Test for Influenza

Submitted on March 27, 2012

Influenza is a viral respiratory infection. It tends to be seasonal, and causes headaches, chills, blocked nose, muscle pains, cough, and sore throat. Influenza symptoms are more intense and tend to last longer than the regular flu or cold. Influenza is of two types: Type A and Type B. Much like other respiratory air or water borne diseases, influenza is also contagious. Typically, an influenza Type A virus infection in a family could lead to a dominant outbreak of Type A in that particular season. However, this isn't a hard and fast rule, and at times there are a mix of Type A and B viruses in the community.

Reasons for Influenza Tests

The doctor orders an influenza test to detect whether the patient has influenza, and its type, if present. The test is also undertaken to chalk out a treatment plan. At times the influenza test is ordered to check if the flu has spread in the neighborhood.

When to go for this test

When you display flu-like symptoms, especially in the flu season, the then doctor orders an influenza test. The test will detect influenza within a time frame of 48 hours. Therefore, if you have flu-like symptoms, it's best to get the test done as soon as the first signs develop.


Testing for influenza is largely dependent on respiratory secretions. Identifiable virus is only shed in the first couple of days of the illness, so the testing has to be done in this period. The most common test used to detect the influenza virus is a rapid influenza antigen test, wherein the nasal secretions are tested. Depending on the type of test, it can detect the presence of influenza as well as the type. The only disadvantage of this test is that it can at times read positive even if the patient is not infected.

There are other methods which are more sensitive. Labs can stain nasal secretions with fluorescent antibodies that help highlight the virus. Within a few hours of collecting the sample, it is easy to distinguish whether the influenza is Type A or Type B.

The key to influenza testing is in the collection of the sample. The ideal sample is a nasal aspirate, but often, samples used for influenza testing are nasal washes or nasopharyngeal swabs.

One method involves inserting a syringe with some sterile saline into the nose, and then with some gentle suction, the saline along with the mucus is sucked into a cup. The NP swab or nasopharyngeal swab involves the patient tipping his/her head, while the Q-tip shaped swab is gently inserted into one nostril, rotated and then withdrawn. Although this sounds painful, it isn't. At best you will feel ticklish and it could make your eyes water. The NP swab is usually administered on adults, whereas the former method, i.e. the nasal wash or aspirate, is used on children. In certain cases, doctors also use a throat swab, but the virus contained in this will be much lower than in an NP aspirate, and hence, is not recommended for rapid tests.

Test results

A positive test for influenza is most indicative of influenza Type A or B, but this result does not indicate how severe the symptoms are, and it doesn't tell the doctor if there could be further complications.

A negative influenza test, despite flu-like symptoms, means that you have some other kind of viral infection, or it could also mean that the rapid tests did not obtain sufficient virus in the sample to provide a conclusive result. Inconclusive reports can occur if you've tested for influenza several days after the occurrence of symptoms.

While all this sounds very grim, one must note that with or without treatment an influenza infection usually goes away within a week or two. It might take longer for the fatigue or sore throat to disappear. In certain cases, influenza causes secondary complications, especially for those with pre-existing respiratory ailments. These include ailments like pneumonia and sepsis which can be critical and should be treated immediately.