Influenza Testing During the 2009-2010 Flu Season

How can I know for certain if I have the flu this season?

To know for certain, a test specific for flu would need to be performed.  But most people with flu symptoms do not need a test for 2009 H1N1 flu because the test results usually does not change how you are treated.

What kinds of flu tests are there?

A number of flu tests are available to detect influenza viruses. The most common are called “rapid influenza diagnostic tests” that can be used in outpatient settings. These tests can provide results in 30 minutes or less. Unfortunately, the ability of these tests to detect the flu can vary greatly. Therefore, you could still have the flu, even though your rapid test result is negative. In addition to rapid tests, there are several more accurate and sensitive flu tests available that must be performed in specialized laboratories, such as those found in hospitals or state public health laboratories. All of these tests are performed by a health care provider using a swab to swipe the inside of your nose or the back of your throat. These tests do not require a blood sample. For more information, see Seasonal Influenza Testing.

How well can these tests detect the flu?

Rapid tests vary in their ability to detect flu viruses. Depending on the test used, their ability to detect 2009 H1N1 flu can range from 10% to 70%. This means that some people with a 2009 H1N1 flu infection have had a negative rapid test result. (This situation is called a false negative test result.) Rapid tests appear to be better at detecting flu in children than adults. None of the rapid tests currently approved by the Food and Drug Administration (FDA) are able to distinguish 2009 H1N1 flu from other flu viruses. 

Will my health care provider test me for flu if I have flu-like symptoms?

Not necessarily. Your health care provider may diagnose you with flu based on your symptoms and their clinical judgment or they may choose to use an influenza diagnostic test. Depending on their clinical judgment and your symptoms, your healthcare provider will decide whether testing is needed and what type of test to perform. CDC has provided recommendations for clinicians this season to help with testing decisions. This season, most testing will be done in people who are seriously ill (hospitalized patients) and patients where testing may impact treatment decisions. In most cases, if a healthcare provider suspects you have the flu, the test results will not change their treatment decisions. 

Who is being tested for flu this season?

This season CDC has provided Interim Recommendations for Clinical Use of Influenza Diagnostic Tests During the 2009-10 Influenza Season which recommends that the following people receive influenza diagnostic testing: 1) people who are hospitalized with suspected flu and 2) people such as pregnant women or people with weakened immune systems, for whom a diagnosis of flu will help their doctor make decisions about their care. CDC expects that most people with flu symptoms this season will not require testing for 2009 H1N1 because the test results usually do not change how you are treated. Additional people may be recommended for testing based on the clinical judgment of their health care provider.

How will I know what strain of flu I have or if it’s 2009 H1N1 (formerly known as Swine Flu)?

You may not be able to find out definitively what flu virus you have. Currently available rapid influenza diagnostic tests cannot distinguish between 2009 H1N1 and seasonal influenza A viruses. Most people with flu symptoms this season will not require testing for 2009 H1N1 because the test results usually do not change how you are treated.  As of September 2009, more than 99% of circulating influenza viruses in the United States are 2009 H1N1. Therefore, at this time, if your health care provider determines that you have the flu, you most likely have 2009 H1N1. As the season progresses, different influenza viruses may circulate and updated national information on circulating influenza viruses is available in the FluView U.S. Weekly Influenza Surveillance Report.

There are laboratory tests available that can tell the difference between 2009 H1N1 and other strains of flu, but these can take one to several days to provide results and this season, CDC has recommended that this testing be focused on 1) people who are hospitalized with suspected flu; 2) people such as pregnant women or people with weakened immune systems, for whom a diagnosis of flu will help their doctor make decisions about their care.

Why can’t I get a more accurate laboratory test to find out if I had flu or what kind of flu I had?

The most accurate laboratory tests, such as real-time reverse transcriptase polymerase chain reaction (rRT-PCR) are only available in certain laboratories, and these tests can take several days to obtain results. This season, CDC is focusing use of these tests on people who are hospitalized or for other reasons explained in the question “Who is being tested for flu this season?”