Flu diagnosis

How do you get a flu test?

If you think you have the flu, it’s important that you talk to your healthcare provider to receive the proper diagnosis^[9] and treatment. When testing for the flu, your provider will conduct a physical exam and ask questions to determine your signs and symptoms of the flu. If it’s flu season and many cases of influenza have occurred in your area, your provider may not even need to test you for the virus before determining your course of treatment. However, in some cases (such as when the cause of your symptoms is uncertain), they may suggest that you get tested for the flu.

PCR tests for the flu

One type of test, a polymerase chain reaction (PCR) test, is becoming common in hospitals and labs. The test is rapid and more sensitive than other tests, and may even be able to identify the strain of influenza you have. A PCR test requires a sample (sometimes a nasal swab) for analysis and identification of the presence of a virus.

Rapid flu tests (RIDTs)

RIDTs, or rapid influenza diagnostic tests, are the most common tests used to diagnose the flu. RIDTs, unlike PCR tests, cannot identify different strains of the flu. These tests, as their name suggests, are fast, generally providing results within about 10–15 minutes. They are not as accurate as other flu tests, however, and sometimes provide false negatives (you have the flu but your test comes back as negative). Like a PCR test, RIDTs require a swab from the inside of your nose or the back of your throat.

Rapid molecular assays

Rapid molecular assays work by detecting a virus’ genetic material. Using a nasal or throat swab as a sample, these tests produce results in 15–20 minutes. Rapid molecular assays are more accurate than RIDTs. If you are uncertain about your symptoms, you should always contact your provider to see whether you have the flu or a different type of infection.

What is the flu vaccine?

The seasonal flu vaccine is an injection or nasal spray^[10] that provides protection from the strains of the flu virus that research has indicated will be the most common in the upcoming flu season. The nasal spray is not recommended for pregnant women, children ages 2–4 with asthma or breathing problems, or people with weakened immune systems. Flu vaccines work by causing antibodies to develop, generally about two weeks after vaccination. These antibodies will recognize and protect against infection by the specific viral strains found in the vaccine. Each year, the viral strains are predicted before the beginning of the flu season—it is those specific strains that the vaccine will protect you against. Flu vaccines do not contain live flu viruses, and because of this, they cannot give you the flu. Traditional flu vaccines are called “trivalent” vaccines because they protect you against three viruses: influenza A (H1N1), influenza A (H3N2), and an influenza B virus. “Quadrivalent” vaccines also protect against an additional influenza B virus. The CDC recommends^[11] that everyone age 6 months or older receive the flu vaccine once per year. Being vaccinated against the flu can greatly reduce incidences of missed work or school, and was shown in a 2017 study by the CDC to reduce the risk of death in young children^[12]. Though the annual flu vaccine is not 100% effective, it is the best step you can take towards defending yourself against the flu.

Side effects from the flu shot

The flu vaccine in the injectable form is often referred to as the “flu shot.” The most common side effects of the flu shot include:

• Soreness, redness, tenderness, or swelling at the injection site
• Low-grade fever
• Muscle aches and soreness
• Headache
• Nausea

Signs of serious life-threatening side effects from the flu shot, though rare, include:

• Breathing problems
• Hoarseness or wheezing
• Hives
• Paleness
• Weakness
• Fast heartbeat
• Dizziness

These symptoms usually occur within minutes to hours after receiving the shot, and can be the result of an allergy to an ingredient in the vaccine (such as egg protein).

When is flu season?

While influenza viruses are in circulation year-round^[13], flu season in the United States occurs in the fall and winter. Influenza activity usually begins to increase in October and reaches its peak between December and February. The flu can sometimes be active as late in the season as May. The impact and severity of the flu varies from season to season. Multiple other respiratory infections are in high circulation at the same time as the flu. Some of these viruses, such as rhinovirus (the most common cause of colds) and respiratory syncytial virus (RSV). Because some of these infections can cause symptoms similar to those of the flu, it’s important that you see your healthcare provider to determine what kind of infection you have.

Flu or cold?

Like the flu, the common cold is a viral respiratory infection. (meaning they infect the nose and throat). While both conditions infect the nose and throat, the flu and the common cold are caused by different viruses. Some of the signs and symptoms of the common cold overlap with those of the flu. Because their symptoms are often similar, it can be difficult to tell which illness you have based on symptoms alone- especially early on. The early stages of the flu may feel like a really bad cold. Generally, however, the flu is worse than the common cold and has much more intense symptoms. While colds are usually mild, the flu can lead to serious infections and complications that may require hospitalization. The common cold generally doesn't require a trip to your healthcare provider. However, if your symptoms become severe or you think that you may have the flu, you should schedule a visit. These are some of the differences between the symptoms of the common cold and the flu:

Flu vs. the “stomach bug”

A common misconception about the flu is that it always causes vomiting and diarrhea. While these symptoms can occur in children with the flu, they are not characteristic of it in adults. What people generally call a “stomach flu” is not actually the flu at all, but viral gastroenteritis^[15], known as the “stomach bug.” Gastroenteritis is most frequently contracted through person-to-person contact with someone who is infected or by ingesting contaminated food or water. Unlike the flu, there is no effective treatment for viral gastroenteritis. Symptoms of the “stomach bug” include:

• Watery diarrhea (bloody diarrhea indicates a different, more severe infection)
• Abdominal pain and cramping
• Nausea and/or vomiting
• Occasional muscle aches or headache
• Low-grade fever

Symptoms of gastroenteritis may appear 1–3 days after infection, and can range from mild to severe. These symptoms may last for 1–2 days, but can persist for up to 10 days.