In April of this year, Warde Medical Laboratory switched to a fourth-generation screening platform for detection of infection by human immunodeficiency virus (HIV). The fourth generation platform — available for some time in Europe but only recently FDA approved in the US — differs from third generation platforms in the addition of detection of the HIV-1 p24 antigen in the initial ELISA screen. (1)

Fourth generation screening platforms also continue to detect antibodies directed against HIV-1, HIV-2, and HIV-O variant as was the case with the previous generation of HIV screening methods. In addition to updating the initial screen for HIV, Warde has also changed the positive screen follow up with testing that follows a diagnostic algorithm proposed by the U.S. Centers for Disease Control and Prevention (CDC) and the Association of Public Health Laboratories (APHL), and formally adopted by the Clinical Laboratory Standards Institute (CLSI) (Figure 1). (2)

According to this algorithm, the previous step of Western Blot confirmation is replaced by a rapid test for discrimination of HIV-1 from HIV-2 infection (“Multi-Spot”, BioRad), and, if necessary in the case of discordant results, nucleic acid testing (NAT) for HIV-1. Currently, two manufacturers (Abbott and BioRad) make FDA-approved fourth generation screening platforms. Warde is using the BioRad Evolis platform for this purpose. The rapid HIV-1/HIV-2
test as adopted by the CDC/APHL/CLSI algorithm is currently manufactured only by BioRad as the MultiSpot assay.

Figure 1. A new algorithm for HIV screening, as proposed by CDC/ APHL, and adopted by CLSI (Figure adapted from Styer et al, 2011)

There are several advantages to the new diagnostic algorithm.

First, the inclusion of p24 antigen detection in the initial screen allows for the earlier detection of new HIV infection by days or weeks compared to third-generation platforms. Studies have shown that the highest rate of HIV transmission occurs during a period of p24 antigenemia (generally detectable 2 to 3 weeks after initial infection) that precedes the development of detectable anti-HIV antibodies (which may take 3 to 12 weeks to develop to detectable levels). (3)  It is hoped that the ability to detect HIV infection during this early window period may help to slow the rate of HIV transmission among the population.

Second, the new algorithm allows for the rapid distinction between antibodies directed against HIV-1 from antibodies directed against HIV-2. Currently, HIV-2 infection (still quite rare in North America) may cross-react with the HIV-1 Western blot, causing false-indeterminate or false-positive results ( Figure 1). (4)

Third, the MultiSpot assay will likely decrease the cost and turnaround time associated with determination of HIV status in infected individuals. While Western blot has high specificity and positive predictive value for HIV-1 infection, it is relatively insensitive compared to newer methods, and there is the potential for cross-reactivity with HIV-2. (4)  Western blot may also have a relatively high rate of false-indeterminate results, especially in low-prevalence populations. Indeterminate results on HIV-1 Western blot (i.e. one or more bands present, but not fulfilling CDC criteria for a positive result) may indicate either a cross-reacting antibody in an HIV-negative patient, cross-reacting antibodies in an HIV-2 positive patient, or early HIV-1 infection. Therefore, indeterminate Western blot results currently are issued with a recommendation to re-test in 4 weeks to assess for the acquisition of additional bands that may indicate early HIV infection. While this has the potential to detect early HIV infection, it can also cause undue anxiety in HIV-negative patients who may harbor cross-reacting antibodies to HIV. Indeed, in a study performed by CDC of blood donors who screened positive for HIV and had indeterminate Western blot results due to antibodies against HIV GAG proteins, only 5 of 209 eventually seroconverted over a 2-year period, and those who did seroconvert did so on their first repeat sample. (5)  At the time, the CDC indicated that individuals with persistent indeterminate Western blot results without acquisition of new bands should be considered HIV negative.

In the new algorithm, a repeatedly positive fourth generation screen will be tested via MultiSpot. If MultiSpot reveals antibodies to HIV-1, then the tested sample is considered positive for HIV-1. If the MultiSpot reveals antibodies to HIV-2, then the tested sample is considered positive for HIV-2. If the MultiSpot is negative in the setting of a positive fourth generation screen, then the algorithm proceeds to HIV nucleic acid testing (NAT). If NAT is positive for HIV-1, then the patient is considered positive for HIV-1. If NAT is negative for HIV-1, then the patient is considered negative for HIV-1. (6,7)

Referring clinicians will have to make individual determinations regarding any potential need for HIV-2 nucleic acid testing
in patients with positive initial 4th generation screens but non-reactive MultiSpot assays and negative HIV-1 NAT results. At present there is not an FDAapproved NAT assay for detection of primary HIV-2 infection, but non-FDA approved HIV-2 NAT is available as a sendout through Warde. Clients should keep in mind, however, that HIV-2 infection remains quite rare in individuals native to North America. 90% of reported HIV-2 cases in the US are from individuals who have immigrated from African nations, mainly in west Africa. Fewer than 170 cases of HIV-2 infection were confirmed by CDC between 1987 and 2009 (constituting only 0.01% of the more than 1.4 million cases of HIV infection during that time period), and only 3% of these cases (5 individuals) were born in the United States. (8)

Since NAT requires a sample that has not been previously tested (due to the sensitivity of NAT and concerns for contamination), and since NAT should be done within 72 hours of sample acquisition, Warde will report samples with positive 4th generation screens and negative MultiSpot assays as “HIV INDETERMINATE,” with a recommendation to pursue HIV-1 NAT on a repeat sample.

Results for specimens sent to Warde/MCL for primary screening for HIV will be reported in one of four ways, as outlined in Figures 2 through 5.

Since CDC/APHL/CLSI guidelines still allow for alternative diagnostic algorithms that include Western blot confirmation, Warde will continue to offer Western blot testing to those clients that wish to pursue this test after a positive HIV screen in their own laboratory. However, when a sample is sent to Warde for primary screening, positive screens will be tested via MultiSpot, rather than by Western blot.