Screening for HIV Infection
Stephen L. Boswell
In the United States, it is estimated that more than 1.1 million people are infected with human immunodeficiency virus (HIV) and that approximately 235,000 persons are unaware of being HIV positive. The advent of accurate and increasingly convenient screening tests, highly effective antiretroviral therapy, and improved outcomes with early or prophylactic treatment argue strongly for screening, especially with a large reservoir of patients remaining undiagnosed. The early identification of HIV infection through routine screening has become an important mission for primary care practices.
As noted, the high prevalence of infected persons who remain undiagnosed poses substantial personal and public health risks. Those at highest risk of infection with HIV are men with a history of sex with other men or bisexual contact (˜60% of newly identified infections); intravenous drug users and their sexual contacts (˜12% newly identified infections); and the sexual contacts of homosexual or bisexual men. Approximately 30% of newly identified infections report heterosexual contact as the most likely mode of transmission. The risk of directly acquiring HIV infection through processed blood or selected blood products (plasma and clotting factor concentrates) has dramatically decreased since 1985 as a consequence of the widespread screening of those who donate blood and plasma, the use of serologic tests for HIV, and the viral inactivation of various plasma products.
When assessing the behavioral risk factors for acquiring HIV infection in an individual patient, the following questions are particularly useful in clinical practice. Has the patient
had sex with someone known to have HIV or AIDS?
had a history of sexually transmitted diseases?
had sex with many men or women or had sex with someone who has had sex with many men or women?
had sex with someone who has used needles to take drugs?
shared needles or works to take drugs?
If the answer to any of these questions is yes, then the patient and physician should give serious consideration of having HIV antibody testing performed.
Viral Dynamics and the Immunologic Response to HIV
After transmission of HIV, there is usually a 2- to 8-week period before the acute retroviral syndrome develops. This flu-like illness occurs in 50% to 90% of infected individuals, lasts 2 to 4 weeks, and resolves spontaneously. Coincident with the acute illness, high-grade viremia develops: Titers of 50 to 100 million particles per milliliter have been reported. Viral titers begin to fall 2 to 3 weeks after the onset of acute symptoms, usually before an HIV antibody response can be detected. This observation suggests that a response other than HIV antibody may be responsible for the initial control of virus replication. Recent evidence suggests a central role for HIV-specific cell-mediated immunity in the control of virus replication during primary HIV infection. Serial measurement of proliferative responses to HIV antigens using CD+ lymphocytes from acutely infected individuals demonstrated a strong inverse correlation between the generation of HIV-specific CD4+ lymphocyte helper cells and plasma HIV RNA levels. Similarly, among long-term nonprogressors, the highest HIV-specific proliferative responses had the lowest plasma HIV RNA levels.
Although the presence of symptoms is common among those who are acutely infected, the infection often goes unrecognized in the primary care setting because of the similarity of the symptoms to those of other common illnesses. Physicians should maintain a high level of suspicion for HIV infection in all patients presenting with a compatible syndrome. A careful evaluation of the patient’s risk behaviors during the 3-month period prior to presentation should be obtained for all such patients. When appropriate, laboratory testing should be performed. This testing may include measurement of HIV RNA by HIV polymerase chain reaction or HIV bDNA and HIV antibody. Although HIV RNA measurement is the preferred method of testing, a test for p24 antigen can be used when RNA testing is not readily available. A negative p24 antigen test does not rule out acute infection, however.
HIV Antibody Assays
Diagnosis of HIV infection is most frequently accomplished by detection of specific antibodies against viral antigens in serum. Tests for detection in saliva and urine have been developed. An important limitation of some of the HIV screening assays used in the United States is that they may not detect all species of HIV. A second species, HIV-2, largely confined to West Africa, is seen sporadically among immigrants from this region or their contacts. Attention should be given to the assay being used when testing individuals at risk for HIV-2 acquisition.
Serologic Enzyme Immunoassays
Although enzyme immunoassays (EIAs) are easily automated, inexpensive, and well suited for testing large numbers of samples, they can nonspecifically bind antibodies and produce false-positive results in patients exposed to other infections or vaccines. The assays have improved from first-generation tests that used viral lysates as a source of viral antigens to today’s more advanced tests using recombinant DNA proteins coated on paper strips, beads, and microplate wells. The most advanced assays have reported sensitivity and specificity of greater than 99.9% in serum from individuals with known HIV infection and uninfected controls.
Rapid Serologic Screening Tests
These have been developed for office use, including HIV antigen–coated gelatin or latex particle agglutination assays. These tests can be performed quickly (in some cases <10 minutes), but they still require Western blot confirmation. Data suggest that some particle agglutination assays may be slightly less sensitive and specific than EIA in moderate-or high-titer samples. Additional rapid screening assays using dipsticks and other technologies are available. Based on comparison with EIAs, several of these rapid assays appear significantly less sensitive when testing low-antibody titer samples. This may be a particular problem during primary HIV infection when antibodies against selected antigens may not yet be present or may be present at very low titer. In general, EIA with confirmatory Western blot is the preferred screening test.
Other Rapid Tests
These assays use whole blood, plasma, or serum. Several are CLIA-waived or of sufficient simplicity to enable performance in an office setting. A complete list of U.S. Food and Drug Administration (FDA)-approved rapid HIV antibody screening tests and their characteristics is available at http://www.cdc.gov/hiv/topics/testing/rapid/rt-comparison.htm.
Tests of Other Body Fluids
Body fluids other than blood may be used to determine an individual’s HIV status. Oral fluids and urine may contain antibodies against HIV. The FDA has approved a test using oral fluid. The OraQuick test collects oral mucosal transudate via a cotton swab and can yield a result within 20 to 40 minutes; a version of this test will soon be available for in-home use. It is hoped that the availability of testing that does not require trained personnel to administer or interpret will significantly increase the number of people getting tested for HIV. Currently, oral testing is not as accurate as blood testing. Therefore, a positive oral test should be considered a preliminary positive and should be confirmed with a blood test.
Confirmatory Testing—Western Blot Test
A positive EIA test should be confirmed by Western blot testing. Relative to EIA, the Western blot is slower, more labor intensive, and much more expensive, but it is able to measure specific HIV antibodies and thus enhance the specificity of testing. It detects antibodies directed at specific HIV proteins (core, envelope, and polymerase) after their separation by gel electrophoresis. Interpretation of reactive and nonreactive tests is based on data from clinical studies submitted to FDA for licensure.
The FDA criteria for Western blot interpretation
Positive—antibodies to multiple virus-specific protein bands are present
Indeterminate—fewer antibody bands are present on the blot
Negative—no antibody bands are present on the blot
When these criteria are used for interpreting test results, the probability of either a false-positive or a false-negative result is extremely small. It should be noted, however, that as many as 15% to 20% of tests on persons at low risk for HIV infection may be described as indeterminate. Sera from persons recently infected with HIV also may produce an indeterminate Western blot pattern. For such persons, a repeat Western blot on a second specimen obtained after the initial specimen often yields a positive blot pattern within 6 months.
Conversely, follow-up testing of uninfected persons whose serum had an indeterminate blot pattern on initial testing usually will show no change in the banding pattern. Serum from some HIV-infected persons who have advanced immunodeficiency may have an indeterminate pattern because of a loss of antibodies to nonenvelope proteins. In spite of the existence of a licensed Western blot test, many laboratories continue to use unlicensed tests because of cost and the stringent criteria required for interpreting the licensed test. The performance characteristics of the unlicensed tests have not been uniformly subjected to the same rigorous scrutiny required for licensure by the FDA. Although recommendations for standardization have been published, the extent to which these are followed is unknown. Information about production standards, interlot variability, or validation of criteria used for interpretation often is not available. The absence of standardization and appropriate quality controls may result in a lower sensitivity or specificity and, thus, a higher probability of inaccurate results.
Other Tests
Tests for virus and viral components are readily available in the United States. These tests include virus culture, p24 antigen assays, polymerase chain reaction assays, and signal amplification assays. In general, these assays are not used for the initial diagnosis of HIV infection in the adult.
Virus Culture
Culturing virus is time consuming, costly, difficult to standardize, and relatively insensitive.
Assays for p24 Antigen
Assays for p24 antigen are hampered by the low prevalence of antigen during the asymptomatic phase of HIV infection.
Polymerase Chain Reaction and Signal Amplification Assays
These assays have a relatively high false-positive rate and are not suitable for screening in most circumstances.
Surrogate Markers—CD4+ Lymphocyte Count
These do not test directly for virus or antibody. The most common, the CD4+ lymphocyte count, is useful for assessing prognosis and therapeutic decision making (see Chapter 13), but it should not be used as an indirect method of determining whether a patient is HIV infected.
Test Interpretation—Minimizing False Positives and False Negatives
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