The Test Sample
What is being tested?
Influenza (the flu) is a viral respiratory infection that tends to be seasonal, usually beginning in late fall and disappearing in early spring. Influenza testing detects the presence of the virus in a sample of respiratory secretions.
Influenza is a common respiratory illness that affects 30 to 50 million Americans each season. Symptoms like headache, fever, chills, muscle pains, exhaustion, a stuffy nose, sore throat, and a cough tend to be more severe and longer lasting than the flu-like symptoms caused by the common cold.
Two types of influenza virus, A and B, cause annual flu pandemics and most epidemics. Type C can cause mild respiratory illness and is not thought to cause epidemics. There are numerous subtypes of influenza A viruses, and they are named using two designations, H and N. The most common influenza A viruses currently infecting humans have the subtypes H1N1 and H3N2. (For more on this, see the condition article on Influenza.)
It is common for health practitioners to diagnose and treat the flu without laboratory testing, especially during peak influenza season and when influenza has already been documented in an area. However, influenza testing can help rule out other illnesses and reduce the chances of people using unnecessary antibiotics, while increasing the chances that they will receive anti-viral therapy early in the illness, when it is most effective.
Also, if there is an outbreak of respiratory illness in a confined setting like a hospital, nursing home, or school, diagnostic testing will help determine the cause of the outbreak. Influenza testing also helps local and state health departments, and the Centers for Disease Control and Preventio (CDC), track influenza in communities. Since the flu virus changes every year, testing also helps the CDC to monitor the subtypes and strains of flu that are circulating that year, to collect information for developing flu vaccines, and to monitor strains for resistance to anti-viral drugs.
There are several different kinds of influenza tests and they serve different purposes. For example, rapid diagnostic tests have become more widely available and can help with individual treatment decisions. However, they are still not very sensitive and can result in misleading, false-negative results.
Some, but not all, rapid diagnostic tests can distinguish between the two different types of influenza subtypes. However, they cannot identify which subtype, or influenza strain, someone has. Therefore, molecular tests (polymerase chain reaction, PCR) and viral cultures are still very important because they identify specific viral strains, something necessary for things like identifying which strains to include in the next year's flu vaccine and finding new types of influenza A that could be a pandemic threat.
How is the sample collected for testing?
Sample collection technique is critical in influenza testing, and different kinds of influenza tests rely on different collection techniques. The best sample is usually a nasal aspirate, but the most frequently used sample is a swab from the nasopharynx.
Health practitioners usually use nasopharyngeal swabs for adults but may choose to do a nasal wash or aspirate for a child. In some circumstances, a health practitioner may use a throat swab, but this contains less of the virus than a nasopharyngeal aspirate and so may not be appropriate for use in rapid testing where sensitivity is a concern.
For an aspirate, the person collecting the sample will use a syringe to push a small amount of sterile saline into the nose, then apply gentle suction to collect the resulting fluid (saline and mucus). To preserve the organisms in the sample, the sample is put into a special container, referred to as "viral transport media" or VTM, for delivery to the laboratory.
The nasopharyngeal swab is collected by having the person tip his or her head back, then a Dacron swab (like a long Q-tip® ) is gently inserted into one of the nostrils until resistance is met (about 1 to 2 inches in), then rotated several times and withdrawn. This is not painful, but it may tickle a bit and cause the eyes to tear.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.