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Diagnostic Accuracy of the Point-of-Care Xpert HIV-1 Viral Load Assay in a South African HIV Clinic

2016; Lippincott Williams & Wilkins; Volume: 72; Issue: 2 Linguagem: Inglês

10.1097/qai.0000000000000978

1944-7884

Nigel Garrett, Paul K. Drain, Lise Werner, Natasha Samsunder, Salim S. Abdool Karim,

HIV/AIDS drug development and treatment

To the Editors: INTRODUCTION The World Health Organization (WHO) recommends starting antiretroviral therapy (ART) for all HIV-positive individuals regardless of CD4 cell count.1 Although 15 million people are already receiving ART, these new guidelines make nearly 20 million more people eligible for lifelong ART.1 However, as ART coverage expands, successful treatment requires routine HIV viral load monitoring to ensure treatment adherence and control of drug resistance. Therefore, simple, cost-effective models of care need to focus on maintaining viral suppression and improving retention in care, while not increasing the burden on HIV care providers and laboratories.2 Current WHO guidelines recommend plasma viral load monitoring at 6 months after ART initiation followed by annual testing,1 and define treatment failure as 2 consecutive viral load levels above 1000 copies per milliliter, despite continued adherence support interventions. The ambitious UNAIDS 90-90-90 target aims to achieve viral suppression in 90% of people on ART by 2020.2 In practice, performing routine laboratory-based viral load testing for the 15 million people receiving ART3 has been a challenge, particularly in resource-limited settings. In areas hardest hit by the HIV epidemic, laboratory services are overburdened, results management systems are often weak, and some patients still lack access to viral load testing altogether. The consequences of weak health care systems are undiagnosed virological failure, late treatment switches, and the potential spread of HIV drug resistance. A point-of-care HIV viral load test could broaden access to routine viral load monitoring, and decentralize HIV care, so that limited clinic resources can be prioritized to manage more complex patients. Clinic-based HIV viral load monitoring could allow same day adherence counseling, simplify stable patient management, and improve early detection of virological failure, all at a reduced cost. To assess clinic-based HIV viral load testing further, we conducted a cross-sectional validation study of 2 novel HIV viral load tests within a South African HIV clinic. METHODS We report on our early clinical experience with the Xpert HIV-1 Viral Load (VL) and Xpert HIV-1 Qual assays, which are both processed on the GeneXpert System (Cepheid, Sunnyvale, CA). The Xpert HIV-1 VL is a fully automated real-time molecular cartridge-based assay, measuring a linear range of 40–10 million copies per milliliter of HIV RNA, which received European CE-In-Vitro Diagnostic (IVD) regulatory approval in December 2014. The Xpert HIV-1 Qual is a molecular, cartridge-based assay that detects total nucleic acid (DNA and RNA) and provides a qualitative result (HIV detectable or undetectable). Both assays can be operated by a health care worker in a clinical setting and provide a result within 90 minutes. We performed a total of 42 Xpert HIV-1 VL assays on plasma samples and 20 Xpert HIV-1 Qual assays on whole blood samples collected consecutively from known HIV-positive South African women who attended for routine study visits in the Centre for the AIDS Programme of Research in South Africa (CAPRISA) 002 study. This study has been following HIV-positive women from acute infection through chronic infection, and while receiving ART since 2004.4 Of the 31/42 fresh (not frozen) plasma samples, 5 were obtained within 1 year of HIV acquisition, 12 during chronic infection from women not receiving ART, 11 were obtained from virologically suppressed women on ART, and three from women who were failing ART. We also evaluated an additional 11/42 frozen samples, of which 2 were collected from women with early infection not on ART and 9 from women failing ART. For both assays, samples were collected in 5 ml EDTA tubes. For the Xpert HIV-1 VL assay, specimens were first centrifuged at 1200 rpm for 10 minutes before transfer of 1 ml of plasma into the assay's cartridge chamber using a sterile pipette. For the Xpert HIV-1 Qual assay, 100 ml of EDTA whole blood was transferred to the assay's cartridge chamber. Each assay was then loaded into the GeneXpert System for analysis. We performed HIV viral load testing with the Roche Taqman version 2 assay (Roche Diagnostics, Risch-Rotkreuz, Switzerland) as the gold standard diagnostic test. To minimize bias, the Xpert HIV-1 VL and Qual assays were performed in the clinic and the Taqman assays in a central laboratory by different technicians. We also stratified results between fresh versus frozen samples, and determined if the Xpert HIV-1 VL assay misclassified any patients above or below a viral load threshold of 1000 copies per milliliter. Ethical approval for this evaluation was granted by the Biomedical Research Ethics Committee of the University of KwaZulu-Natal. RESULTS The median age of the 62 HIV-positive women sampled was 33 years (interquartile range (IQR) 28–37) and their median CD4 cell count was 609 cells per cubic millimeter (IQR 458–703). Overall, Xpert HIV-1 VL and Taqman viral load results had excellent correlation across the viral load spectrum (Spearman ρ = 0.94, P < 0.001) (Fig. 1A). A Bland–Altman plot showed a mean difference between Taqman and Xpert results of −0.10 log copies per milliliter (95% limits of agreement −0.59 to 0.39) with slightly higher values on Xpert (Fig. 1B). The number and proportion of Taqman results correctly categorized by Xpert HIV-1 VL into relevant viral load strata ( 4 log copies per milliliter) were 14/14 (100%), 7/8 (88%), and 18/20 (90%), respectively.FIGURE 1: Correlation between Xpert HIV-1 viral load and Roche Taqman version 2 assays using correlation curves and Bland–Altman plots for overall results (A, B), fresh samples only (C, D), and frozen samples only (E, F).Results were similar when stratified by fresh versus frozen samples. The correlation was strong for the 31 fresh samples [Spearman ρ = 0.96, P < 0.001; mean difference 0.10 log copies per milliliter (0.30 to −0.50)] (Figs. 1C, D). The 11 frozen samples showed a lower coefficient, but similar mean difference [Spearman ρ = 0.71; mean difference 0.11 log copies per milliliter (0.61 to −0.83)] (Figs. 1E, F). Twelve women on ART had a detectable HIV viral load of >1000 copies per milliliter measured with the Taqman version 2 assay. The median viral load was 4.37 log copies per milliliter (range 3.11–4.99). All 12 of these women had detectable viral load results on the Xpert HIV-1 VL (median viral load 4.52 log copies per milliliter, range 2.90–5.36). Among the 42 women tested, only 1 person was misclassified using a viral load threshold of 1000 copies per milliliter. This woman had a Taqman result of 1302 copies per milliliter and an Xpert HIV-1 VL result of 792 copies per milliliter. In addition, 20 samples were tested on the qualitative Xpert HIV-1 Qual. Of these samples, 13 participants had detectable viral loads (median log viral load 3.83; IQR 2.98–4.66) and 7 were virologically suppressed. All except 1 participant with a low viral load of 523 copies per milliliter on Taqman version 2 were correctly identified by the qualitative assay (N = 19/20; 95% sensitivity). This misclassified person had a viral load of 200 copies per milliliter 1 month before testing and an undetectable (<40 copies per milliliter) viral load 1 month after testing. DISCUSSION In summary, the Xpert HIV-1 VL showed good correlation with an established laboratory-based viral load assay, and could be a reliable tool for clinic-based viral load monitoring. The strength of the correlation was slightly weaker when comparing stored specimen. The Xpert HIV-1 Qual may play a role in the diagnosis of HIV, either in early infant diagnosis, as a confirmatory test after antibody-based testing, or for the detection of acute HIV infection in antibody-negative patients with symptoms indicative of acute infection.5 Although Xpert HIV-1 Qual requires lower blood volumes, our study adds evidence that it is not suitable for monitoring HIV-infected patients because it detects total nucleic acids. Recent data from South Africa have revealed that many clinics are struggling with the large number of patients and the rapid ART scale-up.6 For example, more than 25% of patients initiating ART are lost to follow-up after 1 year, and less than half of patients on ART have a recorded 12-month viral load result.6 Moreover, of those patients who receive a viral load result, 25% do not achieve virological suppression, the ultimate goal of ART delivery. This high level of treatment failure suggests that clinic facilities are struggling to manage and retain patients, and care pathways for stable patients on ART may be too complex, consuming both patient and staff time and vital resources. The Xpert HIV-1 VL and other point-of-care viral load assays could play an important role in streamlining pathways for stable patients on ART. Patients would be able to receive test results during the same visit, thereby increasing the number of patients on ART with viral load results. Furthermore, a more satisfactory patient experience could potentially improve retention in care and prove to be cost effective. The Xpert HIV-1 VL is currently available for $16.90/cartridge ex-factory, which could rise up to ∼$20/cartridge, if taxes are added (personal communication with Cepheid). Similar to ART provisions in Africa in the early 2000s, reducing the cost further would allow public funds to maximize the number of tests performed and patient benefit. The successful scale-up of the Xpert MTB/RIF assay and the existing infrastructure in South Africa could serve as a blueprint, if larger studies confirm its utility. This first clinic-based validation of the Xpert HIV-1 VL in a well-characterized cohort provides some early evidence that this tool may be able to fill an important gap in the rapid scale-up of ART globally. However, additional studies will be required to assess the strength of this assay in patients with low-level or rebounding viremia, where decisions to switch treatment are often made. In addition, implementation of this assay would require careful consideration of the potential drawbacks of point-of-care technology, such as the need for ongoing training of site staff, the ability to manage a large volume of tests in clinics with a large patient population, and the potential lack of quality control,7 especially when used in more remote settings, where the technology could have the greatest impact. Nevertheless, the prospect of a potentially simpler, cheaper, and more patient-centered care model is appealing, and studies looking at the implementation of Xpert HIV-1 VL and other point-of-care viral load assays to replace traditional care pathways should be prioritized. ACKNOWLEDGMENTS The authors thank all the CAPRISA 002 Study participants who are continuing to make an important personal contribution to HIV research. The authors are grateful to Marlene Venter, Keenan Govender, and Jessica Naidoo for processing the Xpert HIV-1 viral load samples.