Flu Tests 101

Positive Rapid Flu Test for Type A Influenza    (C is Control)

Flu season is in full force. If you have high fevers, intense headaches, screaming muscles, razor blade throat pain and/or a wicked cough, should you go get a flu test? How reliable are these rapid tests anyway?

Let's cut to the chase: if you have flu symptoms and a POSITIVE rapid flu test (from a swab lovingly probed up inside your nose), YOU HAVE INFLUENZA. These positive tests are extremely accurate- typically 98-99%.  This means they are very SPECIFIC tests- if the test says you have flu, there is a 98% chance that you really do have the flu, so the likelihood of a false positive is very low.

HOWEVER, if you have flu symptoms and your rapid flu test result is NEGATIVE, you might still have the flu...and your doctor should rely on her clinical judgement. Why? Because these rapid flu tests are very SPECIFIC, but much less SENSITIVE. Sensitivity reflects the percentage of the time that the test is positive when the disease is actually there. Rapid Influenza Diagnostic Tests (RIDTs) average a sensitivity of about 62%, meaning the test turns positive 62% of the time in someone who is truly infected. Rephrased, up to one third of the time when a person has the flu, the test could miss it. Newer digital immunoassays (DIAs), and rapid nucleic acid amplification tests (NAATs) may have significantly higher sensitivities (as noted in this 2017 Systematic Review) but for now these tests have a lower share of the market, and many current studies are industry backed. Meanwhile, most of us in primary care have the less sensitive RIDTs in our practice, which are still quite clinically useful.

Why do doctors even use these tests if they are not super sensitive? Because it affects our treatment plan. If we have confirmation that someone has the flu (with a positive test), then we can feel more confident in NOT prescribing an antibiotic, and limit our conversation to pros/cons of taking an antiviral medication as well as medications for symptomatic relief. This step greatly reduces unnecessary antibiotics, which is critical in an era of increasing antibiotic resistance.
Additionally, knowing a patient definitely has the flu helps us advise them about returning to work/school/activities, since we know flu stays infectious for up to a WEEK. (So STAY HOME at least several days, AND if you have fever, wait until you are fever-free for at least 24 hours without taking tylenol or advil!)

If you had the flu vaccine, will that affect your rapid flu test?  No*. 

(*In prior years when the nasal spray flu vaccine was recommended, there was a chance that you could have a false positive test. Studies have shown that in the first several days after receiving the nose spray vaccine, people may test positive for the flu (even though they are not infected.)

Who should be treated with SUSPECTED illness?The CDC encourages doctors to treat all high risk individuals with suspected flu with antiviral medications. Who is "high risk": anyone hospitalized, patients in nursing homes, and people with immune problems, significant chronic problems (lung or heart disease, for example) or with severe illness such as pneumonia. However, HIGH RISK also includes all otherwise healthy children under the age of two years,  adults over the age of 65, and women who are pregnant or have just delivered their baby within the past two weeks. There are also other less common subgroups that are high risk such as people who are very obese (BMI >40), American Indians or Alaska natives, and youth under 19 who are on chronic aspirin therapy.

Who should be treated with CONFIRMED illness? 
Anyone in the high risk categories above should be treated with antiviral medications if they have a positive test, because treatment definitely decreases severe complications, hospitalization and even death.

Otherwise healthy, active adults who test positive for the flu should have a discussion with their physician about taking the antiviral medications. When taken within 48 hours of the onset of flu symptoms, these drugs have been shown to decrease the duration of the flu by one or two days...which may make a big difference to a student, parent or employee. However, these medications are not without side effects, and for some people, they may aggravate flu symptoms- particularly nausea. In controlled studies, the antiviral medications only cause about 10% more nausea/headaches/fatigue than placebo, but there is no doubt that in a minority of patients, the medicine causes more side effects than they are worth. However, since you have the FLU when you are taking the medications- you already likely HAVE nausea, headache, fatigue, etc- so it is often difficult to sort out. In my family, half of us can tolerate the antiviral medications with zero problems, and half...barely keep the pills down long enough to know.

BOTTOM LINE: If you have flu symptoms, get to the doctor as soon as possible (especially if you would consider taking antiviral medications), and understand that a positive test is REALLY positive, but a negative test does not definitely rule out the possibility of the flu. 

AND...that ounce of prevention...remember to GET YOUR FLU SHOT NOW to reduce your chance of getting the flu (or to at least minimize your symptoms if you do catch it.)