Swabbing the throat to obtain a sample for testing
Laboratory tests can be divided into three main categories: screening, diagnostic and therapeutic. Screening tests are those that are performed to screen a patient, or a group of patients, for a particular disease. Physician office laboratories and emergency department staff usually perform these tests, although some are also performed at health fairs and in other outpatient settings. Diagnostic tests are usually more complex than screening tests, and they are performed at more advanced laboratories. Diagnostic tests are usually more accurate than screening tests. Therapeutic tests are performed in order to aid in the delivery of a treatment. They are also complex and more involved than screening tests, yet they are usually of similar complexity as diagnostic tests.
Screening for Influenza Infection
When a patient presents to a health care provider with flu-like symptoms (fever and cough and/or sore throat), the health care provider may want to test for influenza to rule it out as a diagnosis. A sample of the mucous membrane from the patient is taken with a swab. Once at the lab, the sample is processed with chemicals to extract and expose the flu virus, if it is present. The solution containing the virus is then tested using antibodies against the flu. Depending on the test, if the flu virus is present on the sample, the antibodies will attach to it and cause a change in color or turbidity of the solution (a positive result). If there is no virus present, then there will be no reaction (a negative result). Screening tests such as these can only detect the presence of the virus and whether it is type A or type B. However, they are good tests for giving the provider a clue as to whether a patient has influenza or another respiratory condition, and the results are usually available in about 15 minutes.
Diagnostic Testing for Influenza
In order to test for the particular strain of influenza, a more complex test is used. At most large laboratories, this is the real-time PCR (polymerase chain reaction) test. The PCR test uses chemicals to open the virus and expose its RNA (genetic material). Once exposed, the RNA is analyzed for sequences that serve as a "fingerprint" of the virus, telling the laboratory staff the type of virus and the strain when that fingerprint is compared to known fingerprints. Occasionally, the PCR method will yield positive results for influenza but no strain information. When this happens, even more involved tests are done, such as growing the virus in a tissue culture and looking at it under the microscope. Viruses for which no known fingerprint is available are usually called "novel" because they have not been detected before. This was the case in April 2009 with the H1N1 ("swine") influenza virus.
Therapeutic Testing for Influenza
Therapeutic tests for influenza include additional PCR tests and tissue cultures to detect flu viruses that are resistant to antiviral medication. Resistance to antiviral medication in flu viruses comes from genetic mutations. These mutations are seen in the fingerprint of the virus and are compared to known mutations. These results help health care providers give the appropriate antiviral treatment to patients.
Special Considerations
In the case of a screening test, a negative result does not necessarily rule out the disease. There are times when these tests are unable to correctly detect the presence of a virus either because of the nature of the test or because of an error in the testing. Because they are performed at larger laboratories by more skilled personnel, diagnostic and therapeutic tests are less prone to error. However, the final say as to whether someone has the flu rests solely on the health care provider. It is the provider's responsibility to diagnose a patient and offer the best course of treatment.
The Future of Testing
Medical technology, like other technologies, is constantly evolving. The future of laboratory testing for influenza includes more accurate screening tests that are less susceptible to errors. Also on the horizon are portable PCR and viral culture tests that can be carried in a box to the scene of a mass infection, reducing the transport of potentially dangerous samples across populated centers. Technologies yet unknown to scientists may still be discovered.
Tags: health care, care provider, health care provider, screening tests, than screening, than screening tests, virus present
