Tests for Specific Viral Illnesses

Influenza

When there is an outbreak of the flu, your doctor may want to confirm the presence of influenza by viral culture. A viral culture should be obtained within the first four days of illness by swabbing the inside of the nose and throat. The sample is sent to the lab and, if present, influenza virus is identified after five to 10 days. Rapid tests provide results in about 30 minutes. They are only done if the test result affects further treatment. False-positive (and true-negative) results are more likely when there are not many cases of the flu in the community, which is generally at the beginning and end of the influenza season. False-negative (and true-positive) results are more likely to occur when disease prevalence is high in the community, which is typically at the height of the influenza season.

Influenza tests do not need to be done on all patients. For individual patients, tests are most useful when they are likely to give a doctor results that will help with diagnosis and treatment decisions. During a respiratory illness outbreak in a closed setting (e.g. hospitals, nursing home, cruiseship, boarding school, summer camp) however, testing for influenza can be very helpful in determining if influenza is the cause of the outbreak.

Infectious mononucleosis

The "mono spot test," also called heterophil antibody test, is a quick screening method used to detect Epstein Barr virus (EBV), the virus that causes infectious mononucleosis. The immune system produces antibodies against EBV that can be detected with the mono test from two to nine weeks after infection. A sample of blood is mixed on a glass slide with other reactants. If the blood clumps, the test is considered positive.

HIV

There is no cure for HIV/AIDS. However, early detection and monitoring immune function help guide treatment. Opportunistic infections are preventable with the proper combination of medications given at the right time. The three tests used to diagnose HIV are the ELISA, Western blot and viral load. The ELISA test detects the presence of antibodies against HIV proteins. Since the ELISA may be false positive and react with non-HIV antibodies in conditions such as lupus and syphilis, a "Western blot" is done to confirm a diagnosis.

The viral load test measures the amount of virus when HIV is first diagnosed. The test is repeated every four weeks once therapy has started and continues every four months, or as recommended by the doctor. The combination of continued viral load testing and CD-4 white blood cell monitoring helps to guide treatment decisions. A rise in the amount of detectable virus (increase viral load) signifies that treatment may no longer be effective, requiring modification of treatment. An effective change of treatment is confirmed with a decreased follow-up viral load test.

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This article was reviewed and updated June 2007.