Seasonal flu viruses can be divided into two main types: A and B.
They cause the same miserable symptoms, so, if you catch the flu, you can't tell which type is the culprit just by how you feel. In fact, you can't even be sure your symptoms are really caused by the flu. That's what diagnostic tests are for.
If the flu is going around, though, and you have garden-variety flulike symptoms, confirmation with a test probably isn't necessary. You need to rest, drink plenty of fluids and, if necessary, take pain or fever relievers. In a week to 10 days, you should be over it.
But in the hospital, your doctor most likely will want to know whether you have influenza A or B or a bacterial infection. Some antiviral medications are approved only for influenza A, while bacterial infections are treated with antibiotics.
Plus, public health officials want to track flu viruses so they can be sure people are getting vaccinated against the right types. And they need to be on the lookout for viruses that could become pandemic, like H1N1 (a subtype of influenza A) in 2009 or so-called avian flu, highly contagious in birds but never confirmed in the USA.
This season, the CDC has introduced a more streamlined test that incorporates two previous versions. It still uses polymerase chain reaction, which takes a single bit of viral DNA and makes thousands or millions of copies.
The CDC is distributing the test free to state and international labs, but large hospitals and commercial labs also do PCR tests, says the CDC's Michael Shaw.
PCR testing is important for flu surveillance, says Vanderbilt University infectious-disease specialist William Schaffner, president of the National Foundation for Infectious Diseases. "It tells us with great precision what the circulating strains are."
It takes hours to get the results of PCR testing; many doctors use rapid tests that provide results in under a half-hour in their offices. But these, using a sample swabbed from the nose, check only for type A, and they often miss even that.
"If clinicians know that influenza is in the community, they're pretty safe just diagnosing on the basis of symptoms," Shaw says.
When to seek help
If your flu symptoms are mild and you're otherwise healthy, you don't need to see a doctor or take antiviral medication. But you should seek medical care if:
Your symptoms are severe.
You have other ailments, such as chronic heart, lung or kidney conditions, that could increase your risk of serious complications from the flu.
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