Types of HIV Tests

There are two broad categories of HIV tests: screening tests and confirmatory tests. Using these two types of test together can lead to highly accurate and reliable diagnosis of HIV infection.

Screening tests

Screening tests are used for initial testing because they are easier to perform than confirmatory tests, well suited to testing large numbers of samples, and less costly. They are highly sensitive and result in few false negatives (i.e., most infected people test positive). However, screening tests are not as specific as confirmatory tests, so in a small percentage of cases the test result will be positive even if the person is not infected. Therefore, providers should never give results from screening tests that have not been verified through a confirmatory test.

ELISA tests

The most common screening tests are enzyme-linked immunosorbent assay (ELISA) tests. These tests measure antibodies to HIV. Different types of ELISA tests are available. Most require serum specimens, though one uses urine and another uses an oral specimen.

Serum tests. Traditional screening tests use a blood sample. About two dozen types of ELISA tests are in use around the world.

Urine tests. An ELISA test for detecting HIV in urine samples has been approved for use in the U.S.; however, its biggest drawback is that there is no approved confirmatory test for urine samples (in other words, if the urine ELISA results are positive, a blood sample must then be drawn for confirmatory testing).

Oral tests. OraSure (produced by SmithKline Beecham) is an HIV test that uses mucosal transudate as the sample. (Although some call this a saliva test, the sample is not saliva, but an oral sample called mucosal transudate.) The sample is collected by placing the special collection device between the cheek and gum. The specimen is then sent to a lab for ELISA testing. Positive ELISA results can be confirmed using the Western blot test. These tests are more expensive than blood tests. OraScreen, a similar test marketed for home use, is available in some countries, but it is not approved for use in the U.S.

Rapid serologic tests

Rapid serologic tests provide results in less than 30 minutes. These tests also measure antibodies to HIV, but by different mechanisms than ELISA tests, including agglutination tests, immunocomb tests, immunodot tests, and immunochromatographic membrane tests. Most rapid tests are kits that include all of the necessary supplies. These tests are relatively simple, involve a limited number of steps, and are quite accurate when performed correctly. (Most rapid tests require refrigeration.)  While the inherent sensitivity and specificity of ELISA tests may be greater than those of some of the rapid tests, the field performance of rapid tests is often as good as or better than the ELISA because the former is simpler and easier to do in a low-resource setting. One rapid HIV test is approved for use in the U.S. (Single Use Diagnostic System for HIV-1, or SUDS, manufactured by Murex Diagnostics, Inc., Norcross, Georgia, U.S.).

HIV Dipstick Test Kit (developed by PATH) The is a rapid (results in approximately 20 minutes), inexpensive (less than $0.50/test) test that requires no specialized equipment. Sensitivity is more than 99%, and specificity is more than 98%. The dipsticks are licensed in many countries around the world and are currently being produced in Thailand, India, Argentina, and Indonesia. They are not licensed for use in the U.S.

Confirmatory tests

A confirmatory test is done when the results of a screening test are positive. The confirmatory test is expensive and labor intensive and requires subjective interpretation, but it is very specific (in other words, false-positive results are extremely rare). The Western blot test is considered by most to be the “gold standard” for confirmation of positive screening test results. This test also measures antibodies to HIV, but it is more specific than screening tests and false positives are minimal. Another, less commonly used confirmatory test is the immunofluoresence assay (IFA). Positive results from ELISA or rapid tests are commonly confirmed using a Western blot.

Alternate testing strategies

Other testing strategies besides a screening test followed by a confirmatory test have been proposed by the WHO and UNAIDS for use in low-resource settings where the Western blot may not be readily available or affordable. These strategies include using a combination of two screening tests (ELISA or rapid tests) without using the Western blot. Studies have shown that the use of two screening tests together can give results similar to, or in some cases better than, the use of a screening test followed by a confirmatory test, at a much lower cost. It is important to note that results will vary depending on the combination of screening tests used, so it is necessary to evaluate the intended combination before undertaking widespread implementation.

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