Reduction of the diagnostic window with a new combined p24 antigen and human immunodeficiency virus antibody screening assay

Combination assay detecting both human immunodeficiency virus (HIV) p24 antigen and anti-HIV antibodies opens a second diagnostic window

Fourth-generation human immunodeficiency virus (HIV) screening immunoassays reduce the diagnostic window between infection and diagnosis by the inclusion of HIV p24 antigen detection together with HIV antibody detection in the same test. We compared third- and fourth-generation HIV immunoassays and a dedicated HIV p24 antigen test for detection of a case of HIV seroconversion. This demonstrated a second diagnostic window using the fourth-generation assay due to a decline of HIV p24 antigen prior to the detection of HIV antibody. However, HIV p24 antigen was detected in the same sample by the dedicated HIV p24 antigen test, as was HIV proviral DNA. Although it is likely to be rare, this phenomenon has also been reported for other fourth-generation HIV immunoassays and has implications for the reported diagnostic windows of these assays.

Comparative evaluation of the VIDAS HIV DUO Ultra assay for combined detection of HIV-1 antigen and antibodies to HIV

The aim of the study was to evaluate the performance of the combined antigen and antibody HIV screening assay VIDAS HIV DUO Ultra (BioMérieux, Marcy l'Etoile, France) in comparison with two other combined tests: the former version of the same test (VIDAS HIV DUO, BioMérieux) and the AxSYM HIV Ag/Ab Combo assay (Abbott Laboratories, Rungis, France). A prospective study was performed on serum specimens received on a routine basis for HIV testing: 1443 blood samples were tested with the three assays. Sensitivity was 100% for the three tests. Specificity assessed on repeated false-positive samples was 99.86, 99.03 and 99.65% for VIDAS HIV DUO Ultra, VIDAS HIV DUO and AxSYM HIV Ag/Ab Combo, respectively. In addition, 14 seroconversion panels were tested with the VIDAS DUO Ultra and AxSYM HIV Ag/Ab Combo assays. For four of these panels, a positive signal was detected one blood sampling point earlier with the VIDAS DUO Ultra assay, corresponding to a higher sensitivity of the HIV antigen test. These results indicate that the VIDAS HIV DUO Ultra exhibits an improved specificity with comparison to the former version of this assay and an excellent sensitivity for early detection of HIV seroconversion.

A 10-minute, US Food and Drug Administration-approved HIV test

It is now an established fact that rapid HIV tests have important applications in a number of testing situations. Currently, four rapid HIV assays have been approved by the US Food and Drug Administration, including the one discussed in this review. A rapid, lateral flow HIV assay was evaluated at the University of Maryland, USA, one of several sites involved in the clinical evaluation. Samples used in the evaluation totaled 9000 and consisted of serum, plasma and venous whole-blood sets of samples from 3000 study subjects that included population groups considered to be at high risk for HIV infection (n=1000), from HIV-positive individuals (n=1000), and from population groups considered at low risk for HIV infection (n=1000). US Food and Drug Administration-licensed screening and confirmatory assays were used as reference tests. The rapid assay exhibited a sensitivity of 100% across all three sample media and exhibited a specificity of 99.8% using whole blood and plasma and 99.7% using serum. This rapid assay is an excellent addition to the existing US Food and Drug Administration-approved rapid HIV tests, and provides versatility by allowing the testing of venipuncture and fingerstick whole-blood samples in addition to serum and plasma.

Advances in laboratory testing for HIV

In 2004, the diagnosis of established human immunodeficiency virus (HIV) infection can be made with close to 100% assurity. The extraordinarily engineered performances of HIV-screening assays are unprecedented. The well-established confirmatory tests performed by well-versed laboratories using criteria that are well understood in order to interpret the results of these tests give highly accurate outcomes of diagnostic testing strategies. Furthermore, the ability to monitor the progress of the infection and the viral pathogenesis is possible through the use of tests that quantify viral load or the peripheral CD4+ T-cells and other lymphocyte sub-type levels. Newer laboratory testing mechanisms, such as assessment of reverse transcriptase activity and sophisticated cell staining and flow cytometric analyses, have been used to map disease processes and progress on a research level and may be used in future to fine-tune therapy and to follow disease progression in even greater detail. Regulation of all HIV tests is of the highest level in Australia. In-house tests will be expected to conform to the levels specified for commercially produced tests.

Evaluation of the sensitivity and specificity of six HIV combined p24 antigen and antibody assays

In this study, we evaluated the performance of six HIV combined p24 antigen and antibody (Ag/Ab) assays versus two third-generation anti-HIV antibody assays. The assays were evaluated using p24 antigen panel of 31 HIV-1 subtypes (n = 124), 25 HIV-1 seroconversion panels (n = 176), HIV-1 antibody positive samples including group M subtypes and group O (n = 559), HIV-2 positive samples (n = 110), and unselected HIV negative samples from four French private laboratories (n = 1005). The results showed that overall HIV combined Ag/Ab assays present better performance, when compared to antibody-only assays. However, some differences were observed in the sensitivity of the six HIV combined Ag/Ab assays evaluated. The AxSYM and Murex Combo assays had the best sensitivity score in this study and reduced the window period by 2.0-2.35 days relative to antibody only assays and 1-2.17 days relative to the other combined Ag/Ab assays. Among combined HIV Ag/Ab assays, Genscreen Plus and AxSYM Combo presented the highest specificity, with 99.9% and 99.8%, respectively.

Evaluation of Four Simple/Rapid Assays and Two Fourth-Generation ELISAs for the Identification of HIV Infection on a Serum Panel Representing the HIV-1 Group M Genetic Diversity in Cameroon

The performance of 4 rapid and simple assays: Camstix-HIV 1+2 (Camdiagnostix, Yaounde, Cameroon); Determine HIV 1+2+0 (Abbott Laboratories, Tokyo, Japan); Genie II HIV-1/HIV-2 (Bio-Rad, Marnes la Coquette, France); ImmunoComb II HIV 1 & 2 BiSpot (Orgenics, Yavne, Israel); and 2 fourth-generation ELISAs: Enzygnost HIV Integral (Dade Behring, Marburg, Germany) and Genscreen plus HIV Ag-Ab (Bio-Rad, Marnes la Coquette, France) currently used in Cameroon to detect HIV infections were evaluated on a local serum panel. A total of 503 samples were collected, using the Camstix-HIV 1+2 assay. Overall, 280 samples were confirmed HIV positive, 181 were negative, and 42 were indeterminate. All positive samples belonged to group M: CRF02_AG (73.5%), A1 (7.1%), A2 (1.2%), G (4.7%), F2 (5.1%), D (1.6%), CRF11 (1.6%), CRF06 (1.2%), and CRF01_AE (1.6%). Sensitivity, specificity, test efficiency, and positive and negative predictive values were calculated both including and excluding indeterminate samples. Except for Genie II and ImmunoComb II (98.9 and 99.3%, respectively), sensitivities were 100% for the remaining 4 tests. Specificities, efficiencies, and positive predictive values of all assays were negatively affected by the addition of HIV-indeterminate samples in the calculations. These data show the importance of prior test evaluations on local serum panels and in field conditions before a national policy for HIV screening is decided on and stress also the need to use tests and algorithms that can reduce the high number of HIV-indeterminate results in Africa.

Virological exploration of individuals with discordant HIV screening tests

Screening for HIV infection can use many algorithms. When two different HIV antibody assays are used, discordant results may occur. To discriminate between HIV seroconversion, HIV variant infection and false positive reactivity, 30 consecutive subjects with two discordant HIV antibody-screening assays were extensively investigated for HIV infection. No subject had HIV seroconversion or reached HIV seropositivity criteria after a follow-up of 3 months. By contrast 36% became HIV negative by the use of both HIV screening assays. p24 Antigen, HIV-1 RNA, HIV-1 DNA, HIV-2 DNA assays and HIV isolation by sensitive culture were unable to identify HIV infection in the 30 subjects with discordant HIV screening assays. The data suggest that the use of two HIV screening assays increase false-positive HIV results without increasing clinical sensitivity. To compliment follow-up of HIV screening, early testing for HIV RNA could be useful to identify or eliminate a recent infection.

Multicenter evaluation of a new, automated enzyme-linked immunoassay for detection of human immunodeficiency virus-specific antibodies and antigen

A collaborative multicenter study was conducted to evaluate the sensitivity, specificity, and precision of a three-step, fully automated, qualitative microparticle-based enzyme-linked immunoassay (AxSYM HIV Ag/Ab Combo; Abbott Laboratories), designed to simultaneously detect (i). antibodies against human immunodeficiency virus type 1 (HIV-1) and/or type 2 (HIV-2) and (ii). HIV p24 antigen. A significant reduction in the HIV seroconversion window was achieved by combining detection of HIV antibodies and antigen into a single assay format. For 22 selected, commercial HIV seroconversion panels, the mean time of detection with the combined-format HIV antigen-antibody assay was reduced by 6.15 days compared to that with a similar third-generation single-format HIV antibody assay. The quantitative sensitivity of the combination assay for the p24 antigen (17.5 pg/ml by use of the p24 quantitative panel VIH SFTS96') was nearly equivalent to that of single-format antigen tests. The combination assay demonstrated sensitive (100%) detection of anti-HIV immunoglobulin in specimens from individuals in CDC stages A, B, and C and from individuals infected with different HIV-1 group M subtypes, group O, or HIV-2. The apparent specificity for hospitalized patients (n = 1938) was 99.90%. In a random population of 7900 volunteer blood donors, the specificity (99.87%) was comparable to that of a third-generation single-format HIV antibody assay (99.92%) on the same donor specimens. In addition, the combination assay was robust to potential interfering specimens. The precision of the combination was high, with intra- and interrun variances of <or=9.3% for each precision panel specimen or assay control and <or=5.3% for the negative assay control.

Reduction of the diagnostic window in three cases of human immunodeficiency-1 subtype E primary infection with fourth-generation HIV screening assays

BACKGROUND AND OBJECTIVES

The influence of genetic variability on the sensitivity of serological diagnosis of human immunodeficiency virus (HIV) infection has, to date, been poorly investigated. The aim of the present study was to assess whether fourth-generation assays for the combined detection of HIV antigen and antibodies to HIV (anti-HIV) permit a reduction of the diagnostic window in comparison to third-generation antibody enzyme immunoassays (EIAs), which so far have shown a poor sensitivity for detection of HIV-1 non-subtype B primary infections.

MATERIALS AND METHODS

Three patients with primary HIV-1 subtype E (CRF01-AE) infection were tested with different third- and fourth-generation assays, stand-alone HIV antigen (Ag) EIAs and reverse transcription-polymerase chain reaction (RT-PCR). Additionally, virus lysates from HIV-1 Group M and O and HIV-2, at concentrations of p24 Ag close to the detection limit of licensed HIV Ag EIAs, were investigated with fourth-generation EIAs and HIV Ag EIAs.

RESULTS

In the first blood donor, the most sensitive fourth-generation assay detected HIV-1 infection 11 days earlier than five of the eight third-generation antibody assays. Fourth-generation EIAs, with a high sensitivity for HIV antigen, detected HIV-1 subtype E infection simultaneously or 4 days later than HIV-1 RT-PCR on pooled samples. Low concentrations of virus lysates of different HIV-1 subtypes A-H and group O, tested positive with fourth-generation EIAs, with a high sensitivity of the antigen-detection module.

CONCLUSIONS

Fourth-generation EIAs, especially those with a high sensitivity for HIV-1 p24 antigen, reduce the diagnostic window for primary HIV-1 subtype E infection in comparison with third-generation antibody-screening assays. These preliminary data from seroconversions and virus lysates indicate that the genetic diversity of HIV-1 does not represent a major challenge for the most sensitive EIAs of this new assay generation.

Rapid confirmation of HIV infection

OBJECTIVE

To investigate the utility of a prototype rapid confirmatory HIV assay which offers specific results in 5 min and has applications in a variety of important testing situations.

METHODS

The performance of the rapid confirmatory assay was assessed with 849 blood samples, including serum, plasma, venipuncture whole blood, and peripheral blood collected via fingerstick. Included were over 700 HIV Western blot (WB)-confirmed antibody positive sera, and others which were classified as negative or indeterminate by WB. The analytic sensitivity of the rapid confirmatory assay was assessed using 13 HIV-1 seroconversion panels, and all results were compared to those of an FDA-licensed WB reference test.

RESULTS

The rapid test exhibited 100% concordance with the reference test when testing HIV-1 WB-confirmed positive samples, and 92.3% concordance with samples having WB-inconclusive results. The sensitivity for confirming recent seroconversion was as good, or better than, the FDA-licensed HIV-1 WB in 10/13 panels. The rapid assay performed accurately with whole blood collected from fingerstick, and exhibited excellent precision and reproducibility.

CONCLUSION

We conclude that this rapid HIV confirmatory assay, the first of its kind, demonstrates proof of principle for the accurate confirmation of HIV-1 infection and offers important advantages in public health and clinical testing venues.

Multicenter evaluation of a new automated fourth-generation human immunodeficiency virus screening assay with a sensitive antigen detection module and high specificity

Fourth-generation assays for the simultaneous detection of human immunodeficiency virus (HIV) antigen and antibody that were available on the international market until now have antigen detection modules with relatively poor sensitivity and produce a higher rate of false-positive results than third-generation enzyme immunoassays (EIAs). The new Cobas Core HIV Combi EIA with an improved sensitivity for HIV p24 antigen was compared to alternative fourth- and third-generation assays, the p24 antigen test, and HIV type 1 (HIV-1) RNA reverse transcriptase PCR (RT-PCR). A total of 94 seroconversion panels (n = 709 sera), samples from the acute phase of infection after seroconversion (n = 32), anti-HIV-1-positive specimens (n = 730) from patients in different stages of the disease, 462 subtyped samples from different geographical locations, anti-HIV-2-positive sera (n = 302), dilutions of cell culture supernatants (n = 62) from cells infected with different HIV-1 subtypes, selected performance panels from Boston Biomedica Inc., 7,579 unselected samples from blood donors, 303 unselected daily routine samples, 997 specimens from hospitalized patients, and potentially interfering samples (n = 1,222) were tested with Cobas Core HIV Combi EIA. The new assay showed a sensitivity comparable to that of the Abbott HIV-1 AG Monoclonal A for early detection of HIV infection in seroconversion panels. The mean time delay of Cobas Core HIV Combi EIA (last negative sample plus 1 day) in comparison to that for HIV-1 RT-PCR for 87 panels tested with both methods was 2.75 days. The diagnostic window was reduced with Cobas Core HIV Combi EIA by between 3.6 and 5.7 days from that for third-generation assays. The specificities of Cobas Core HIV Combi EIA in blood donors were 99.84 and 99.85% (after repeated testing). Overall, 30 repeatedly reactive false-positive results out of 10,031 HIV-negative samples were obtained with Cobas Core HIV Combi EIA. Our results show that a fourth-generation assay with improved specificity such as Cobas Core HIV Combi EIA is suitable for blood donor screening because of its low number of false positives and because it detects HIV p24 antigen with a sensitivity comparable to that of single-antigen assays.

Significant closure of the human immunodeficiency virus type 1 and hepatitis C virus preseroconversion detection windows with a transcription-mediated-amplification-driven assay

While the present generation of serology-based assays has significantly decreased the number of human immunodeficiency virus type 1 (HIV-1) and hepatitis C virus (HCV) infections acquired by transfusion, the possibility of infected donations escaping detection still exists. The average seronegative viremic window duration during which immunological assays are unable to detect the virus is estimated to be between 16 and 22 days for HIV-1 and approximately 70 days for HCV. Significant reduction of detection window duration was demonstrated using a nucleic acid amplification assay, the Procleix HIV-1/HCV Assay, which utilizes transcription-mediated amplification technology to simultaneously detect HIV-1 and HCV RNAs. For 26 commercially available HIV-1 seroconversion panels tested, specimens were reactive in the HIV-1/HCV assay at the same time as or earlier than in serological assays. Overall, the HIV-1/HCV assay was able to reduce the detection window duration by an average of 14 days and 6 days compared to tests relying on recognition of HIV-1 antibody and p24 antigen, respectively. For 24 commercially available HCV seroconversion panels tested, the specimens were reactive in the HIV-1/HCV assay at an earlier blood sampling date than in serological assays, reducing the detection window duration by an average of 26 days. Similar results were obtained in testing the HIV-1 and HCV seroconversion panels in the virus-specific HIV-1- and HCV-discriminatory assays, respectively. In conclusion, the HIV-1/HCV assay and corresponding discriminatory assays significantly reduced detection window durations compared to immunoassays.

Evaluation of a new combined antigen and antibody human immunodeficiency virus screening assay, VIDAS HIV DUO Ultra

Combined antigen and antibody screening (fourth-generation) assays reduce the diagnostic window period between the time of human immunodeficiency virus (HIV) infection and laboratory diagnosis by 4 days, on average, in comparison to antibody-only (third generation) enzyme immunoassays (EIAs). The aim of the present study was to assess whether the new VIDAS HIV DUO Ultra (Biomérieux, Marcy-l'Etoile, France) showed an improved sensitivity and specificity in comparison to licensed fourth-generation assays. A total of 16 seroconversion panels, 15 cell culture supernatants infected with different HIV type 1 (HIV-1) subtypes, and 257 potentially cross-reactive serum samples were tested with VIDAS DUO HIV Ultra, Genscreen Plus HIV Ag-Ab, Enzygnost HIV Integral, Enzymun-Test HIV Combi, Genscreen HIV 1/2, version 2 (third-generation EIA), and Genetic Systems HIV-1 Ag EIA (p24 antigen assay). VIDAS HIV DUO Ultra showed a comparable sensitivity to the single p24 antigen assay in seroconversion panels and a dilution series of virus lysates. The diagnostic window was reduced with VIDAS HIV DUO Ultra by 3.82 days, on average, in comparison with the fourth-generation assay with the lowest sensitivity of the antigen detection module. HIV-1 infection was detected 5.88 days earlier than with third-generation EIA. The mean time delay between reverse transcription-PCR and VIDAS HIV DUO Ultra was only 2.31 days. The specificity of fourth-generation assays after retesting ranged between 98.1 and 100%. In conclusion, VIDAS HIV DUO Ultra can replace single-antigen screening for laboratory diagnosis and screening of HIV infection in blood donors. There was no evidence for a second diagnostic window due to impaired sensitivity of the antibody detection module of all the fourth-generation EIAs evaluated in the present study. The specificity after initial and/or repeated testing of VIDAS HIV DUO Ultra was equivalent to that of a third-generation assay.

Evidence for a diagnostic window in fourth generation assays for HIV

BACKGROUND

The routine HIV screening essentially depends on the detection of HIV specific antibodies. However, HIV p24 antigen can be detected in individuals with recent HIV infection about 2-18 days prior to seroconversion. New fourth generation HIV screening assays combine the detection of anti-HIV antibodies with the simultaneous detection of HIV p24 antigen. This may result in a reduction of the diagnostic window after primary infection.

OBJECTIVES

The performance of two novel fourth generation assays in routine diagnostic was evaluated.

STUDY DESIGN

We compared two third generation, two fourth generation and one antigen HIV assays in a case with acute primary HIV infection.

RESULTS

In our case, the HIV infection was detected 11 days earlier with the fourth generation assays compared to third generation assays. Interestingly, after the initial reactive results the fourth generation assays became negative resulting in a second diagnostic window. During this second diagnostic window neither third nor fourth generation HIV assays were reactive. This second diagnostic window was caused by the absence of HIV specific antibodies and the decline of HIV p24 antigen concentrations below the detection limits of the fourth generation assays.

CONCLUSIONS

Fourth generation assays markedly improve the diagnosis of recent HIV infections but the possibility of a second diagnostic window must be considered.

Seven human immunodeficiency virus (HIV) antigen-antibody combination assays: evaluation of HIV seroconversion sensitivity and subtype detection

In this study, we evaluated the performance of two prototype human immunodeficiency virus (HIV) antigen-antibody (Ag-Ab) combination assays, one from Abbott Laboratories (AxSYM HIV Ag-Ab) and the other from bioMerieux (VIDAS HIV Duo Ultra), versus five combination assays commercially available in Europe. The assays were Enzygnost HIV Integral, Genscreen Plus HIV Ag-Ab, Murex HIV Ag-Ab Combination, VIDAS HIV Duo, and Vironostika HIV Uniform II Ag-Ab. All assays were evaluated for the ability to detect p24 antigen from HIV-1 groups M and O, antibody-positive plasma samples from HIV-1 groups M and O, HIV-2, and 19 HIV seroconversion panels. Results indicate that although all combination assays can detect antibodies to HIV-1, group M, subtypes A to G, circulating recombinant form (CRF) A/E, and HIV-1 group O, their sensitivity varied considerably when tested using diluted HIV-1 group O and HIV-2 antibody-positive samples. Among combination assays, the AxSYM, Murex, and VIDAS HIV Duo Ultra assays exhibited the best antigen sensitivity (at approximately 25 pg of HIV Ag/ml) for detection of HIV-1 group M, subtypes A to G and CRF A/E, and HIV-1 group O isolates. However, the VIDAS HIV Duo Ultra assay had a lower sensitivity for HIV-1 group M and subtype C, and was unable to detect subtype C antigen even at 125 pg of HIV Ag/ml. The HIV antigen sensitivity of the VIDAS HIV Duo and Genscreen Plus combination assays was approximately 125 pg of HIV Ag/ml for detection of all HIV-1 group M isolates except HIV-1 group O while the sensitivity of Vironostika HIV Uniform II Ag-Ab and Enzygnost HIV Integral Ag-Ab assays for all the group M subtypes was >125 pg of HIV Ag/ml. Among the combination assays, the AxSYM assay had the best performance for detection of early seroconversion samples, followed by the Murex and VIDAS HIV Duo Ultra assays.

Fourth-generation enzyme-linked immunosorbent assay for the simultaneous detection of human immunodeficiency virus antigen and antibody

The VIDAS HIV DUO Ultra, a fourth-generation immunoassay under development for the simultaneous detection of human immunodeficiency virus type 1 (HIV-1) p24 antigen and antibodies to HIV-1 and HIV-2, was evaluated. The enzyme-linked fluorescence immunoassay, performed on the automated VIDAS instrument, is claimed to detect early and established HIV infection. The assay was challenged with a total of 2,847 samples that included 74 members of 10 seroconversion panels, 9 p24 antigen-only-reactive members of a panel of group M clades, 503 consecutively collected samples from individuals seeking care in the University of Maryland Medical System, 1,010 samples from U.S. blood donors, 1,141 samples from patients in a high-incidence population in Trinidad, 83 samples from a clinic for sexually transmitted diseases in the Bahamas, 10 confirmed HIV-1 group O samples, and 16 confirmed HIV-2 samples from the Cote d'Ivoire. Reference tests were U.S. Food and Drug Administration-licensed HIV antibody screening, p24 antigen tests, HIV confirmatory assays, and the Roche Diagnostics Amplicor HIV-1 Monitor. The VIDAS HIV DUO Ultra demonstrated 100% sensitivity and 99.5% specificity overall, with a 99.7% specificity in low-risk individuals. The analytical sensitivity, as assessed by seroconversion panels and p24 antigen in samples, was equivalent to the sensitivity of the reference assays used to characterize these panels. The VIDAS HIV DUO Ultra is accurate, offers potential advantages over conventional HIV testing for time and cost savings, has walk-away capability, and correctly identifies both early and established HIV infections.

Assessment of the performance of a rapid, lateral flow assay for the detection of antibodies to HIV

Rapid HIV assays have recently been shown to have important applications for various testing situations, including early identification of infected individuals, to allow intervention strategies in a clinically relevant time frame. A rapid, lateral flow, HIV-1/2/O assay was evaluated using 2,000 serum or plasma samples from various risk groups and geographic locations, including HIV-1 and HIV-2 positive sera from five countries. Two U.S. Food and Drug Administration (FDA)-licensed screening assays and a FDA-licensed confirmatory assay were used as reference tests. The rapid assay exhibited a near-perfect sensitivity (99.2%) and an excellent specificity (99.9%). Moreover, its analytical sensitivity was found to be better than most FDA-licensed enzyme-linked immunosorbent assays (ELISAs), detecting infection at the same time as the most sensitive ELISA in two of five seroconversion panels, and at the same time or earlier than four of five ELISAs in all five panels. We conclude that this rapid assay is a suitable test for the detection of HIV infection that could be particularly useful in developing countries where facilities may not support the use of instrumentation.

Shortening of the diagnostic window with a new combined HIV p24 antigen and anti-HIV-1/2/O screening test

Because antibodies to the human immunodeficiency virus (HIV) are absent in the very early phase of HIV infection, there remains a slight residual risk for HIV transmission by blood donations by viremic but antibody negative donations. To shorten the diagnostic window between infection and the detection of antibodies, Enzygnost HIV Integral (Dade Behring, Germany) was developed. With this new test, HIV p24 antigen and HIV antibodies can be detected simultaneously in a single test. In a multicenter study the new screening assay has been compared with various tests that detect only HIV antibodies or HIV p24 antigen and with assays which permit a simultaneous detection of HIV antigen and HIV antibodies. The new assay showed 100% sensitivity for the detection of antibodies to HIV-1, groups M (n=1102) and O (n=55), and HIV-2 (n=289). In 23 out of 52 seroconversion panels, seroconversion was detected 2-18 days earlier with the new combined antigen/antibody test compared to single antibody tests. All samples from a viral load panel (n=451), all samples containing p24 antigen (n=302), and all but one of the cell culture supernatants (n=38) infected with various HIV-1 subtypes or HIV-2 were identified reliably by the new test. The specificity of the assay for 4002 unselected blood donors was 99.78% initially and 99.80% after retesting. Potentially interfering factors had no systematic influence on specificity. By testing for p24 antigen, which is present prior to the onset of antibody production in some cases of recent HIV infection, the new assay reduces the diagnostic window as compared to third generation screening assays, thus permitting an earlier diagnosis of HIV infection.

[Screening tests combined with p24 antigen and anti-HIV antibodies in early detection of HIV-1]

The five available p24 Ag/anti-HIV combined tests were compared to the six third-generation anti-HIV assays mainly used in blood transfusion centers. Among 70 selected HIV-1 positive samples (12 samples from early infected blood donors and 58 from ten commercial panels), 59 were positive with at least one assay. False negative results were observed for zero to six samples with p24 Ag/Ab assays versus seven to 19 with antibody (Ab) tests. In five cases, one or more combined assays gave a positive signal later than the most sensitive Ab screening test. One sample with a high p24 Ag titer was missed by one combined test. The mean time delay between the most sensitive test and the second one was 0.3 to 2 days. The p24 Ag limit of detection was investigated with seven dilutions of the HIV Ag reference. The threshold of the p24 Ag detection was found to be between 65 and 250 pg/mL of HIV Ag. For four of the five combined assays, p24 Ag detectability was assessed with dilutions of infected culture cell supernatants from 13 HIV-1 different genotype strains exhibiting HIV Ag titers from 300 to 450 pg/mL. One of the four combined assays gave negative results but close to the cut-off for three supernatant dilutions (1 B, 1 F, 1 HIV-1/O) and one missed the HIV-1/O dilution. The p24 Ag/Ab combined assays permit an earlier diagnosis of HIV infection than third generation assays even if the yield in terms of reduction of the window period is moderate. They are less sensitive than p24 Ag screening assays for the detection of this marker. Consequently, the p24 Ag/Ab assays have not been used for the diagnosis of a primary infection instead of p24 Ag screening tests. They must be considered only as good tools for the detection of HIV infection.

Characterization of a discontinuous epitope of the human immunodeficiency virus (HIV) core protein p24 by epitope excision and differential chemical modification followed by mass spectrometric peptide mapping analysis

A combination of epitope excision, epitope extraction, and differential chemical modification followed by mass spectrometric peptide mapping was used for the characterization of a discontinuous epitope that is recognized by the mouse anti-HIV-p24 monoclonal antibody 5E2.A3. In epitope excision, the protein is first bound to an immobilized antibody and then digested with proteolytic enzymes. In epitope extraction, the protein is first digested and subsequently allowed to react with the antibody. After epitope excision of the p24-5E2.A3 complex with endoproteinase Lys-C, a large fragment remained affinity bound corresponding to amino acids 1-158 of HIV-p24 (fragment 1-158). Further digestion, however, resulted in loss of affinity. Moreover, no affinity-bound fragments were observed after an epitope extraction experiment. These data from the epitope excision and extraction experiments suggest that the epitope is discontinuous. For the further characterization of the epitope, amino groups in the epitope-containing fragment were acetylated in both the affinity bound and free states followed by mass spectrometric analysis. Two successive acetylation reactions were performed: (1) the first used a low molar excess of acetic anhydride, and (2) the second, after separation from the antibody, a high molar excess of its hexadeuteroderivative. This isotopic labeling procedure, in combination with high resolution mass spectrometry, allowed the precise determination of relative reactivities of amino groups. In this study, no differences were observed in the ranking of the relative reactivities of five lysine residues. However, the N-terminal amino group was found to be part of the discontinuous epitope.

Improved performance of seroconversion with a 4th generation HIV antigen/antibody assay

Recently a new fourth generation microELISA for large scale blood screening has been described in which HIV p24 Ag detection was integrated in an anti-HIV-1/-2 and anti-HIV-1 group O assay based on a direct assay format: (Vironostika HIV Uni-Form II Ag/Ab (Van Binsbergen et al., (1998)). When compared to the third generation a-HIV assay (Vironostika HIV Uni-Form II plus O), the seroconversion window was narrowed with more than one week. A more precise window estimation based on seroconversion series with short sampling time intervals of up to 7 days, is described in parallel with that for the single HIV p24 Ag assay. It was found with 10 relevant seroconversion series that the HIV p24 Ag assay closes the seroconversion window with 6.2 days as compared to the 3rd generation a-HIV assay, while a window reduction of 4 days was found with Vironostika HIV Uni-Form II Ag/Ab. These seroconversion data show that with the new assay two-thirds of the current p24 Ag window is closed. There was no statistical difference in seroconversion sensitivity between the HIV p24 Ag and the new assay format. An extended evaluation of the new assay with subtype HIV-1 M and group O p24 Ag strains and with anti-HIV antibodies obtained from individuals infected with different HIV-1 subtypes showed that all subtypes of HIV-1 M and HIV-1 group O p24 antigen were detected as well as of HIV antibody.

Multicenter evaluation of a new rapid automated human immunodeficiency virus antigen detection assay

Although human immunodeficiency virus (HIV) antigen assays are of limited value for monitoring antiretroviral therapy, they play an important role for confirmatory testing of fourth generation HIV screening enzyme immunoassay (EIA) reactive samples. In a multicenter study, a new automated rapid p24 antigen assay, Elecsys HIV Ag (Roche Diagnostics Boehringer Mannheim GmbH, Penzberg, Germany), was compared to FDA licensed tests (Abbott HIV-1 Ag monoclonal and Coulter HIV-1 p24 antigen assay). In the evaluation 27 seroconversion panels were included, sera from the acute phase of infection, single and follow-up samples from HIV antibody positive patients, dilution series of HIV antigen positive standards, sera and cell culture supernatants infected with different HIV-1 subtypes (A-H, and O) HIV-2 and recombinant HIV-1 (gag/env) isolates. To challenge the specificity of the new assay, 2565 unselected blood donors, sera from pregnant women, dialysis and hospitalized patients and 407 potentially cross-reactive samples were investigated. Acute HIV infection was detected in three to eight seroconversion panels earlier with Elecsys HIV Ag than with the alternative assays. Higher numbers of serum samples from HIV infected patients tested positive by Elecsys HIV Ag than with the comparative assays. All HIV-1 subtypes and HIV-2 isolates were recognized with Elecsys HIV Ag. Abbott HIV-1 Ag monoclonal and Coulter HIV-1 p24 antigen assay showed a variable sensitivity for the different HIV-1 subtypes. The specificity of Elecsys HIV Ag and Coulter HIV-1 p24 antigen assay were 99.8 and 99.93%, respectively. All the eight sera that were false reactive by Elecsys HIV Ag were tested negative with the Elecsys HIV Ag Neutralization Test. In conclusion, Elecsys HIV Ag was more sensitive than the alternative assays and showed a high specificity in combination with the neutralization assay. The very short incubation time of 18 min and the fully automated procedure of Elecsys HIV Ag which permits direct testing from the primary patient blood collection tube, represent a major improvement for routine laboratory diagnosis in comparison to the alternative assays.