The current HIV testing algorithm, which was recommended by the Centers for Disease Control and Prevention (CDC) in 1989, indicates ‘no positive test results should be given to clients/patients until a screening immunoassay has been repeatedly reactive on the same specimen and a supplemental, more specific test such as the western blot has been used to validate those results’ . The western blot detects anti-HIV antibody in a human serum sample infected with HIV; however, it cannot detect acute infections (period prior to detectable antibody), which have been associated with a higher probability of disease transmission compared with established infections [2–4]. The HIV-1 western blot also misclassifies many HIV-2 infections as HIV-1, which is problematic because HIV-2 infections do not respond to many first-line antiretroviral agents, including nonnucleoside reverse transcriptase inhibitors and some protease inhibitors . In 2010, an alternative laboratory HIV diagnostic testing algorithm was proposed  (Fig. 1) that is designed to detect early infections, reduce indeterminate results, and identify HIV-2 infections [7–10]. The alternative diagnostic algorithm involves screening with a sensitive fourth-generation antigen/antibody HIV-1/2 immunoassay, or if unavailable, a third-generation HIV-1/2 immunoassay. When the screening immunoassay is repeatedly reactive, it is followed with an HIV-1/HIV-2 antibody differentiation test. If the differentiation test is reactive, the result is positive for either HIV-1 or 2 antibodies or both. However, when the HIV antibody differentiation test results are negative, an HIV-1 nucleic acid test (NAT) is used to resolve infection status. Persons with a positive NAT and a negative differentiation test are considered to have acute HIV-1 infection.
To date, HIV NAT has not been used widely for diagnosis due to its labor requirements, cost, and uncertainty about whether acute infections would be identified in certain populations [4,11]. Recently, the Food and Drug Administration (FDA) approved fourth-generation immunoassays that detect p24 antigen and HIV-1 and HIV-2 antibodies . These assays have the ability to detect more than 80% of acute HIV infections otherwise detectable only by NAT [13–15]. The commercial availability of fourth-generation HIV-1/2 assays will make simultaneous screening for both acute and established HIV infections feasible for most clinical laboratories. However, the specificity of these screening tests must be evaluated in low prevalence settings because of cost implications associated with NAT to resolve false-positive fourth-generation immunoassay screening test results. In this study, we evaluated the performance of the FDA-approved fourth-generation assay, the GS HIV Combo Ag/Ab immunoassay (Bio-Rad Laboratories, Redmond, Washington, USA) , as part of the alternative laboratory HIV diagnostic testing algorithm compared to the current algorithm (repeatedly reactive third-generation immunoassay/HIV-1 western blot). The evaluation was conducted using specimens from a low prevalence population, persons with established infections, and seroconverters.
Quest Diagnostics obtained three sets of de-identified residual serum/plasma specimens and processed them at their Lenexa, Kansas facility: 10 014 specimens from life insurance applicants, a population that typically has low HIV prevalence (<0.1%) ; 493 previously tested GS HIV-1 western blot-positive specimens from life insurance applicants; and 20 previously tested GS HIV-1 western blot-indeterminate specimens submitted for diagnostic testing. In addition, CDC obtained 230 serial plasma specimens from 26 US donors early in the process of HIV seroconversion (presumably infected with subtype B) from BBI-SeraCare Diagnostics (West Bridgewater, Massachusetts, USA) and Zeptometrix Inc. (Buffalo, New York, USA).
At Quest Diagnostics, all specimens (except those from the HIV seroconversion panels) were tested according to package insert instructions with both the GS HIV-1/HIV-2 Plus O third-generation immunoassay  and the GS HIV Combo Ag/Ab fourth-generation immunoassay (Bio-Rad Laboratories) using the EVOLIS Automated Microplate System (Bio-Rad Laboratories). Specimens that were initially reactive with the third-generation or fourth-generation immunoassay were tested in duplicate on the same assay, and specimens reactive on one or both of the repeat tests were considered to be repeatedly reactive. Specimens repeatedly reactive by third-generation or fourth-generation immunoassay were tested with a rapid HIV-1/HIV-2 differentiation assay, Multispot HIV-1/HIV-2 Rapid test (Bio-Rad Laboratories) , and if negative, by the APTIMA HIV-1 RNA Qualitative assay (Gen-Probe Incorporated, San Diego, California, USA) . Specimens with HIV-2-positive results on the differentiation assay were also tested with HIV-2 immunoassay (Bio-Rad Laboratories) and research use only (RUO) QualiCode HIV-1/2 Western Blot Kit (QualiCode; Immunetics, Boston, Massachusetts, USA).
At the CDC, specimens from the HIV seroconversion panels were tested manually by GS third-generation and fourth-generation immunoassays in singlet, Multispot and APTIMA. Because all specimens used in this study were unlinked from personal identifiers, this study was determined by the CDC to be research not involving identifiable human participants.
Data management and analysis
Test results generated at Quest Diagnostics were recorded in an Excel database that was subsequently provided to CDC via secure data network. For specimens from low HIV prevalence life insurance applicants, we calculated the specificities of third-generation and fourth-generation immunoassays and used the Mid-P exact test to calculate 95% confidence intervals (CIs). For western blot-positive and western blot-indeterminate specimens, we compared the results of the current and alternative laboratory HIV diagnostic testing algorithms. Samples from all specimen sets were considered to be from HIV-infected individuals if they were either western blot-positive or NAT-positive. Analyses were performed using SAS 9.2 (SAS Institute Inc., Cary, North Carolina, USA) statistical software.
For seroconversion panels, the analyses were conducted as described previously . Differences in the number of HIV-1 infections detected by the current and alternative laboratory HIV diagnostic algorithms were statistically analyzed using McNemar's test with one degree of freedom and continuity correction.
Specimens from a low prevalence population
Thirteen (0.13%) of 10 014 specimens from life insurance applicants were repeatedly reactive on at least one third-generation or fourth-generation immunoassay (Table 1). Of these, two specimens repeatedly reactive by both third-generation and fourth-generation immunoassays were HIV-1-positive by western blot and Multispot (i.e., considered HIV-1-positive by both algorithms). Two third-generation immunoassay and nine fourth-generation immunoassay results were false-positive based on available testing data (Table 1): one specimen repeatedly reactive only by third-generation immunoassay and eight repeatedly reactive only by fourth-generation immunoassay were negative by western blot, Multispot, and NAT; one specimen repeatedly reactive only by third-generation immunoassay and another repeatedly reactive only by fourth-generation immunoassay were western blot-indeterminate and negative by Multispot and NAT. Specificities of third-generation (10 010/10 012) and fourth-generation immunoassays (10 003/10 012) were 99.98% (95% CI 99.93–100%) and 99.91% (95% CI 99.84–99.96%), respectively. Specificity values for both assays were within the confidence bounds reported within the product package inserts for low-risk populations. No acute HIV infections were identified among specimens from these life insurance applicants.
HIV-1 western blot-positive specimens
All 493 HIV-1 western blot-positive specimens were third-generation and fourth-generation repeatedly reactive (i.e., classified HIV-1-positive by current algorithm). With the alternative algorithm, 491 (99.6%) were HIV-1-positive by Multispot and two (0.4%) were HIV-2-positive by Multispot, HIV-2 immunoassay, and HIV-2 western blot.
HIV-1 western blot-indeterminate specimens
Twelve (60%) of the 20 HIV-1 western blot-indeterminate specimens were repeatedly reactive by third-generation immunoassay, but not by fourth-generation immunoassay, and were negative by Multispot and NAT (i.e., classified as HIV-negative by the alternative diagnostic testing algorithm). Eight (40%) of the 20 western blot-indeterminate specimens were repeatedly reactive by third-generation and fourth-generation immunoassay. Of these, six were negative by Multispot and NAT (i.e., classified as negative by the alternative algorithm), and two were reactive by Multispot (i.e., classified as antibody-positive by the alternative algorithm) but NAT-negative. One of these two HIV-1 western blot-indeterminate specimens had p24 and p55 western blot bands present, and the other had a gp160 band.
Specimens from HIV-1 seroconverters
The third-generation immunoassay was reactive with 102 seroconverter specimens and the fourth-generation immunoassay with 131 (Table 2). The use of Multispot as a supplemental test improved the correct classification of HIV-1 infections compared with the western blot regardless of the screening immunoassay used. A total of 90 specimens were positive using Multispot as the supplemental test, whereas only 56 were confirmed positive by the western blot (Table 2). Two seroconverters with reactive NAT results at earlier bleeds had a total of four subsequent specimens that were negative by NAT but positive by Multispot and positive (n = 3) or indeterminate (n = 1) by western blot. Multispot was negative in 12 seroconverter specimens that were reactive by third-generation immunoassay and 41 that were reactive by fourth-generation immunoassay. Use of the entire alternative algorithm including NAT further improved detection of HIV-1 infections compared with the western blot, and correctly classified as positive 102 seroconverter specimens with the third-generation immunoassay and 130 with the fourth-generation immunoassay compared with 56 using the current algorithm with either immunoassay (Table 3).
The GS HIV Combo Ag/Ab fourth-generation immunoassay reduced the diagnostic window compared with the third-generation immunoassay [7,15], enabling detection of acute HIV infections only otherwise detected by amplified RNA methods. This screening assay also performed with high specificity in a low HIV prevalence population . Similar performance characteristics have been observed in studies of the only other FDA-approved fourth-generation immunoassay on the US market, the Abbott ARCHITECT HIV Ag/Ab Combo . In the current study, the alternative laboratory HIV testing algorithm using the GS HIV Combo Ag/Ab fourth-generation immunoassay not only correctly resolved the infection status of specimens from individuals with established HIV infections that had been detected using the HIV-1 western blot assay, but also identified two HIV-2 infections in specimens misclassified as HIV-1 by western blot.
When used as part of the alternative algorithm in populations with persons at high risk of HIV during their early infection period, fourth-generation immunoassays will enable testing programs to detect HIV infections earlier. Even when a third-generation immunoassay is used as the initial test in the alternative algorithm, more early infections will be detected than in the current algorithm when a third-generation immunoassay is used with western blot. Individuals in the acute stage of HIV infection are usually highly viremic, resulting in more virus shedding at mucosal sites, and are therefore more likely to transmit HIV infection through bodily secretions . Estimates of the proportion of total HIV transmissions attributable to early infections range widely from 6 to 49.4% [3,22,23]. To use the alternate algorithm to its full advantage to detect early infections and reduce transmissions, laboratories must be prepared to conduct NAT testing with rapid turnaround and public health systems must be in place to conduct timely linkage to medical care and partner services . Detection of early HIV infection may also play a key role in the success of treatment as prevention by offering available antiretroviral drugs to high-risk people before or immediately after HIV exposure or as a means of secondary prevention .
The Bio-Rad GS fourth-generation immunoassay demonstrated high specificity in a very low-prevalence population. The specificity is similar to that obtained with third-generation immunoassays . However, with the western blot-indeterminate specimens, more false-positive results appeared to occur with third-generation immunoassay compared with the fourth-generation immunoassay, potentially due to cross reaction with the p24 antigen present in the third-generation assay, but not the fourth. This study demonstrated that few NATs will be required to resolve specimens with false-positive screening assay results when the alternative HIV diagnostic testing algorithm is utilized. Further, in our study, the alternative algorithm correctly identified the HIV infection status among all HIV-1 western blot-positives, highlighting its utility in a variety of settings in the USA. In addition, even though HIV-2 is rare in the USA , the alternative algorithm identified HIV-2 infections that would otherwise have been misclassified as HIV-1 infections by the HIV-1 western blot, as has been noted in other studies [10,27]. With the availability of supplemental testing technologies that differentiate HIV-1 from HIV-2 and widespread use of the alternative algorithm, it is likely that more HIV-2 infections will be recognized.
The alternative HIV diagnostic testing algorithm has previously been shown to work well in high-risk individuals  and in persons with established HIV-1 infection when used with a third-generation screening immunoassay . In this study of over 10 000 specimens, excluding the seroconversion specimens, the alternative algorithm correctly resolved the infection status of all specimens except two. These two specimens were repeatedly reactive by third-generation and fourth-generation immunoassay, and HIV-1 western blot-indeterminate, Multispot HIV-1-reactive, and NAT-negative, indicating discordance between serologic and virologic markers of infection. These specimens highlight the difficulty of resolving the true infection status without follow-up testing with a subsequent sample. It is possible that they represent false-positive results by the alternative algorithm. Alternatively, they might be similar to the four study specimens from seroconverters that were HIV-1-positive by serology but NAT-negative during follow-up. Other studies have shown that 3–5% of HIV-1 antibody-positive specimens [28,29] are negative by NAT. Persons classified as HIV antibody-positive by the alternative algorithm (positive third-generation or fourth-generation immunoassay and Multispot results) who have a negative NAT after entering care will need further diagnostic testing .
There are several theoretical limitations associated with these analyses. First, factors beyond assay performance may have been responsible for the limited number of false-positive and false-negative results. False-positive results may have occurred due to contamination of samples or specimen mix up, and false-negative NAT results in stored, frozen specimens tested retrospectively may have happened due to degradation of viral RNA by microbes or multiple freeze-thaws [31–33]. Second, for the HIV-1 western blot-indeterminate specimens, we did not have the ability to obtain follow-up samples and, thus, the ultimate infection status is not known with certainty. Third, even though our study demonstrated the excellent performance of the alternative laboratory HIV diagnostic testing algorithm in selected specimen sets, we did not assess its performance with known HIV-2-positive specimens or in an actual population at high risk for acute infection. Finally, although the performance of the alternative diagnostic testing algorithm was assessed in this study using APTIMA, the only NAT approved by the FDA for diagnosis, it may be of additional benefit to assess the performance of quantitative HIV RNA assays, which are used widely in clinical settings for therapeutic monitoring .
The Bio-Rad fourth-generation HIV-1/2 immunoassay demonstrated high sensitivity for both early and established infections, and high specificity in a low HIV prevalence population. When coupled with the alternative supplemental tests, testing jurisdictions will gain the ability to corroborate reactive screening test results for early infections, identify HIV-2 infections, and reduce indeterminate results. These features make this algorithm a good alternative to the current HIV testing algorithm, which utilizes western blot or immunofluorescence assay (IFA) as the definitive supplemental test. However, as with any diagnostic testing, low numbers of false-positive results may occur that will warrant additional diagnostic testing.
The authors would like to acknowledge the technical staff of the Quest Diagnostics, Lenexa, Kansas facility for processing and testing these specimens.
Conflicts of interest
No financial disclosures were reported by the authors of this article.
The findings and conclusions in this article are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention or the US Department of Health and Human Services. The use of trade names and commercial sources is for identification only and does not imply endorsement by the US Department of Health and Human Services.
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Keywords:© 2013 Lippincott Williams & Wilkins, Inc.
fourth-generation immunoassay; HIV testing algorithms; specificity