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Tests and Diagnosis for Cold and Flu

By Nancy Baxi, M.D. ; Updated August 14, 2017

Most cases of cold and flu are diagnosed clinically, which means taking notes on the patient's history and a thorough physical exam are all that is needed. However, if a patient is very sick or the symptoms aren’t classic, further testing (such as a blood test taken in the lab) may be needed. Generally speaking, doctors are looking to make sure there isn’t a secondary, more severe underlying illness.

Clinical Diagnosis

The physical exam is performed mainly to rule out other things. The vital signs should be evaluated for fast heart rate or low blood pressure or an elevated temperature (and in the elderly, sometimes a low temperature). The breathing rate and pattern also should be evaluated. The eyes, ears, mouth, throat and tonsils are examined. The exam for a patient with a cold may be completely normal, or there may be nasal discharge or a red throat. The ears may be normal or appear full from fluid in the eustachian tube. The inside of the nose may have a characteristic allergy pattern if someone has nasal allergies. The sinuses may be normal or tender to the touch. The neck exam may reveal swollen lymph nodes, but people with allergies and either bacterial or viral infections can also have swollen lymph nodes in the neck. The doctor may also check the lungs by listening to the patient’s breathing.

Children can have multiple colds in a row, so it is important to distinguish whether the child is having one cold right after the other, or if it’s the same cold that keeps getting worse.

Rule Out Other Illnesses

The clinical exam will also help with ruling out other diagnoses. The doctor will look for evidence of other infections, such as infection of the ears, sinuses, throat, tonsils (tonsillitis), throat (strep throat), bronchi (bronchitis) and lungs (pneumonia) as well as for pertussis (whooping cough). For those with the flu, sometimes a lab test is needed to rule out other illnesses.


Blood tests are generally not helpful for the cold or flu, but sometimes the doctor may order them to help rule out other illnesses, depending on the patient.

The flu can be tested by checking for the influenza antigen. The test can be conducted either through a nasal wash or a throat swab. This test does not take long, and getting the results can be as quick as 30 minutes. When the test is positive, it is usually correct, but if the test is negative, it is not always right. This means physicians should not just rely on the test, but also examine the patient for symptoms. A viral culture is then sent for backup, but this takes several days while the lab tries to see if there is a virus growing. If the test is negative but the patient is quite ill and the doctor still suspects that the patient has the flu, the doctor may begin antiviral medication at this point.

If a patient has enlarged, tender lymph nodes, a red throat, swollen tonsils, a fever and a dry cough, they should be tested for strep throat. If a patient has a fever, cough, shortness of breath and wheezing sounds on a lung exam, the doctor may order a chest X-ray to evaluate for pneumonia.

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