Real-world application of the Xpert® HBV viral load assay on serum and dried blood spots

As we strive towards the WHO goal of elimination of viral hepatitis as a public health threat by 2030, implementation of reliable, accurate diagnostic assays is crucial to identify those at risk of disease progression and those at risk of transmission. Ironically those at greatest risk of chronic hepatitis B are often in resource-poor regions with limited access to testing, collection, storage, and/or transportation of peripheral blood. The Xpert® HBV Viral Load assay provides an easy to use, convenient means of measuring load on GeneXpert platforms. In this study, the Xpert assay is evaluated against four commercially available high-throughput assays for Hepatitis B virus (HBV) loads. In addition application of dried blood spots (DBS) for estimation of viral load is assessed on real-world samples collected from a remote Pacific Island, Kiribati. A total of 107 serum/plasma samples were tested in the Xpert HBV load assay and compared with the Abbott m2000, Alinity m, and Roche Cobas CAP/CTM and 6800. Fifty-three DBS were tested in the Xpert assay and compared with matching serum samples. Overall 82% serum/plasma samples demonstrated good correlation between the Xpert and Roche and Abbott assays, to within 0.5 log10 IU/ml. The greatest discrepancies were seen at the limits of quantification of all assays. About 85.4% DBS gave estimable viral loads to within 1 log10 IU/ml of the serum load. The Xpert HBV viral load assay is recommended for all settings but particularly useful for resource-poor settings. Utility of DBS with the Xpert assay provides a simple means for testing in remote settings.

Rapid, random-access, and quantification of hepatitis B virus using the Cepheid Xpert HBV viral load assay

BACKGROUND

Monitoring viral load (VL) is an essential part of the management of patients chronically infected with hepatitis B virus (HBV). The commercial HBV VL assays currently available are generally performed on high-throughput platforms for batch wise testing of plasma samples, with relatively long turn-around-times. Rapid VL testing could provide immediate input to clinical decision making.

METHODS

One hundred two stored plasma samples from 102 patients who were previously tested for HBV VL by the Cobas Ampliprep/Taqman or Cobas 4800 (Roche, Pleasanton, CA), were analyzed by the recently introduced Cepheid Xpert HBV Viral Load Assay. Thirty-one of the 102 samples were negative for HBV DNA and 71 out of 102 samples had a detectable VL. HBV DNA loads ranged from <20 to 5E8 IU/mL. HBV genotypes (A, B, C, D, E, and G) were known for 52 of the VL positive samples. Correlation of VL results between both assays was determined by the Pearson correlation coefficient (r2 ). The level of concordance was assessed using the Bland-Altman analysis.

RESULTS

HBV VLs correlated well between both assays, across all genotypes (Pearson correlation coefficient r2  = 0.987). Six samples exceeded a 0.5  log difference between assays. Bland-Altman analysis demonstrated a mean of the difference of -0.107 log and a standard deviation of 0.271 log.

CONCLUSION

High correlation was observed between the Roche Cobas HBV Viral Load tests and the Xpert HBV Viral Load Assay, thus enabling rapid, random access, and accurate HBV VL assessment.

Amplification-free detection of SARS-CoV-2 with CRISPR-Cas13a and mobile phone microscopy

The December 2019 outbreak of a novel respiratory virus, SARS-CoV-2, has become an ongoing global pandemic due in part to the challenge of identifying symptomatic, asymptomatic, and pre-symptomatic carriers of the virus. CRISPR diagnostics can augment gold-standard PCR-based testing if they can be made rapid, portable, and accurate. Here, we report the development of an amplification-free CRISPR-Cas13a assay for direct detection of SARS-CoV-2 from nasal swab RNA that can be read with a mobile phone microscope. The assay achieved ∼100 copies/μL sensitivity in under 30 min of measurement time and accurately detected pre-extracted RNA from a set of positive clinical samples in under 5 min. We combined crRNAs targeting SARS-CoV-2 RNA to improve sensitivity and specificity and directly quantified viral load using enzyme kinetics. Integrated with a reader device based on a mobile phone, this assay has the potential to enable rapid, low-cost, point-of-care screening for SARS-CoV-2.

Stimulant Use and Study Protocol Completion: Assessing the Ability of Men Who Have Sex with Men to Collect Dried Blood Spots for Laboratory Measurement of HIV Viral Load

Stimulant use is associated with higher HIV viral load (VL) and sexual HIV transmission risk among men who have sex with men (MSM) living with HIV. There is little research on willingness of drug users living with HIV to fully participate in studies, especially those involving self-collection of biomarker data. This study presents findings from an at-home dried blood spot collection study measuring laboratory-quantified VL among U.S. HIV-positive MSM who reported high-risk sexual behavior and/or suboptimal antiretroviral therapy (ART) adherence to assess the association between drug-use behavior and (1) ability to complete a study protocol and (2) VL outcomes. Among recruited participants (n = 766), 35% reported stimulant drug use (amphetamines, cocaine, crack, crystal meth, ecstasy, or a combination of stimulant drugs), 39% reported using other drugs (heroin, marijuana, prescription opioids, and others), and 27% reported no drug use in the past 3 months. In all, 61% of enrolled participants completed the study protocol. Stimulant drug users were less likely (ARR 0.84; 95% CI 0.72-0.98) to complete the protocol than other drug users. Furthermore, other drug users were significantly less likely than non-drug users (ARR 0.52; 95% CI 0.28-0.97) to have an HIV VL result ≥ 1500 copies/mL. This study provides important estimates regarding the likelihood of participation in biomedical research activities among HIV-positive MSM with varying drug-use behaviors, showing that it is feasible to conduct such biomedical studies with drug-using MSM who report high-risk sexual behavior and struggle with their ART adherence.

Optimizing viral load testing access for the last mile: Geospatial cost model for point of care instrument placement

INTRODUCTION

Viral load (VL) monitoring programs have been scaled up rapidly, but are now facing the challenge of providing access to the most remote facilities (the "last mile"). For the hardest-to-reach facilities in Zambia, we compared the cost of placing point of care (POC) viral load instruments at or near facilities to the cost of an expanded sample transportation network (STN) to deliver samples to centralized laboratories.

METHODS

We extended a previously described geospatial model for Zambia that first optimized a STN for centralized laboratories for 90% of estimated viral load volumes. Amongst the remaining 10% of volumes, facilities were identified as candidates for POC placement, and then instrument placement was optimized such that access and instrument utilization is maximized. We evaluated the full cost per test under three scenarios: 1) POC placement at all facilities identified for POC; 2)an optimized combination of both on-site POC placement and placement at facilities acting as POC hubs; and 3) integration into the centralized STN to allow use of centralized laboratories.

RESULTS

For the hardest-to-reach facilities, optimal POC placement covered a quarter of HIV-treating facilities. Scenario 2 resulted in a cost per test of $39.58, 6% less than the cost per test of scenario 1, $41.81. This is due to increased POC instrument utilization in scenario 2 where facilities can act as POC hubs. Scenario 3 was the most costly at $53.40 per test, due to high transport costs under the centralized model ($36 per test compared to $12 per test in scenario 2).

CONCLUSIONS

POC VL testing may reduce the costs of expanding access to the hardest-to-reach populations, despite the cost of equipment and low patient volumes. An optimal combination of both on-site placement and the use of POC hubs can reduce the cost per test by 6-35% by reducing transport costs and increasing instrument utilization.

Comparative evaluation of three FDA-approved HIV Ag/Ab combination tests using a genetically diverse HIV panel and diagnostic specimens

BACKGROUND

HIV Ag/Ab combination assays are recommended by CDC for routine screening and several HIV Ag/Ab combination tests are now FDA-approved. Maintaining high specificity and consistent sensitivity across diverse HIV strains is critical for these assays to accurately detect HIV infection and expedite delivery of patient results.

OBJECTIVES

To evaluate performance of three FDA-approved HIV tests: ARCHITECT HIV Combo (Abbott), ADVIA Centaur HIV Combo (Siemens) and BioPlex HIV Ag-Ab (Bio-Rad).

STUDY DESIGN

Sensitivity and specificity were evaluated using an extensive panel of 28 HIV infected human specimens and 17 cultured virus isolates representing multiple genotypes, 6 seroconversion panels, 4 human samples with acute infection, WHO p24 standard and 4020 clinical specimens.

RESULTS

The p24 limit of detection (LOD) for the WHO standard was 0.19IU/ml, 0.70IU/ml, and 1.77IU/ml in BioPlex, ARCHITECT, and Centaur respectively. The distribution of LODs across 15 HIV-1 isolates was substantially narrower in ARCHITECT (5-33pg/ml) than in BioPlex (11-198pg/ml) and Centaur (6-384pg/ml). All assays detected antibodies to the majority of HIV-1 and HIV-2 variants. However, reduced sensitivity was observed for Centaur in detection of antibodies to HIV-1 group M (CRF02_AG), O and N variants. BioPlex and ARCHITECT showed better seroconversion sensitivity than Centaur, detecting one bleed (3-7 days) earlier in 4 (BioPlex) and 3 (ARCHITECT) of 6 seroconversion panels. ARCHITECT demonstrated the highest specificity (99.90-100%) compared to BioPlex (99.80%) and Centaur (99.42%).

CONCLUSIONS

The overall performance of ARCHITECT and BioPlex was superior to Centaur, especially for detection of acute HIV infection.

A European multicientre study on the comparison of HIV-1 viral loads between VERIS HIV-1 Assay and Roche COBAS® TAQMAN® HIV-1 test, Abbott RealTime HIV-1 Assay, and Siemens VERSANT HIV-1 Assay

BACKGROUND

Viral load monitoring is essential for patients under treatment for HIV. Beckman Coulter has developed the VERIS HIV-1 Assay for use on the novel, automated DxN VERIS Molecular Diagnostics System.^¥ OBJECTIVES: Evaluation of the clinical performance of the new quantitative VERIS HIV-1 Assay at multiple EU laboratories.

STUDY DESIGN

Method comparison with the VERIS HIV-1 Assay was performed with 415 specimens at 5 sites tested with COBAS^® AmpliPrep/COBAS^® TaqMan^® HIV-1 Test, v2.0, 169 specimens at 3 sites tested with RealTime HIV-1 Assay, and 202 specimens from 2 sites tested with VERSANT HIV-1 Assay. Patient monitoring sample results from 4 sites were also compared.

RESULTS

Bland-Altman analysis showed the average bias between VERIS HIV-1 Assay and COBAS HIV-1 Test, RealTime HIV-1 Assay, and VERSANT HIV-1 Assay to be 0.28, 0.39, and 0.61 log₁₀ cp/mL, respectively. Bias at low end levels below 1000cp/mL showed predicted bias to be <0.3 log₁₀ cp/mL for VERIS HIV-1 Assay versus COBAS HIV-1 Test and RealTime HIV-1 Assay, and <0.5 log₁₀cp/mL versus VERSANT HIV-1 Assay. Analysis on 174 specimens tested with the 0.175mL volume VERIS HIV-1 Assay and COBAS HIV-1 Test showed average bias of 0.39 log₁₀cp/mL. Patient monitoring results using VERIS HIV-1 Assay demonstrated similar viral load trends over time to all comparators.

CONCLUSIONS

The VERIS HIV-1 Assay for use on the DxN VERIS System demonstrated comparable clinical performance to COBAS^® HIV-1 Test, RealTime HIV-1 Assay, and VERSANT HIV-1 Assay.

Long spacer arm-functionalized magnetic nanoparticle platform for enhanced chemiluminescent detection of hepatitis B virus

A simple and cost-effective platform based on conjugating long spacer arms (LSA) onto magnetic nanoparticles (MNPs) was developed to enhance the chemiluminescent (CL) detection of pathogens. The modification method is both convenient and practical because it utilizes the commercially available macromolecule, carboxymethylated glucan (CMG), as the LSA. CMG-MNPS are designed to have low steric hindrance and high suspension properties, which allow for facile modification and hybridization reactions that enhance the CL sensitivity and detection. The infectious pathogen, hepatitis B virus (HBV) was selected for feasibility testing on this platform. The biotinylated amplicon of HBV, obtained by polymerase chain reaction (PCR), was hybridized to DNA probes functionalized on CMG-MNPs. The magnetic complexes were then incubated with streptavidin-alkaline phosphatase (SA-AP) to form linkages through biotin-streptavidin interactions. Finally, the magnetic complexes were mixed with 3-(2'-spiroadamantyl)-4-methoxy-4-(3"-phosphoryoxy)-phenyl-1,2-dioxetane (AMPPD) to generate CL signals that were proportional to the concentration of the HBV target. The detection of HBV with CMG-MNPs was more sensitive than that with the conventional carboxylated MNPs (CMNPs, succinic anhydride-modified MNPs). When optimized, the novel method showed high specificity and a detection limit of 0.5 pM. This new platform shows promise for the early clinical diagnosis of infectious diseases.

Highly Sensitive Electrical Detection of HIV-1 Virus Based on Scanning Tunneling Microscopy

A highly sensitive immunosensor based on scanning tunneling microscopy (STM) was developed for the first time to detect living material such as HIV-1 virus by gold (Au) nanoparticle and fragmented antibody complex. Fragmented antibodies were pre-immobilized on the Au surface, then HIV-1 virus like particles (HIV-1 VLPs) and Au-nanoparticle and fragmented antibody complexes were applied to develop sandwich assay. The developed surface morphology and the current profile of fabricated immunosensing element were characterized by Raman spectroscopy and investigated with STM. The power spectrum derived from the current profile was found to be related with concentrations of HIV-1 VLPs. Using the electrical detection method based on current mapping profile of STM, living material such as virus, HIV-1 VLPs, was able to be detected successfully. The proposed technique can be a promising method to construct the highly sensitive and efficient sensor for detecting viruses and other living materials.

Immunosensor based on the ZnO nanorod networks for the detection of H1N1 swine influenza virus

This paper presents an immunosensor fabricated on patterned zinc oxide nanorod networks (ZNNs) for detecting the H1N1 swine influenza virus (H1N1 SIV). Nanostructured ZnO with a high isoelectric point (IEP, approximately 9.5) possesses good absorbability for proteins with low IEPs. Hydrothermally grown ZNNs were fabricated on a patterned Au electrode (0.02 cm2) through a lift-off process. To detect the H1N1 SIV, the sandwich enzyme-linked immunosorbent assay (ELISA) method was employed in the immunosensor. The immunosensor was evaluated in an acetate buffer solution containing 3,3',5,5'-tetramethylbenzidine (TMB) via cyclic voltammetry at various H1N1 SIV concentrations (1 pg/mL-5 ng/mL). The measurement results of the fabricated immunosensor showed that the reduction currents of TMB at 0.25 V logarithmically increased from 259.37 to 577.98 nA as the H1N1 SIV concentration changed from 1 pg/mL to 5 ng/mL. An H1N1 SIV immunosensor, based on the patterned ZNNs, was successfully realized for detecting 1 pg/mL-5 ng/mL H1N1 SIV concentrations, with a detection limit of 1 pg/mL for H1N1 SIV.

Real-time optical detection of single human and bacterial viruses based on dark-field interferometry

The rapid and sensitive detection and characterization of human viruses and bacteriophage is extremely important in a variety of fields, such as medical diagnostics, immunology and vaccine research, and environmental contamination and quality control. We introduce an optical detection scheme for real-time and label-free detection of human viruses and bacteriophage as small as ~24 nm in radius. Combining the advantages of heterodyne interferometry and dark-field microscopy, this label-free method enables us to detect and characterize various biological nanoparticles with unsurpassed sensitivity and selectivity. We demonstrate the high sensitivity and precision of the method by analyzing a mixture containing HIV virus and bacteriophage. The method also resolves the distribution of small nano-impurities (~20-30 nm) in clinically relevant virus samples.

Label-free virus detection using silicon photonic microring resonators

Viruses represent a continual threat to humans through a number of mechanisms, which include disease, bioterrorism, and destruction of both plant and animal food resources. Many contemporary techniques used for the detection of viruses and viral infections suffer from limitations such as the need for extensive sample preparation or the lengthy window between infection and measurable immune response, for serological methods. In order to develop a method that is fast, cost-effective, and features reduced sample preparation compared to many other virus detection methods, we report the application of silicon photonic microring resonators for the direct, label-free detection of intact viruses in both purified samples as well as in a complex, real-world analytical matrix. As a model system, we demonstrate the quantitative detection of Bean pod mottle virus, a pathogen of great agricultural importance, with a limit of detection of 10 ng/mL. By simply grinding a small amount of leaf sample in buffer with a mortar and pestle, infected leaves can be identified over a healthy control with a total analysis time of less than 45 min. Given the inherent scalability and multiplexing capability of the semiconductor-based technology, we feel that silicon photonic microring resonators are well-positioned as a promising analytical tool for a number of viral detection applications.

Comparison of MagNA Pure 96, Chemagic MSM1, and QIAamp MinElute for hepatitis B virus nucleic acid extraction

This study was designed to compare two automated systems and one manual system for hepatitis B virus (HBV) nucleic acid extraction. The two automated systems were the MagNA Pure 96 system (Roche Applied Science, Manheim, Germany) and the Chemagic system (Chemagen, Baesweiler, Germany), and the manual system was the QIAamp system (Qiagen, Hilden, Germany). Sixty-eight samples that were within the detection range of the Cobas Ampliprep/Cobas TaqMan (CAP/CTM) platform (Roche Molecular Systems, Manheim, Germany) were selected. Extracted viral nucleic acids from the three systems were quantified using an AccuPower HBV Quantitative PCR kit (Bioneer, Daejon, Korea). The MagNA Pure 96 system and QIAamp system did not detect viral loads in one sample. The Chemagic system did not detect low viral loads in nine samples (range, 26-290 IU/mL by the CAP/CTM platform). Comparisons of the viral loads of the samples from the MagNA Pure 96 system, the Chemagic system, and the QIAamp system with those from the CAP/CTM platform yielded correlation coefficients of 0.977, 0.914, and 0.967, respectively. Comparisons of the MagNA Pure 96 system and the Chemagic system with the QIAamp system yielded correlation coefficients of 0.987 and 0.939, respectively. The MagNA Pure 96 system demonstrated better performance than the Chemagic system for HBV nucleic acid extraction. The MagNA Pure 96 system demonstrated comparable performance with the QIAamp system.

Effects of temperature on the susceptibility of insect cells to infection by baculoviruses

Three insect cell lines were tested for susceptibility to baculovirus infection by use of a typical endpoint assay procedure. Cell lines from Spodoptera frugiperda (IPLB-Sf21AE), Lymantria dispar (IPLB-LdEIta), and Heliothis virescens (IPLB-HvE6s) in 96-well tissue culture plates were each infected with dilutions of extra cellular virus suspensions of the Autographa californica nucleopolyhedrovirus (AcMNPV). In addition, the L. dispar and H. virescens cells were also infected with L. dispar nucleopolyhedrovirus, and Helicoverpa zea nucleopolyhedrovirus, respectively. Each cell/virus combination was incubated at three temperatures: 22, 27 and 32 degrees C and wells were scored for positive infection (presence of occlusion bodies in cell nuclei) at 2 to 4 day intervals for up to 4 weeks. The resulting data were analyzed by the Spearman-Kärber method, providing virus titers for each combination of virus, cell line, and temperature. The results were categorized by accuracy (assuming the highest titer achieved was the most accurate) and by rapidity of maximum titer. AcMNPV reached the highest titer in each line at 22 degrees C although equivalent titers were reached with both AcMNPV and HzSNPV in the HvE6a line at all three temperatures. This line actually reported about 100-fold less AcMNPV than the other two lines with the same virus sample. Alternatively, the Sf21AE and LdEIta lines reached 10-fold higher titers at the lowest temperature as compared with the higher temperatures, although also at a slower rate.

Rapid enumeration of respiratory viruses

Virus detection and enumeration has become increasingly important in fields ranging from medicine and biotechnology to environmental science. Although there are a wide variety of techniques that can be used to count viruses, there is demand for a rapid and more accurate means for virus enumeration. In this work, the performance of a flow cytometer that was designed and custom-built specifically for rapid detection of single viruses was evaluated. The instrument, designated a single nanometric particle enumerator (SNaPE), was characterized and calibrated using fluorescent polystyrene nanospheres. The reliability of the instrument with respect to virus enumeration was demonstrated for three medically relevant viruses, adenovirus-5, respiratory syncytial virus, and influenza A, treated with a fluorescent nucleotide stain. In each case, the SNaPE yielded a virus particle concentration consistent with, but slightly lower than, transmission electron microscopy (TEM) results, as expected. In addition, on the basis of calibration of signal intensity, the average peak height for a given virus was correlated with genome size, as expected. In contrast to time-consuming analyses such as TEM and plaque titers, SNaPE analysis of pure virus samples (including sample handling, data collection, and data processing) can be completed within 1 h.

Suppression of plasma virus load below the detection limit of a human immunodeficiency virus kit is associated with longer virologic response than suppression below the limit of quantitation

Suppression of human immunodeficiency virus type 1 plasma virus load (PVL) to <20 copies/mL is associated with a longer virologic response after initiation of antiretroviral therapy. The relationship between duration of virologic response and PVL nadir according to a less sensitive assay was explored. When compared with subjects with a PVL nadir >500 copies/mL, the relative risks of PVL rising above 1000 copies/mL for participants in the INCAS trial and the British Columbia Drug Treatment Program with a PVL nadir below the limit of detection (LOD) were 0.04 (95% confidence interval [CI], 0.02-0.09) and 0.06 (95% CI, 0.03-0.12), respectively. The corresponding relative risks for persons with a detectable but not quantifiable PVL nadir were 0.25 (95% CI, 0.13-0.50) and 0.54 (95% CI, 0.25-1.19). The relative risks of virologic failure associated with a PVL nadir detectable but not quantifiable and a PVL nadir below the LOD were statistically different (P<.0001) in both data sets.

Failure to quantify viral load with two of the three commercial methods in a pregnant woman harboring an HIV type 1 subtype G strain

The level of HIV-1 RNA in plasma has become one of the most important markers in the follow-up of HIV-infected patients. Three techniques are commercially available: both the Amplicor HIV Monitor and the NASBA HIV-1 RNA QT are target amplification methods, whereas the Quantiplex HIV RNA assay is a branched DNA signal amplification technique. Detection in both target amplification techniques is based on a single primer pair and a single probe in the gag region, whereas multiple probes capture the pol region of the viral RNA in the branched DNA assay. We investigated the discrepant observation of an undetectable viral load in an immunodeficient pregnant HIV-1-infected patient of African origin with no prior antiretroviral treatment. Although clinical progression was present in this patient with tuberculosis and a low CD4 cell count, viral load determinations with both the Amplicor Monitor and NASBA assays revealed no detectable RNA levels. The presence of HIV-1 RNA in the plasma of the patient was demonstrated by an in-house RNA-PCR. Subsequent HIV-1 RNA quantification with the branched DNA method revealed a high viremia (460,000 copies/ml). DNA sequence analysis of the gag gene identified a subtype G HIV-1 strain (HIV-1BL). To our knowledge this is the first report of a patient harboring an HIV-1 genotype of the main group with a high viral load as quantified by the branched DNA assay, but undetectable with the two commercial HIV RNA amplification techniques because of genetic divergence. In the case of discrepant low viral loads determined by one amplification technique in patients with advanced clinical stage one should use an alternative quantification technique for confirmation.

Effects of specimen collection, processing, and storage conditions on stability of human immunodeficiency virus type 1 RNA levels in plasma

To define the optimal blood collection parameters for plasma human immunodeficiency virus type 1 (HIV-1) viral load testing, plasma HIV-1 RNA levels were quantitated with the NASBA HIV-1 RNA QT System from blood specimens that were collected, processed, and stored under a variety of conditions that might have affected HIV-1 RNA stability. We determined that when whole blood was processed within 2 h of specimen collection the levels of HIV-1 RNA detected in EDTA-, heparin-, and acid citrate dextrose (ACD)-anticoagulated plasma samples were comparable. The levels of HIV-1 RNA in serum specimens (mean = 4.126 log units) were significantly lower (P < 0.01) than the levels in corresponding plasma samples (mean = 4.501 log units). One cycle of freeze-thaw (-70 degrees C) did not significantly reduce the level of HIV-1 RNA detected in EDTA-, heparin-, or ACD-anticoagulated plasmas. The EDTA-anticoagulated plasmas showed the smallest decrease in HIV-1 RNA copies (0.050 log units). HIV-1 RNA levels decreased over a 6-month time period in serum as well as in EDTA-, ACD-, and heparin-anticoagulated plasmas stored at -70 degrees C. However, the only significant decreases were for serum (mean decrease = 0.317 log units) and heparin-anticoagulated samples (mean decrease = 0.384 log units). A comparison of the levels of HIV-1 RNA in cell-free plasma collected in VACUTAINER EDTA Plasma Preparation Tubes and in standard VACUTAINER EDTA tubes determined that HIV-1 RNA levels were stable for up to 30 h after collection when stored at either room temperature (mean standard deviation [SD] = +/- 0.101 log units) or at 4 degrees C (mean SD = +/- 0.102 log units) as cell-free plasma or as EDTA-anticoagulated whole blood (mean SD = +/- 0.109 log units). These data indicate that EDTA-anticoagulated plasma is the most suitable and stable matrix for HIV-1 RNA quantitation.