A new quantitative HIV load assay based on plasma virion reverse transcriptase activity for the different types, groups and subtypes

In vitro replicative potential of an HIV-1/MO intergroup recombinant virus compared to HIV-1/M and HIV-1/O parental viruses

Genetic recombination is one of the major evolution processes of HIV-1. Despite their great genetic divergence, HIV-1 groups M and O can generate HIV-1/MO intergroup recombinants. The current description of 20 HIV-1/MO unique recombinant forms suggests a possible benefit of the recombination. The aim of this work was to study in vitro the replicative potential of HIV-1/MO recombinant forms. This analysis was based on a simple recombination pattern, [Ogag/pol-Menv], harboring a breakpoint in Vpr. A chimeric infectious molecular clone, pOM-TB-2016 was synthesized from HIV-1/M subtype B and HIV-1/O subgroup T and recombinant viruses were obtained by transfection/co-culture. To compare the replicative potential of these viruses, two markers were monitored in culture supernatants: Reverse Transcriptase (RT) activity and P24 antigen concentration. The results showed a superiority of the group M parental virus compared to group O for both markers. In contrast, for the recombinant virus, RT activity data did not overlap with the concentration of P24 antigen, suggesting a hybrid behavior of the recombinant, in terms of enzyme activity and P24 production. These results highlighted many hypotheses about the impact of recombination on replicative potential and demonstrated again the significant plasticity of HIV genomes and their infinite possibility of evolution.

A highly sensitive aptamer-based HIV reverse transcriptase detection assay

Although many new assays for HIV have been developed, several labs still use simple and reliable radioactivity-based reverse transcriptase (RT) nucleotide incorporation assays for detection and quantification. We describe here a new assay for detection and quantitation of HIV RT activity that is based on a high affinity DNA aptamer to RT. The aptamer is sequestered on 96-well plates where it can bind to RT and other constituents can be removed by extensive washing. Since the aptamer mimics a primer-template, upon radiolabeled nucleotide addition, bound RT molecules can extend the aptamer and the radioactive signal can be detected by standard methods. In addition to being procedurally simple, the assay demonstrated high sensitivity (detection limits for RT and virions were ≤6400 molecules (∼4 × 10^-8 units) and ∼100-300 virions, respectively) and was essentially linear over a range of at least 10⁴. Both wild type and drug-resistant forms of HIV-1 RT were detectable as was HIV-2 RT, although there were some modest differences in sensitivity.

Assessment of the Cavidi ExaVir Load Assay for Monitoring Plasma Viral Load in HIV-2-Infected Patients

HIV plasma viral load is an established marker of disease progression and of response to antiretroviral therapy, but currently there is no commercial assay validated for the quantification of viral load in HIV-2-infected individuals. We sought to make the first clinical evaluation of Cavidi ExaVir Load (version 3) in HIV-2-infected patients. Samples were collected from a total of 102 individuals living in Cape Verde, and the HIV-2 viral load was quantified by both ExaVir Load and a reference in-house real-time quantitative PCR (qPCR) used in Portugal in 91 samples. The associations between viral load and clinical prognostic variables (CD4⁺ T cell counts and antiretroviral therapy status) were similar for measurements obtained using ExaVir Load and qPCR. There was no difference between the two methods in the capacity to discriminate between nonquantifiable and quantifiable HIV-2 in the plasma. In samples with an HIV-2 viral load quantifiable by both methods (n = 27), the measurements were highly correlated (Pearson r = 0.908), but the ExaVir Load values were systematically higher relative to those determined by qPCR (median difference, 0.942 log₁₀ copies/ml). A regression model was derived that enables the conversion of ExaVir Load results to those that would have been obtained by the reference qPCR. In conclusion, ExaVir Load version 3 is a reliable commercial assay to measure viral load in HIV-2-infected patients and therefore a valuable alternative to the in-house assays in current use.

The performance of reverse transcriptase assay for the estimation of the plasma viral load in HIV-1 and HIV-2 infections

Viral load testing for human immunodeficiency virus 1 (HIV-1) in resource-poor settings continues to be a challenge. Although antiretroviral therapy (ART) is being made available in developing countries, monitoring of viral load is not being done on a regular basis. The purpose of this study was to assess the utility of Cavidi version 3.0, which measures the plasma reverse transcriptase (RT) activity and compare its performance with molecular HIV viral load assays. In all, 125 HIV-1 and 13 HIV-2 positive samples were analyzed. The overall sensitivity of the assay was 86.8% and 94.1% for viral load >1000 copies/ml measured by Qiagen Artus HIV-1 RG RT PCR and Abbott RealTime HIV-1 PCR assays, respectively. Compared with the routine molecular viral load assays, Cavidi version 3.0 is inexpensive, user-friendly, the expenditure on infrastructure is minimal, and it can be used for monitoring of both HIV types.

Micro- and nanotechnology for HIV/AIDS diagnostics in resource-limited settings

Thirty-four million people are living with HIV worldwide, a disproportionate number of whom live in resource-limited settings. Proper clinical management of AIDS, the disease caused by HIV, requires regular monitoring of both the status of the host's immune system and levels of the virus in their blood. Therefore, more accessible technologies capable of performing a CD4+ T cell count and HIV viral load measurement in settings where HIV is most prevalent are desperately needed to enable better treatment strategies and ultimately quell the spread of the virus within populations. This review discusses micro- and nanotechnology solutions to performing these key clinical measurements in resource-limited settings.

Highly sensitive plasma RNA quantification by real-time PCR in HIV-2 group A and group B infection

Plasma HIV-2 viral load has been reported as predictive of AIDS in HIV-2 infected patient but the lack of sensitivity of the current HIV2 viral load assay is a limitation for the monitoring of the HIV-2-infected patients.

OBJECTIVE

To validate a new quantification assay based on a synthetic HIV-2 RNA transcript and real-time PCR with primers and probes selected in the LTR region, together with high-performance reagents and a protective RNA carrier.

STUDY DESIGN

We quantified 23 HIV-2 group A and B supernatants and 58 plasma samples with our TAQMAN-PCR assay and compared the results to those of our previously published in a real time reference PCR performed onto Light Cycler technology, the LC-PCR with a detection of 2.0 log10 copies/ml.

RESULTS

The performance of TAQMAN-PCR was significantly improved, yielding a detection limit of 17 RNA copies/ml. There was a major difference (1-5 log10 copies/ml) between LC-PCR and TAQMAN-PCR values for HIV-2 group B supernatants. Twenty-six of 27 plasma samples with levels higher than 2.0 log10 copies/ml in LC-PCR were positive by TAQMAN-PCR. Ten of the 31 plasma samples below the LC-PCR detection limit were quantifiable with the TAQMAN-PCR.

CONCLUSIONS

The new primers and probe in the LTR region can circumvent HIV-2 diversity, making our method suitable for use in HIV-2 group B-infected patients. Use of a high-performance RT enzyme and RNA carrier protection contributed to improving the detection limit. This study confirms that plasma viral load is lower than 17 copies/ml in a large number of HIV-2-infected patients.

Single real-time reverse transcription-PCR assay for detection and quantification of genetically diverse HIV-1, SIVcpz, and SIVgor strains

Although antiretroviral treatment availability has improved, the virological monitoring of patients remains largely uneven across regions. In addition, viral quantification tests are suffering from human immunodeficiency virus type 1 (HIV-1) genetic diversity, fueled by the emergence of new recombinants and of lentiviruses from nonhuman primates. We developed a real-time reverse transcription-PCR (RT-PCR) assay that is relatively inexpensive and able to detect and quantify all circulating forms of HIV-1 and its simian immunodeficiency virus (SIV) precursors, SIVcpz and SIVgor. Primers and a probe were designed to detect all variants of the HIV-1/SIVcpz/SIVgor lineage. HIV-1 M plasma (n = 190; 1.68 to 7.78 log(10) copies/ml) representing eight subtypes, nine circulating recombinant forms, and 21 unique recombinant forms were tested. The mean PCR efficiency was 99%, with low coefficients of intra- and interassay variation (<5%) and a limit of quantification of <2.50 log(10) copies/ml, with a 200-μl plasma volume. On the studied range, the specificity and the analytical sensitivity were 100 and 97.4%, respectively. The viral loads were highly correlated (r = 0.95, P < 0.0001) with the reference method (generic HIV assay; Biocentric) and had no systematic difference, irrespective of genotype. Furthermore, 22 HIV-1 O plasmas were screened and were better quantified compared to the gold-standard RealTime HIV-1 assay (Abbott), including four samples that were only quantified by our assay. Finally, we could quantify SIVcpzPtt and SIVcpzPts from chimpanzee plasma (n = 5) and amplify SIVcpz and SIVgor from feces. Thus, the newly developed real-time RT-PCR assay detects and quantifies strains from the HIV-1/SIVcpz/SIVgor lineage, including a wide diversity of group M strains and HIV-1 O. It can therefore be useful in geographical areas of high HIV diversity and at risk for the emergence of new HIV variants.

Assessment of simple risk markers for early mortality among HIV-infected patients in Guinea-Bissau: a cohort study

BACKGROUND

Decisions about when to start an antiretroviral therapy (ART) are normally based on CD4 cell counts and viral load (VL). However, these measurements require equipment beyond the capacity of most laboratories in low-income and middle-income settings. Thus, there is an urgent need to identify and test simple markers to guide the optimal time for starting and for monitoring the effect of ART in developing countries.

OBJECTIVES

(1) To evaluate anthropometric measurements and measurement of plasma-soluble form of the urokinase plasminogen activator receptor (suPAR) levels as potential risk factors for early mortality among HIV-infected patients; (2) to assess whether these markers could help identify patients to whom ART should be prioritised and (3) to determine if these markers may add information to CD4 cell count when VL is not available.

DESIGN

An observational study.

SETTING

The largest ART centre in Bissau, Guinea-Bissau.

PARTICIPANTS

1083 ART-naïve HIV-infected patients.

OUTCOME MEASURES

Associations between baseline anthropometric measurements, CD4 cell counts, plasma suPAR levels and survival were examined using Cox proportional hazards models.

RESULTS

Low body mass index (BMI≤18.5 kg/m(2)), low mid-upper-arm-circumference (MUAC≤250 mm), low CD4 cell count (≤350 cells/μl) and high suPAR plasma levels (>5.3 ng/ml) were independent predictors of death. Furthermore, mortality among patients with low CD4 cell count, low MUAC or low BMI was concentrated in the highest suPAR quartile.

CONCLUSIONS

Irrespective of ART initiation and baseline CD4 count, MUAC and suPAR plasma levels were independent predictors of early mortality in this urban cohort. These markers could be useful in identifying patients at the highest risk of short-term mortality and may aid triage for ART when CD4 cell count is not available or when there is shortness of antiretroviral drugs.

Inadequate coordination of maternal and infant HIV services detrimentally affects early infant diagnosis outcomes in Lilongwe, Malawi

OBJECTIVE

To assess the continuity of care and outcome of pediatric HIV prevention, testing, and treatment services, focusing on early infant diagnosis with DNA polymerase chain reaction (PCR).

DESIGN

A retrospective observational cohort.

METHODS

Maternal HIV antibody, infant HIV DNA PCR test results, and outcome data from HIV-infected infants from the prevention of mother-to-child transmission, early infant diagnosis, and pediatric HIV treatment programs operating in Lilongwe, Malawi, between 2004 and 2008 were collected, merged, and analyzed.

RESULTS

Of the 14,669 pregnant women who tested HIV antibody positive, 7875 infants (53.7%) received HIV DNA PCR testing. One thousand eighty-four infants (13.8%) were HIV infected. Three hundred twenty (29.5%) children enrolled into pediatric HIV care, with 202 (63.1%) at the Baylor Center of Excellence. Among these, antiretroviral therapy was initiated on 110 infants (54.5%) whose median age was 9.1 months (interquartile range, 5.4-13.8) and a median of 2.5 months (interquartile range, 1.4-5.2) after HIV clinic registration. Sixty-nine HIV-infected infants (34.2%) died or were lost by December 2008. Initiation of antiretroviral therapy increased the likelihood of survival 7-fold (odds ratio, 7.1; 95% confidence interval, 3.68 to 13.70).

CONCLUSIONS

Separate programs for maternal and infant HIV prevention and care services demonstrated high attrition rates of HIV-exposed and HIV-infected infants, elevated levels of mother-to-child transmission, late infant diagnosis, delayed pediatric antiretroviral therapy initiation, and high HIV-infected infant mortality. Antiretroviral therapy increased HIV-infected infant survival, emphasizing the urgent need for improved service coordination and strategies that increase access to infant HIV diagnosis, improve patient retention, and reduce antiretroviral therapy initiation delays.

Performance characteristics of the Cavidi ExaVir viral load assay and the ultra-sensitive P24 assay relative to the Roche Monitor HIV-1 RNA assay

BACKGROUND

The Cavidi viral load assay and the ultra-sensitive p24 antigen assay (Up24 Ag) have been suggested as more feasible alternatives to PCR-based HIV viral load assays for use in monitoring patients infected with HIV-1 in resource-limited settings.

OBJECTIVES

To describe the performance of the Cavidi ExaVir Load™ assay (version 2.0) and two versions of the Up24 antigen assay and to characterize their agreement with the Roche Monitor HIV-1 RNA assay (version 1.5).

STUDY DESIGN

Observational study using a convenience sample of 342 plasma specimens from 108 patients enrolled in two ACTG clinical trials to evaluate the performance characteristics of the Up24 Ag assay using two different lysis buffers and the Cavidi ExaVir Load™ assay.

RESULTS

In analysis of agreement with the Roche assay, the Cavidi assay demonstrated superiority to the Up24 Ag assays in accuracy and precision, as well as sensitivity, specificity, and positive and negative predictive values for HIV-1 RNA ≥ 400, ≥ 1000 and ≥ 5000 copies/mL. Logistic performance curves indicated that the Cavidi assay was superior to the Up24 assays for viral loads greater than 650 copies/mL.

CONCLUSIONS

The results suggest that the Cavidi ExaVir Load assay could be used for monitoring HIV-1 viral load in resource-limited settings.

Global cost modeling analysis of HIV-1 and HCV viral load assays

This review addresses hidden costs associated with the Bayer VERSANT assay, Roche AMPLICOR MONITOR test and COBAS AMPLICOR MONITOR test and how these influence the final per reportable cost to a testing laboratory in resource-rich and -poor countries. An in-depth evaluation and recommendation of the most cost-effective approach for these tests is presented. The analyses demonstrate the need for manufacturers to consider labor and supply costs when marketing a kit in resource-poor countries, noting that marketing strategies need to change. In the absence of any proven monitoring alternative, emphasis is placed on increasing market share to promote significant reduction in kit prices to suit the demands of markets in resource-poor countries. Finally, recommendations are made to improve the overall cost structure of viral load testing. This review is intended as a tool to optimize assay usage in attaining the lowest performance costs by assay and is not to endorse any test, as will become apparent.

Research needs and challenges in the development of HIV diagnostic and treatment monitoring tests for use in resource-limited settings

PURPOSE OF REVIEW

The aim of this article is to review research priorities for current and new technologies to diagnose HIV and to monitor treatment response, including technologies to enumerate CD4 cell counts and quantify plasma viral load, in resource-limited settings.

RECENT FINDINGS

Numerous challenges remain before HIV diagnostic and treatment monitoring technologies can be broadly implemented, especially in rural areas. New technologies that are less costly and complex to use are in development and may be better suited than current technologies for use in resource-limited settings. Investment into research activities is needed for development, evaluation and validation of new technologies. The lack of clarity in the process for evaluation and validation for these technologies affects country and program-level decisions on the appropriateness of technologies for individual settings. Implementation research is needed to assess how best to use CD4 and viral load to guide initiation and management of antiretroviral treatment, as well as how best to scale up diagnosis of HIV serostatus in infants. Studies also need to be conducted to determine if the same CD4 cutoffs can be used in resource-limited settings for initiating antiretroviral therapy and prophylaxis against opportunistic infections due to potential regional differences and the impact of other common co-morbidities on CD4 cell counts.

SUMMARY

Increased availability of antiretroviral therapy in resource-limited settings increases the need for reliable, less costly and simpler to use HIV diagnostic and treatment monitoring technologies. Global leadership is needed to coordinate the research and development necessary to ensure that HIV diagnostic and treatment monitoring technologies are properly evaluated in the setting where they will be used.

HIV-1 viral load and phenotypic antiretroviral drug resistance assays based on reverse transcriptase activity in comparison to amplification based HIV-1 RNA and genotypic assays

BACKGROUND

Amplification based HIV-1 viral load and genotypic resistance assays are expensive, technologically complex and may be difficult to implement in resource limited settings. Inexpensive, simpler assays are urgently needed.

OBJECTIVES

To determine the suitability of the ExaVir Load and ExaVir Drug assays for use in patient monitoring.

STUDY DESIGN

Specimens from 108 adults were used to compare ExaVir Load HIV-1 RT to Amplicor HIV-1 Monitor HIV-1 RNA, and ExaVir Drug phenotype to HIV GenoSure genotype.

RESULTS

HIV-1 RT and HIV-1 RNA levels were comparable (Pearson correlation coefficient 0.83). Most (94%) had detectable results in both assays. The mean difference (HIV-1 RT minus HIV-1 RNA) was -0.21 log(10)cps/mLequiv. Relationship between HIV-1 RT and HIV-1 RNA was not affected by RT mutations, CD4 cell count, or efavirenz (EFV) or nevirapine (NVP) use. Phenotypes were generally consistent with genotype findings for EFV, but not for NVP. Most patients (93.9%) with phenotypic EFV resistance had at least one EFV mutation, while 78.0% of patients with phenotypic NVP resistance had at least one NVP mutation. Eleven of 49 samples tested for EFV susceptibility were found resistant (n=2) or with reduced susceptibility (n=9) despite the absence of genotypic resistance. Eleven of 45 samples tested for NVP susceptibility were found resistant (n=9) or with reduced susceptibility (n=2) with no evidence of genotypic mutations.

CONCLUSIONS

The ExaVir Load assay performed well and may be an alternative to amplification based techniques for HIV-1 RNA quantification. The ExaVir Drug assay for phenotypic resistance testing requires further evaluation, especially for NVP.

Comparative evaluation of the ExaVir Load version 3 reverse transcriptase assay for measurement of human immunodeficiency virus type 1 plasma load

In resource-limited settings, the virological monitoring of antiretroviral therapy is limited by high cost and the lack of infrastructure. The Cavidi ExaVir Load assay employs a simple and inexpensive enzyme-linked immunosorbent assay format to measure human immunodeficiency virus (HIV) reverse transcriptase activity, which correlates with plasma RNA load. The version 3 assay has been described as having improved precision and sensitivity. There are limited data on its performance relative to those of current real-time assays. Our objective was to compare HIV type 1 (HIV-1) RNA load measurement in plasma by ExaVir Load version 3 (designated ExaVir), Abbott M2000sp/M2000rt RealTime HIV-1 assay (designated RealTime), and Roche COBAS Ampliprep/COBAS TaqMan HIV-1 version 1 assay (designated TaqMan). Plasma from 119 patients (34 with subtype B infection, 85 with non-subtype B infection [A-H, CRF01, CRF02, CRF06, CRF12, CRF14, and complex]; 48 subjects were treatment experienced, 71 were naive) and serial dilutions of the second international standard (IS) were tested. Assay relationship and agreement were determined by linear regression, correlation analysis, and the Bland-Altman method. The ExaVir assay quantified 77/83 (92.8%) samples with viral loads of >2.3 log10 copies/ml by the molecular assays. Results were linearly associated and strongly correlated with RealTime and TaqMan measurements (R of 0.94 and 0.92, respectively) for both subtype B (R of 0.97 and 0.95, respectively) and non-subtype B (R of 0.93 and 0.91, respectively) samples. Mean differences were 0.28 and 0.18 log10 copies/ml in favor of the two molecular assays; 7/119 (5.9%) and 5/119 (4.2%) samples were outside the 95% level of agreement. ExaVir underquantified the IS by a mean of 0.2 (range, 0.0 to 0.5) log10 copies/ml. The ExaVir assay showed excellent concordance with real-time molecular assays, offering a suitable option for virological monitoring in settings with limited infrastructure.

Quantitation of human immunodeficiency virus type 1 viral load in plasma using reverse transcriptase activity assay at a district hospital laboratory in Botswana: a decentralization pilot study

Roche COBAS Amplicor monitor version 1.5 assay is considered gold standard for viral load monitoring in Botswana. Due to its demand for elaborate infrastructure, viral load testing has been confined to the national HIV reference laboratories. Cavidi ExaVir Load version 2 assay was considered as a potential alternative to decentralize viral load testing to the rural/remote hospital laboratories and thus increase access to therapy. This study compared the performance of ExaVir Load v2 assay at a district hospital laboratory in Serowe and COBAS Amplicor monitor v1.5 assay at the Botswana Harvard HIV Reference Laboratory using quality assessment samples and plasma from HIV-positive individuals. ExaVir Load v2 and COBAS Amplicor monitor v1.5 assays had very good agreement; Kappa statistic 0.951. The COBAS Amplicor monitor v1.5 and ExaVir Load v2 assays detected HIV-1 RNA in 84 and 86 samples but did not detect HIV-1 RNA in 221 and 219 samples, respectively. The two assays detected HIV-1 RNA concordantly in 82 samples and were strongly correlated (r=0.8554, P<0.0001). ExaVir Load v2 assay provided a simple and reliable alternative viral load system that is adaptable to district hospital laboratories. The cost per test is less than RT-PCR. The ExaVir Load v2 systems have since been placed in four more district and primary hospital laboratories.

Transmitted drug resistance, selection of resistance mutations and moderate antiretroviral efficacy in HIV-2: analysis of the HIV-2 Belgium and Luxembourg database

Background

Guidelines established for the treatment of HIV-1 infection and genotype interpretation do not apply for HIV-2. Data about antiretroviral (ARV) drug efficacy and resistance mutations is scarce.

Methods

Clinical data about HIV-2 infected patients in Belgium and Luxembourg were collected and the effect of ARV therapy on plasma viral load and CD4 counts were analysed. Viral RNA encoding for protease (PR) and reverse transcriptase (RT) from ARV-naïve and treated patients were sequenced.

Results

Sixty-five HIV-2 infected patients were included in this cohort. Twenty patients were treated with 25 different ARV combinations in a total of 34 regimens and six months after the start of ARV therapy, only one third achieved viral load suppression. All of these successful regimens bar one contained protease inhibitors (PIs). Mean CD4 gains in the group of viral load suppressors and the group of patients treated with PI-containing regimens were respectively significantly higher than in the group of non-suppressors and the group of PI-sparing regimens. The most frequent mutations selected under therapy (compared to HIV-2 ROD) were V71I, L90M and I89V within PR. Within RT, they were M184V, Q151M, V111I and K65R. All of these mutations, except K65R and M184V, were also found in variable proportions in ARV-naïve patients.

Conclusion

Despite a high rate of ARV treatment failure, better virological and immunological results were achieved with PI-containing regimens. The analysis of polymorphic positions and HIV-2 specific mutations selected during therapy showed for the first time that transmission of drug resistant viruses has occurred in Belgium and Luxembourg. The high heterogeneity in ARV combinations reflects a lack of guidelines for the treatment of HIV-2 infection.

Use of an HIV-1 reverse-transcriptase enzyme-activity assay to measure HIV-1 viral load as a potential alternative to nucleic acid-based assay for monitoring antiretroviral therapy in resource-limited settings

An inexpensive and technically less-demanding methodology to quantify HIV-1 viral load would be of great value for resource-limited settings, where the nucleic-acid amplification technique (NAAT) is impractical and/or resource-prohibitive. In this study, an HIV-1 reverse-transcriptase enzyme-activity assay (ExaVir Load assay, version 1) was compared with the gold standard RT-PCR assay, Roche HIV-1 Amplicor Monitor, version 1.5. A total of 121 plasma specimens were used for the evaluation. ExaVir Load had a sensitivity of 97 % and a specificity of 71 % in identifying specimens with <400 copies ml−1 in the Roche RT-PCR assay as being less than the detection limit of the assay (5500 copies ml−1). The mean difference (95 % limits of agreement) between Roche RT-PCR and ExaVir Load was –0.23 (−1.59 to 1.13) log10(copies ml−1) by Bland–Altman analysis. Significant negative correlations were seen between CD4+ T-cell counts and the ExaVir Load assay (r=−0.32, P<0.05), and between CD4+ T-cell counts and the Roche RT-PCR (r=−0.38, P<0.01). The present study with HIV-1 showed a strong correlation between the ExaVir Load assay and the RT-PCR assay. Hence, the ExaVir Load assay could be considered for use in resource-limited settings as an alternative viral-load assay to the standard NAAT-based assay after further evaluation with prospective specimens.

Diagnosis of HIV-1 infection in children younger than 18 months in the United States

The objectives of this technical report are to describe methods of diagnosis of HIV-1 infection in children younger than 18 months in the United States and to review important issues that must be considered by clinicians who care for infants and young children born to HIV-1-infected women. Appropriate HIV-1 diagnostic testing for infants and children younger than 18 months differs from that for older children, adolescents, and adults because of passively transferred maternal HIV-1 antibodies, which may be detectable in the child's bloodstream until 18 months of age. Therefore, routine serologic testing of these infants and young children is generally only informative before the age of 18 months if the test result is negative. Virologic assays, including HIV-1 DNA or RNA assays, represent the gold standard for diagnostic testing of infants and children younger than 18 months. With such testing, the diagnosis of HIV-1 infection (as well as the presumptive exclusion of HIV-1 infection) can be established within the first several weeks of life among nonbreastfed infants. Important factors that must be considered when selecting HIV-1 diagnostic assays for pediatric patients and when choosing the timing of such assays include the age of the child, potential timing of infection of the child, whether the infection status of the child's mother is known or unknown, the antiretroviral exposure history of the mother and of the child, and characteristics of the virus. If the mother's HIV-1 serostatus is unknown, rapid HIV-1 antibody testing of the newborn infant to identify HIV-1 exposure is essential so that antiretroviral prophylaxis can be initiated within the first 12 hours of life if test results are positive. For HIV-1-exposed infants (identified by positive maternal test results or positive antibody results for the infant shortly after birth), it has been recommended that diagnostic testing with HIV-1 DNA or RNA assays be performed within the first 14 days of life, at 1 to 2 months of age, and at 3 to 6 months of age. If any of these test results are positive, repeat testing is recommended to confirm the diagnosis of HIV-1 infection. A diagnosis of HIV-1 infection can be made on the basis of 2 positive HIV-1 DNA or RNA assay results. In nonbreastfeeding children younger than 18 months with no positive HIV-1 virologic test results, presumptive exclusion of HIV-1 infection can be based on 2 negative virologic test results (1 obtained at > or = 2 weeks and 1 obtained at > or = 4 weeks of age); 1 negative virologic test result obtained at > or = 8 weeks of age; or 1 negative HIV-1 antibody test result obtained at > or = 6 months of age. Alternatively, presumptive exclusion of HIV-1 infection can be based on 1 positive HIV-1 virologic test with at least 2 subsequent negative virologic test results (at least 1 of which is performed at > or = 8 weeks of age) or negative HIV-1 antibody test results (at least 1 of which is performed at > or = 6 months of age). Definitive exclusion of HIV-1 infection is based on 2 negative virologic test results, 1 obtained at > or = 1 month of age and 1 obtained at > or = 4 months of age, or 2 negative HIV-1 antibody test results from separate specimens obtained at > or = 6 months of age. For both presumptive and definitive exclusion of infection, the child should have no other laboratory (eg, no positive virologic test results) or clinical (eg, no AIDS-defining conditions) evidence of HIV-1 infection. Many clinicians confirm the absence of HIV-1 infection with a negative HIV-1 antibody assay result at 12 to 18 months of age. For breastfeeding infants, a similar testing algorithm can be followed, with timing of testing starting from the date of complete cessation of breastfeeding instead of the date of birth.

Evaluation of two commercially available alternatives for HIV-1 viral load testing in resource-limited settings

There is an urgent need for low-cost assays for HIV-1 quantitation to ensure adequate follow-up of HIV-infected patients on antiretroviral therapy (ART) in resource-limited countries. Two low-cost viral load assays are evaluated, a reverse transcriptase activity assay (ExavirLoad v2, Cavidi) and a real-time reverse transcriptase PCR assay (Generic HIV viral load, Biocentric). Both tests were compared with the ultrasensitive HIV Amplicor Monitor assay. Samples were collected in Mombasa, Kenya, from 20 HIV-1 seronegative and 150 HIV-1 seropositive individuals of whom 50 received antiretroviral treatment (ART). The ExavirLoad and the Generic HIV viral load assay were performed in a local laboratory in Mombasa, the Amplicor Monitor assay (version 1.5, Roche Diagnostics) was performed in Ghent, Belgium. ExavirLoad and Generic HIV viral load reached a sensitivity of 98.3% and 100% and a specificity of 80.0% and 90.0%, respectively. Linear regression analyses revealed good correlations between the Amplicor Monitor and the Generic HIV viral load (r=0.935, p<0.001) with high accuracy (100.1%), good precision (5.5%) and a low percent similarity coefficient of variation (5.4%). Bland-Altman analysis found 95% of the samples within clinically acceptable limits of agreement (-1.19 to 0.87logcopies/ml). Although, the ExavirLoad also showed a good linear correlation with the Amplicor Monitor (r=0.901, p<0.001), a problem with false positive results was more significant. The cost per test remains relatively high (US$ 30 for ExavirLoad and US$ 20 for the Generic HIV viral load). Hence, false positive results and the need for an expensive PCR instrument for the Generic HIV viral load assays still limit the implementation of these tests in less equipped, less experienced laboratories.

Diagnosis of pediatric human immunodeficiency virus infection in resource-constrained settings

The majority of children infected with human immunodeficiency virus live in resource-constrained settings and die without an established diagnosis. Definitive laboratory diagnosis in children younger than 12-18 months requires virologic testing; however, antibody testing is often the only option available. Antibody testing provides a definitive diagnosis in older children but is frequently not used. Children meeting clinical criteria should be treated regardless of availability of laboratory diagnoses.

HIV-1 viral load assays for resource-limited settings

The authors discuss studies on the low-cost viral load assays that are currently available and their potential for use in resource-limited settings.

Reverse transcriptase viral load correlates with RNA in SIV/SHIV-infected macaques

Monitoring of viral load in macaques has usually been carried out using in-house PCR-based methods. A novel viral load (VL) kit (ExaVir Load) based on the measurement of lentivirus reverse transcriptase (RT) activity provides a potential alternative to methods that measure plasma viral RNA. RT is a fundamental and conserved activity of all retroviruses and the method should theoretically detect RT from all lentiviruses. To test this we compared VL measured by a commercially available RT kit with an in-house QC RT-PCR in macaques infected with SIV and SHIV. Both RT and RNA levels were measured over time in both sets of macaques. Results indicated that the relationship between both tests was strong for SIV and SHIV (r = 0.95 and r = 0.92, p < 0.0001, respectively). The VL trends also followed each other, indicating that both techniques measured the same process of viral replication. Furthermore, the RT load obtained using standardized control plasma samples supplied by NIBSC gave values close to the designated VL. However, when comparing RT load with QC RT-PCR a consistently three to five time higher level was obtained with the RT assay, highlighting potential differences in assay calibration. Even so, the data suggest that the RT assay is both sensitive and robust for use in the SIV/SHIV macaque model, particularly where molecular-based assays for SIV VL determinations are not easily available. The assay is also a commercially available kit and hence has the potential to reduce the variability seen between laboratories using in-house PCR.

Reverse transcriptase activity for quantitation of HIV-1 subtype C in plasma: relation to RNA copy number and CD4 T-cell count

The present study monitored the changes in human immunodeficiency virus (HIV) viral load using a reverse transcriptase (RT) assay and an HIV-1 RNA based assay, and relates these data to the dynamics of CD4 cell counts. The samples examined originate from a prospective study of HIV-1 subtype C infected, untreated Ethiopians followed twice yearly over a period of up to 5 years. The ExaVir Load test, version 1, was used for isolation and quantitation of HIV-1 RT in plasma. The RT activities recovered were compared to the HIV-1 RNA copy numbers, which had been determined previously by the NucliSens HIV-1 QT Test. There was a significant correlation between the data obtained in the two tests (r = 0.65, P < 0.0001). During follow-up, the median RT and RNA levels increased more or less in parallel up to approximately four times the values at admittance. CD4 cell counts, which had also been determined previously, decreased slowly but continuously from approximately 310 to 190 CD4 cells/ml. In the majority of individual patients, there was an inverse correlation between CD4 T-cell counts and RT activity, and with the RNA copy number, and the data obtained by either test could be used to predict CD4 T-cell counts. The ExaVir Load test thus provides data equivalent to the estimation of the number of HIV-1 RNA copies for the prediction of CD4 T-cell counts. It is based on a simple technique, can be run in any routine diagnostic laboratory, and is a competitive alternative for use in resource limited settings.

Comparison of two human immunodeficiency virus (HIV) RNA surrogate assays to the standard HIV RNA assay

Human immunodeficiency virus (HIV) RNA testing is the gold standard for monitoring antiretroviral therapy in HIV-infected patients. However, equipment and reagent costs preclude widespread use of the assay in resource-limited settings. The Perkin-Elmer Ultrasensitive p24 assay and the Cavidi Exavir Load assay both offer potentially simpler, less costly technologies for monitoring viral load. These assays were compared to the Roche Amplicor HIV-1 Monitor Test, v1.5, using panels of clinical samples (subtype B) from HIV-positive subjects and HIV-spiked samples (subtypes A, C, D, CRF_01AE, CRF_02AG, and F). The Ultrasensitive p24 assay detected 100% of the spiked samples with virus loads of >250,000 copies/ml and 61% of the clinical samples with virus loads of 219 to 288,850 copies/ml. Detection rates were improved substantially if an external lysis buffer was added to the procedure. The Cavidi assay detected 54 to 100% of spiked samples with virus loads >10,000 copies/ml and 68% of the clinical samples. These detection rates were also greatly improved with a newly implemented version of this kit. Coefficients of variation demonstrate good reproducibility for each of these kits. The results from the Cavidi v1.0, Cavidi v2.0, and Perkin-Elmer, and the Perkin-Elmer Plus external buffers all correlated well with the results from the Roche Monitor Test (r = 0.83 to 0.96, r = 0.84 to 0.99, r = 0.58 to 0.67, and r = 0.59 to 0.95, respectively). Thus, the use of these two assays for monitoring patients, together with less-frequent confirmation testing, offers a feasible alternative to frequent HIV RNA testing in resource-limited settings.

A new model to monitor the virological efficacy of antiretroviral treatment in resource-poor countries

Monitoring the efficacy of antiretroviral treatment in developing countries is difficult because these countries have few laboratory facilities to test viral load and drug resistance. Those that exist are faced with a shortage of trained staff, unreliable electricity supply, and costly reagents. Not only that, but most HIV patients in resource-poor countries do not have access to such testing. We propose a new model for monitoring antiretroviral treatment in resource-limited settings that uses patients' clinical and treatment history, adherence to treatment, and laboratory indices such as haemoglobin level and total lymphocyte count to identify virological treatment failure, and offers patients future treatment options. We believe that this model can make an accurate diagnosis of treatment failure in most patients. However, operational research is needed to assess whether this strategy works in practice.

Improved HIV-1 viral load determination based on reverse transcriptase activity recovered from human plasma

A more sensitive version of ExaVir Load, a test that utilizes reverse transcriptase (RT) activity from virions in plasma to determine HIV-1 viral load, is described. The virions were immobilized on a gel that was washed, followed by lysis of the virions, elution of purified RT, and finally RT activity determination. The changes made to the original test were: (1) improved washing of the immobilized virions by addition of a non-lytic detergent to the wash buffer, (2) improved virion lysis procedure, including changes in salt, detergent and pH, (3) the use of larger sample volumes in the RT assay, and (4) prolonged RT reaction time. The alterations gave a tenfold increased sensitivity compared to the original version. The correlation between RT load by the current test and RNA PCR was the same as previously (r=0.90). Using colorimetric product detection, the average detection limit in a panel of 262 patient plasma from Stockholm was 0.5 fg RT/ml, corresponding to approximately 170 RNA copies/ml. None of 54 HIV-1 RNA negative samples exhibited RT. The amount of RT load positive samples were 19% for samples containing 50-400 RNA, 71% for samples with 400-1,500, and 100% among samples with >8,000 copies/ml (according to Roche Amplicor). The sensitivity could be increased further using fluorimetric detection. In conclusion, the modifications of the test described result in an important increase in sensitivity. It can now be regarded as a competitive alternative method for HIV viral load determinations.

Human immunodeficiency virus (HIV) reverse transcriptase activity correlates with HIV RNA load: implications for resource-limited settings

Measurement of human immunodeficiency virus type 1 (HIV-1) plasma RNA levels using Roche AMPLICOR version 1.5 (HIV RNA) is an integral part of monitoring HIV-infected patients in industrialized countries. These assays are currently unaffordable in resource-limited settings. We investigated a reverse transcriptase (RT) assay as a less expensive alternative for measuring viral burden that quantifies RT enzyme activity in clinical plasma samples. A comparison of RT and HIV RNA assays was performed on 29 paired plasma samples from patients living in the United States and 21 paired plasma samples from patients living in Cameroon. RT levels correlated significantly with plasma HIV RNA viral loads in plasma from U.S. patients (r = 0.898; P < 0.001) and Cameroonian patients, a majority of whom were infected with HIV-1 clade type CRF02_AG (r = 0.669; P < 0.01). Among 32 samples with HIV viral load of >2,000 copies/ml, 97% had detectable RT activity. One Cameroon sample had undetectable RNA viral load but detectable RT activity of 3 fg/ml. The RT assay is a simple and less expensive alternative to the HIV RNA assay. Field studies comparing these assays in resource-limited settings are warranted to assess the practicality and usefulness of this assay for monitoring HIV-infected patients on antiretroviral therapy.

Plasma virion reverse transcriptase activity and heat dissociation-boosted p24 assay for HIV load in Burkina Faso, West Africa

BACKGROUND

In resource-limited settings, the requirement for inexpensive, easy-to-perform viral load monitoring has increased with greater antiretroviral drug availability.

OBJECTIVES

To evaluate feasibility, in Burkina Faso, of a simple assay for plasma HIV reverse transcriptase (RT) activity quantification compared to heat dissociation-boosted (HDB) p24 antigen and RNA-based quantifications in plasma samples from HIV-infected patients.

METHODS

: Plasma viraemia was quantified by RT activity, HDB-p24 and RNA copies in 84 samples from 70 HIV-1 group M-infected patients (82% non-B subtype, 93% treatment naive), including serial samples from nine patients.

RESULTS

RT activity detected 86% of plasma samples containing measurable RNA copies; corresponding to 0, 93 and 100% of samples with 1.7-4.0 log(10), 4.1-4.8 log(10) and 4.9-6.7 log(10) RNA copies/ml, respectively. HDB-p24 detected 77% of plasma samples containing measurable RNA copies; corresponding to 27, 80 and 86% of samples with 1.7-4.0 log(10), 4.1-4.8 log(10) and 4.9-6.7 log(10) RNA copies/ml, respectively. Measurement error based on one-way analysis of variance between RT activity and HDB-p24 values with RNA copies showed good agreement with RT activity (ME, <10%), however poorer agreement was obtained with HDB-p24 values (ME, >10%). Patient follow up showed a similar pattern of viraemia with RNA and RT activity assays.

CONCLUSION

Field trials in Burkina Faso support the practical use of plasma RT activity assay as an affordable alternative for HIV viral load determination in regions where RNA detection remains difficult to perform. HDB-p24 use requires further evaluation before being considered as an alternative method in African HIV-infected patient follow up.

Transfer and evaluation of an automated, low-cost real-time reverse transcription-PCR test for diagnosis and monitoring of human immunodeficiency virus type 1 infection in a West African resource-limited setting

There is an urgent need for low-cost human immunodeficiency virus type 1 (HIV-1) viral load (VL) monitoring technologies in resource-limited settings. An automated TaqMan real-time reverse transcription-PCR (RT-PCR) assay was transferred to the laboratory of the Centre de Diagnostic et de Recherches sur le SIDA, Abidjan, Côte d'Ivoire, and assessed for HIV-1 RNA VL testing in 806 plasma samples collected within four ANRS research programs. The detection threshold and reproducibility of the assay were first determined. The quantitative results obtained with this assay were compared with two commercial HIV-1 RNA kits (the Versant version 3.0 and Monitor version 1.5 assays) in specimens harboring mainly the circulating recombinant form 02 strain (CRF02). The clinical evaluation of this test was done in different situations including the early diagnosis of pediatric infection and the monitoring of antiretroviral-treated patients. The quantification limit of our method was 300 copies/ml. The HIV-1 RNA values obtained by real-time PCR assay were highly correlated with those obtained by the Versant kit (r = 0.901; P < 0.001) and the Monitor test (r = 0.856; P < 0.001) and homogeneously distributed according to HIV-1 genotypes. For the early diagnosis of pediatric HIV-1 infection, the sensitivity and specificity of the real-time PCR assay were both 100% (95% confidence intervals of 93.7 to 100.0 and 98.3 to 100.0, respectively), compared to the Versant results. Following initiation of antiretroviral treatment, the kinetics of HIV-1 RNA levels were very comparable, with a similar proportion of adults and children below the detection limit during follow-up with our technique and the Versant assay. The TaqMan real-time PCR (12 dollars per test) is now routinely used to monitor HIV-1 infection in our laboratory. This technology should be further evaluated in limited-resource countries where strains other than CRF02 are prevalent.

Evaluation of two commercially available, inexpensive alternative assays used for assessing viral load in a cohort of human immunodeficiency virus type 1 subtype C-infected patients from South Africa

Although human immunodeficiency virus type 1 (HIV-1) RNA is the acknowledged "gold standard" marker for monitoring disease activity in patients receiving highly active antiretroviral therapy (HAART), it remains unaffordable in resource-constrained settings. The present study investigated two commercially available kits for the detection of HIV-1 viral load markers as more affordable alternatives to HIV-1 RNA quantitation. The greatly improved heat-denatured, signal-boosted HiSens HIV-1 p24 Ag Ultra kit (Perkin-Elmer) and the ExaVir Load Quantitative HIV-RT kit (Cavidi Tech AB) were compared with the Amplicor HIV-1 Monitor (version 1.5) assay (Roche Molecular Systems Inc.). A total of 117 samples containing HIV-1 subtype C were analyzed by all three methodologies. Eighty-nine of these samples represented serial measurements from 20 patients receiving HAART. The remaining samples analyzed were from a group of treatment-naive patients. The association between the p24 antigen assay and the RNA assay was fairly strong (R(2) = 0.686). The association between the reverse transcriptase (RT) quantitation assay and the RNA assay was strong (R(2) = 0.810). Both alternative assays seemed most useful for the serial monitoring of patients receiving HAART (n = 89 plasma samples from 20 patients), as all assays showed a statistically significant downward trend over time, with the trend being either linear or curvilinear. In addition, all three assays showed negative correlations with the CD4 count (CD4 count versus RNA load, r = -0.336 and P = 0.001; CD4 count versus p24 antigen level, r = -0.541 and P < 0.0001; CD4 count versus RT level, r = -0.358 and P = 0.0006). Still of major concern are both the lack of sensitivity and the wide degrees of variability of both assays. However, both assays provide a less expensive alternative to the Roche viral load assay and demonstrate the same trends during treatment.

Evaluation of an ultrasensitive p24 antigen assay as a potential alternative to human immunodeficiency virus type 1 RNA viral load assay in resource-limited settings

An inexpensive enzyme-linked immunosorbent assay method for human immunodeficiency virus type 1 quantitation, ultrasensitive p24 antigen assay (Up24), was compared with RNA viral load assay (VL). Up24 had 100% sensitivity of detection at a viral load of >/=30,000, with sensitivity of 46.4% at a viral load of <30,000 (232 specimens from 65 seropositive subjects). The assay was highly reproducible, with excellent correlation between duplicates and among three laboratories.

Phenotypic Susceptibility to Nonnucleoside Inhibitors of Virion-Associated Reverse Transcriptase From Different HIV Types and Groups

OBJECTIVES

To evaluate a phenotype assay based on plasma reverse transcriptase (RT) to assess HIV susceptibility to nonnucleoside RT inhibitors (NNRTIs). To compare RT-based phenotype with recombinant virus assay (RVA) phenotype- and genotype-based analysis. To assess group O and HIV-2 susceptibility to NNRTIs in correlation with genotype polymorphisms.

METHODS

RT activity was quantified and its susceptibility to efavirenz, nevirapine, and delavirdine measured as drug concentration resulting in 50% inhibition. RT phenotype was compared with genotype analysis. Eighteen plasma samples from 14 group M- and culture supernatants from 4 group M-, 9 group O-, and 7 HIV-2-infected patients were investigated. RT-based and RVA-based phenotypes were compared for identical plasma from 9 group M-infected patients.

RESULTS

RT-based and RVA-based phenotypes were in complete agreement. RT-based phenotype- and genotype-predicted susceptibility were concordant for all but 1 group M samples. One plasma showed susceptibility to 3 NNRTIs by phenotypes, despite the presence of 101E and 106I/V residues. The HIV-2 RTs were totally resistant to the NNRTIs tested. Among HIV-1 group O, 6 were totally resistant to NNRTIs independently of the presence of the 181C mutation and 3 were susceptible to some NNRTIs.

CONCLUSION

Plasma RT-based phenotype could be useful as a simple alternative for monitoring resistance to NNRTIs. This assay is suitable for highly divergent strains. It would be particularly useful for large epidemiologic survey of the natural HIV polymorphism and the potential impact in emergence of drug resistance, particularly to nevirapine, widely used to prevent mother-to-child transmission.

Relationships between infectious titer, capsid protein levels, and reverse transcriptase activities of diverse human immunodeficiency virus type 1 isolates

Most studies on human immunodeficiency virus type 1 (HIV-1) replication kinetics or fitness must rely on a particular assay to initially standardize inocula from virus stocks. The most accurate measure of infectious HIV-1 titers involves a limiting dilution-infection assay and a calculation of the dose required for 50% infectivity of susceptible cells in tissue culture (TCID(50)). Surrogate assays are now commonly used to measure the amount of p24 capsid, the endogenous reverse transcriptase (RT) activity, or the amount of viral genomic RNA in virus particles. However, a direct comparison of these surrogate assays and actual infectious HIV-1 titers from TCID(50) assays has not been performed with even the most conserved laboratory strains, let alone the highly divergent primary HIV-1 isolates of different subtypes. This study indicates that endogenous RT activity, not p24 content or viral RNA load, is the best surrogate measure of infectious HIV-1 titer in both cell-free supernatants and viruses purified on sucrose cushions. Sequence variation between HIV-1 subtypes did not appear to affect the function or activity of the RT enzyme in this endogenous assay but did affect the detection of p24 capsid by both enzyme immunoassays and Western blots. Clear groupings of non-syncytium-inducing (NSI), CCR5-tropic (R5), and SI/CXCR4-tropic (X4) HIV-1 isolates were observed when we compared the slopes derived from correlations of RT activity with infectious titers. Finally, the replication efficiency or fitness of both the NSI/R5 and SI/X4 HIV-1 isolates was not linked to the titers of the virus stocks.

Lack of longitudinal intrapatient correlation between p24 antigenemia and levels of human immunodeficiency virus (HIV) type 1 RNA in patients with chronic hiv infection during structured treatment interruptions

Structured treatment interruptions (STIs) have been proposed as a potential treatment strategy during human immunodeficiency virus type 1 (HIV-1) antiretroviral therapy. This still-experimental intervention requires a close monitoring of patients' plasma viremia and CD4(+)-T-cell counts during the treatment interruption phase. By using signal amplification of a heat-dissociated p24 antigen (p24Ag) assay, we compared p24Ag levels with levels of HIV RNA in plasma. One hundred seventy-four plasma samples were obtained from 51 chronically HIV-infected patients: 117 from patients who underwent STIs and 57 from patients who did not. Partial immune complex dissociation and clearance of those complexes by the erythrocytes were also investigated. A significant association between the two assays was observed (beta = 0.23, 95% confidence interval = 0.18, 0.28; P < 0.0001), but the association was smaller in the subset of samples from patients undergoing STIs. Moreover, discordant results and lack of longitudinal intrapatient correlation between levels of p24Ag and HIV-1 RNA were higher in this group. Incomplete immune complex dissociation and binding of those complexes to erythrocytes could be contributing factors involved in the diminished detection of p24Ag. Therefore, signal amplification of a heat-dissociated p24Ag had a positive association with current HIV RNA assays in a population-based analysis. However, it might not be sensitive enough to monitor longitudinal intrapatient viremia during STIs in patients with high CD4(+)-T-cell counts potentially due to the production of high-affinity anti-p24 antibodies and clearance of immune complexes by erythrocytes.

HIV-1 viral load determination based on reverse transcriptase activity recovered from human plasma

We describe a procedure (ExaVir Load) to carry out human immunodeficiency virus-1 (HIV-1) viral load testing using reverse transcriptase (RT) recovered from HIV-1 virions in plasma. Samples from individuals infected with HIV-1 were treated with a sulphydryl-reactive agent to inactivate endogenous polymerases. Virions were then immobilised on a gel and washed in individual mini columns to remove RT-inhibiting antibodies, antiviral drugs, and other RT inhibitors. Immobilised virions were lysed finally, and the viral RT eluted. The amount of RT recovered was quantified by a sensitive RT activity assay using either colorimetry or fluorimetry to detect DNA produced by RT. The "RT load" values of 390 samples from 302 HIV-1 patients living in Sweden were compared to results obtained with an HIV-1 RNA viral load assay. The correlation between the two tests was r = 0.90, P < 0.0001. Four of 202 samples from healthy blood donors gave low positive values in the RT test. All samples in a panel with 10 HIV-1 subtypes were positive by the RT load. The RT load test provides a technically less demanding and cost-effective alternative to methods based on nucleic acid amplification. Being insensitive to genetic drift occurring in HIV, the assay should be of particular use in resource-limited settings, where different subtypes and recombinant HIV strains occur.