How Colds and the Flu Are Diagnosed

Colds and the flu are primarily diagnosed by your symptoms, but doctors will sometimes also use a rapid test or other lab tests to confirm a flu diagnosis. Figuring out what, specifically, is behind your symptoms can better help you and your doctor develop a treatment plan. 

Self-Checks

For many, it’s unnecessary to go to the doctor to confirm a cold diagnosis, especially if you are otherwise healthy and symptoms are mild. There’s no cure for colds, and treatment typically involves using over-the-counter medications to manage symptoms. So an official diagnosis by a doctor might not change how your illness is treated.

A quick inventory of symptoms is usually enough to determine whether what you have is a cold or something more serious. If you aren’t sure whether you have a cold or the flu, here are some things to look for:

• How quickly did the symptoms appear? Cold symptoms typically appear gradually, whereas flu symptoms often come on abruptly.
• Do you have a fever? Colds rarely cause fevers, but they’re common in flu cases. It is possible, however, to have the flu without a fever.
• Does your body or head ache? Aches and pains in the joints, back, and head are much more common with the flu than colds.
• How tired or weak do you feel? While colds are unpleasant, people can still generally go about their business. The flu, however, can cause so much fatigue or weakness that it is difficult to get out of bed.

Physical Exam

If you do go to a doctor, they will conduct a physical exam to determine whether what you’re experiencing is a cold or the flu.

Doctors will likely ask questions about your symptoms (for example, when they appeared, how severe they are, etc.) and vaccination history (if you received the flu vaccine this season), as well as take some vital signs such as your temperature or heart rate. They might also listen to your lungs and evaluate your breathing to check for potential complications like pneumonia.

Labs and Tests

A physical exam is the primary way doctors diagnose colds and the flu, but they sometimes confirm that diagnosis using other methods like labs and tests.

While not everyone with flu-like symptoms needs to have labs done, diagnostic tests can be particularly helpful when deciding what treatment to recommend or responding to an outbreak in a hospital or school.

Labs and tests used to diagnose influenza include rapid diagnostic tests, assays, viral cultures, and serologic testing.

Rapid Influenza Diagnostic Tests (RIDTs)

Rapid tests are perhaps the most common diagnostic tool doctors use to confirm a flu diagnosis. The test involves using a swab to swipe a sample from inside the nose and testing it for signs of the influenza virus.

Doctors like rapid tests because they are quick; results are usually available within 10 to 15 minutes and can easily be done in a clinic.

The convenience, however, comes with some drawbacks. Rapid tests can’t identify the specific strain causing the infection, and they aren’t as accurate as other lab tests at detecting flu. Someone could get a negative result on the rapid test and still be infected with the virus.

Rapid Molecular Assays

Another kind of rapid tests used to diagnose flu is a rapid molecular assay. This type of test is similarly quick (with results in 15 to 30 minutes), but it’s more accurate than an RIDT. Rapid molecular assays detect the virus’ nucleic acids, or RNA. 

Other assays can be done to determine not just the presence of flu virus but also the specific strain responsible for the infection. This can be helpful when doctors or health officials would like to know whether the case is the result of influenza A or influenza B and whether the subtype is the same as other cases reported in the area.

Viral Culture

While not often used to diagnose individual flu cases, health officials might use viral cultures to get more information about a specific strain circulating in a given area or population. These tests are much slower than rapid tests, though some can provide results in as little as one to three days.

Much like a bacterial culture, viral cultures are done by taking a respiratory sample (a nose or throat swab) and attempting to grow it in the lab so it can be studied.

Scientists use viral cultures to pinpoint what specific virus might be behind a given outbreak or epidemic, spot new strains starting to circulate, and identify the influenza strains that should be included in the next year’s vaccine.

Serologic Testing

Serologic tests are blood tests that look for signs you have been exposed to a given microbe like the influenza virus. This type of test is typically only done by public health agencies or researchers and is not used by doctors to confirm individual flu cases.

Differential Diagnoses

Knowing whether you have a common cold, the flu, or something else entirely might make a difference in what your doctor recommends for treatment. For example, anti-viral medications are available to treat some high-risk individuals for the flu, but they won’t work against other viruses.

Doctors can often tell the difference between colds and the flu by symptoms alone. Both might cause respiratory discomforts like coughing or nasal congestion, but some symptoms are much more common with the flu, such as fever, headache, and body aches, and they’re generally a lot more severe. People with the flu tend to look and feel a lot sicker than those with a common cold.

That said, a lot of things can look like the flu, which is why doctors will often administer some kind of lab or test to confirm the diagnosis before prescribing anti-virals specific to influenza.

If a rapid flu test comes back negative, however, the doctor might still give a flu diagnosis if symptoms closely align with typical flu cases, depending on the accuracy rate of the tests or when they were taken (very early or very late in the illness).

Frequently Asked Questions

How are colds and flu diagnosed?

Most cases of cold or flu can be diagnosed by the characteristic signs and symptoms of these common respiratory infections. It's unlikely that you would be tested for a cold, but rapid influenza tests can confirm a flu diagnosis.

How do you tell if you have a cold or the flu?

The common cold is characterized by the gradual onset of symptoms, including sneezing, stuffy nose, cough, sore throat, and mild to moderate chest discomfort. The flu is recognized by its rapid onset and symptoms like high fever, chills, headache, body aches, fatigue, weakness, cough, and moderate to severe chest discomfort.

When are lab tests used for the flu?

Flu tests aren't always necessary, but they may be done to confirm the diagnosis and direct the appropriate treatment. The test options, which typically involve a nasal and throat swab, include rapid antigen testing, rapid molecular testing, and viral cultures.

What diseases might the flu be mistaken for?

Lab testing is crucial for severely ill people to differentiate influenza from other possible causes. These include:

• Coronaviruses (most specifically COVID-19)
• Echoviruses
• Legionnaire's disease
• Parainfluenza
• Respiratory syncytial virus (RSV)
• Streptococcal pneumonia
• Tuberculosis

What is the difference between the flu and COVID-19?

The flu is caused by an influenza virus, while COVID-19 is caused by the severe acute respiratory syndrome coronavirus 2 (SAR-CoV-2). Although both are transmitted in similar ways and can cause similar symptoms, COVID-19 is generally more contagious, causes illness in different risk groups, and has a higher mortality.