New screening tests: HSV, CMV, HBV, HCV, parvovirus, and HIV
Tips on choosing the right tests and getting valid results
Highly sensitive and specific ELISA assays have been developed for both HIV-1 and HIV-2. Both types of HIV have the same mode of transmission and incur opportunistic infections and other conditions. However, HIV-2 develops more slowly, is generally milder, and appears to be less transmissible than HIV-1.
HIV-2 occurs primarily in West Africa, although cases have been reported in Western Europe and India.
Several generations of assays have been approved for HIV-1 and HIV-2 over the past 2 decades. Today, third-generation assays using enzyme-coupled HIV antigens are standard. These tests are able to detect HIV antibody within 22 days of acute infection.16
Rapid tests (<30 minutes) and saliva tests are available for HIV screening. The IgG antibody is found as a transudate in oral secretions. Therefore, a technique that absorbs and concentrates IgG antibody from the oral cavity can be used to assay for HIV antibodies. An oral assay has been approved, but only for HIV-1 testing.17
Beware of false-positive and false-negative HIV results
Because of the enormous impact of an HIV diagnosis, it is critical to counsel patients that HIV screening assays can be falsely positive or falsely negative. However, compared with tests for the other viral infections covered in this article, HIV testing is perhaps the most sensitive and specific of FDA-approved tests.
False-positive rates range from 1 in 1,000 to 1 in 2,000 tests.
False negatives are less common and usually occur because of the time it takes for a patient to seroconvert after acute infection, which can be up to 1 year after infection.16
Confirm positive results via Western blot
The Western blot test is the gold standard for confirming ELISA-positive HIV results. For this highly sensitive (>99%) test, specific HIV proteins are obtained, separated by electrophoresis, and incubated with the serum along with control specimens. The CDC recognizes 3 results: positive, negative, or indeterminate.
A positive Western blot reacts to 2 bands (from p24, gp41, and gp120/160). A negative result reacts to no bands, and an indeterminate test reacts to only 1 band. Several chronic autoimmune diseases such as systemic lupus erythematosus and Hashimoto’s thyroiditis are associated with p24 reactivity.18
Other HIV tests
In addition to antibody tests, antigen and RNA tests can diagnose and help monitor the course of the disease and response to therapy. The most widely used are the p24 antigen detection test and the PCR assay for HIV-1 RNA.
Finding p24 antigen in the serum precedes the antibody response and may diagnose HIV earlier than ELISA. PCR testing for HIV viral RNA may be positive earlier than the p24 and ELISA antigen tests.19
Dr. Baker reports that he receives grant/research support from GlaxoSmithKline and Merck, is a consultant to GlaxoSmithKline, and is on the speakers bureaus of 3M, GlaxoSmithKline, and Pfizer.
