver patient care. Recent studies have shown that surgical masks may not be as effective¹¹ but are probably sufficient. Lim et al¹² reported that 79 (37%) of 212 workers who responded to a survey experienced headaches while wearing N95 masks. This remains a controversial issue.

Besides getting the flu shot, what can one do to avoid getting influenza or transmitting to others?

• Cover your cough (cough etiquette) and sneeze.
• Practice good hand hygiene.
• Avoid close contact with people who are sick.
• Do not go to school or work if sick.

A recent study of influenza in households suggested that having the person with flu and household contacts wear face masks and practice hand hygiene within the first 36 hours decreased transmission of flu within the household.¹³

The United States does have a national influenza pandemic plan that outlines specific roles in the event of a pandemic, and I urge you to peruse it at www.hhs.gov/pandemicflu/plan/.

RECOGNIZING AND DIAGNOSING INFLUENZA

The familiar signs and symptoms of influenza—fever, cough, muscle aches, and headache—are nonspecific. Call et al¹⁴ analyzed the diagnostic accuracy of symptoms and signs of influenza and found that fever and cough during an epidemic suggest but do not confirm influenza, and that sneezing in those over age 60 argues against influenza. They concluded that signs and symptoms can tell us whether a patient has an influenza-like illness, but do not confirm or exclude the diagnosis of influenza: “Clinicians need to consider whether influenza is circulating in their communities, and then either treat patients with influenza-like illness empirically or obtain a rapid influenza test.”¹⁴

The signs and symptoms of pandemic 2009 H1N1 are the same as for seasonal flu, except that about 25% of patients with pandemic flu develop gastrointestinal symptoms. It has not been more virulent than seasonal influenza to date.

Should you order a test for influenza?

Most people with influenza are neither tested nor treated. Before ordering a test for influenza, ask, “Does this patient actually have influenza?” Patients diagnosed with “influenza” may have a range of infectious and noninfectious causes, such as vasculitis, endocarditis, or any other condition that can cause a fever and cough.

If I truly suspect influenza, I would still only order a test if the results would change how I manage the patient—for example, a patient being admitted to the hospital where isolation would be required.

Pandemic H1N1 will be detected only as influenza A in our current PCR screen for human influenza. The test does not differentiate between seasonal strains of influenza A (which is resistant to oseltamivir) and pandemic H1N1 (which is susceptible to oseltamivir). This means if you intend to treat, you will have to address further complexity.

Testing for influenza

The clinician should be familiar with the types of tests available. Each test has advantages and disadvantages¹⁵:

Rapid antigen assay is a point-of-care test that can give results in 15 minutes but unfortunately is only 20% to 30% sensitive, so a negative result does not exclude the diagnosis. The positive predictive value is high, meaning a positive test means the patient does have the flu.

Direct fluorescent antibody testing takes about 2.5 hours to complete and requires special training for technicians. It has a sensitivity of 47%, a positive predictive value of 95%, and a negative predictive value of 92%.

PCR testing takes about 6 hours and has a sensitivity of 98%, a positive predictive value of 100%, and a negative predictive value of 98%. This is probably the best test, in view of its all-around performance, but it is not a point-of-care test.

Culture takes 2 to 3 days, has a sensitivity of 89%, a positive predictive value of 100%, and a negative predictive value of 88%.

These tests can determine that the patient has influenza A, but a confirmatory test is always required to confirm pandemic H1N1. This confirmatory testing can be done by the CDC, by state public health laboratories, and by commercial reference laboratories.