Thursday, January 15, 2015
How Accurate are Flu Tests?
In primary care offices, we do many types of rapid testing to help us diagnose and treat patients- pregnancy tests, urine tests for infection, rapid strep tests, and of course, rapid flu tests. How accurate are these tests? If a rapid flu test is POSITIVE, it is extremely accurate- 98%. This means they are very SPECIFIC tests- if it says you have it, you have a 98% chance that you really do have the flu, so the likelihood of a false positive is very low. If your rapid flu test result is NEGATIVE, however, you might still have the flu...and your doctor should rely on her clinical judgement. The SENSITIVITY of these tests varies, but averages about 62%, meaning about 1/3 of the time when you DO have the flu, the test could miss it. (The most recently published meta-analysis of rapid flu tests analyzed 26 different rapid test brands, but all had similar data on accuracy-http://www.ncbi.nlm.nih.gov/pubmed/22371850.)
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 until you are fever-free for at least 24 hours!)
If you had the flu vaccine, will that affect your rapid flu test? If you had a shot- no. If your flu vaccine was the intranasal spray, there is 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.)
During epidemic years of the flu (including 2015) the CDC encourages doctors to treat all high risk individuals with suspected flu with antiviral medications, and in fact, last week the CDC put out a special Health Update.
Who should be treated with SUSPECTED illness? Some are obvious- 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 can 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.