Two types of tests are available to look specifically for the genetic material of the virus: qualitative and quantitative.
The qualitative test seeks to detect the presence or absence of HIV but does not indicate how much of the virus is present. It is not performed as often as the quantitative tests, which do tell how much virus is present and are therefore more useful for following a person who is infected with HIV to determine where he or she is in the course of the illness and how effective treatment is.
The qualitative test has about 97 percent sensitivity and about 98 percent specificity. This means that it detects 97 out of 100 people who are infected, and that 98 out of 100 people who test negative truly are not infected. The test is not as accurate as the ELISA/Western blot combination, so it is not routinely used for screening purposes. Instead it is employed in a few select situations, such as diagnosing infection in the newborn. This test may also be useful in those cases in which the ELISA and Western blot assay are inconclusive for adults and an additional test is necessary.
The quantitative tests, or vital load tests, are also known as polymerase chain reaction tests. There are several types, all of which look for the genetic material of the virus and then “amplify” it to readable quantities. These tests measure the quantity of virus circulating in the blood. One reason they are not recommended for screening purposes is that it is possible at different stages in the course of a person’s infection with HIV to have levels of virus that are undetectable by these assays. Thus a “negative” test does not always exclude infection. Similarly a “positive” test may, rarely, be a false positive.
The quantitative tests are thought to have between 86 percent and greater than 98 percent sensitivity. However, new viral load assays are currently being evaluated, and in the near future more sensitive tests may be available that would be more useful for screening purposes. At this time these tests are about five times more expensive than the ELISA and Western blot assay, and this is another factor to consider when deciding on one particular test over another. The quantitative tests are used primarily to follow the course of an HIV-infected person’s illness, and they are useful in determining when medications should be initiated or changed. They can also clarify inconclusive ELISA and Western blot tests and help in identifying infected newborns.
*318\213\8*