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Diagnosing acute HIV infection at point of care: a retrospective analysis of the sensitivity and specificity of a fourth-generation point-of-care test for detection of HIV core protein p24
  1. Naomi Fitzgerald1,
  2. Maria Cross2,
  3. Siobhan O'Shea2,
  4. Julie Fox1
  1. 1Department of Genito-Urinary Medicine, Guy's and St Thomas’ NHS Foundation Trust, London, UK
  2. 2Infection Sciences, Virology Section, Viapath, Guy's and St Thomas’ NHS Foundation Trust, London, UK
  1. Correspondence to Dr Naomi Fitzgerald, Department of HIV, Guy's and St Thomas' NHS Foundation Trust, Westminster Bridge Road, London SE1 7EH, UK; naomi.fitzgerald{at}gstt.nhs.uk

Abstract

Objectives Detection of acute HIV infection is vital in preventing onward transmission. HIV point-of-care testing (POCT) has improved uptake of HIV testing but has been limited to third-generation assays, which only detect chronic HIV infection. Previous evaluation of the fourth-generation Alere Determine HIV-1/2 Ag/Ab Combo POCT showed only 50% sensitivity for HIV core protein p24 (p24 antigen) detection, which is suboptimal for diagnosis of acute HIV infection with limited advantage over third-generation POCT. We aimed to assess the sensitivity of the new Alere HIV Combo POCT to detect acute HIV infection.

Methods Stored samples in samples already identified as p24-positive using standard-of-care fourth-generation assays were randomly selected alongside groups of antibody-positive samples and HIV-negative samples. Each sample was tested using the new Alere POCT according to manufacturer's instructions. Sensitivity and specificity were then calculated.

Results The Alere HIV Combo POCT test demonstrated 88% sensitivity 95% CI (78% to 98%) and 100% specificity 95% CI (99.7% to 100%) for detection of p24 antigen.

Conclusions This new POCT shows improved sensitivity for detection of p24 antigen and may be of value for clinical use in detecting acute HIV infection. Further evaluation of its use in a clinical setting is still required.

  • PRIMARY HIV INFECTION
  • HIV
  • HIV TESTING
  • ANTIGEN DETECTION
  • DIAGNOSIS

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Background

Over a quarter of people living with HIV infection in the UK remain unaware of their HIV infection.1 ,2 This undiagnosed proportion drives HIV transmission, particularly among those who are seroconverting and have extremely high viral loads. The detection of acute HIV infection is vital in preventing onward transmission and for consideration of early antiretroviral therapy in order to improve clinical outcome.3–8

HIV point-of-care testing (POCT) has greatly improved the uptake and acceptability of HIV testing2 ,3 but until now has been carried out using third-generation antibody assays or an insensitive fourth-generation antigen/antibody combination test.9 ,10 Previous laboratory and field evaluation of the Alere Determine HIV-1/2 Ag/Ab Combo POCT showed only 50% sensitivity for p24 antigen detection which is suboptimal for the diagnosis of acute HIV infection with limited advantage over standard third-generation POCT.9 ,10

This study aims to assess the sensitivity of the new Alere HIV Combo POCT to detect acute HIV infection in samples identified as HIV core protein p24-positive using standard-of-care fourth-generation assays.

Methods

Stored samples from individuals previously tested using the Abbott Architect HIV Ag/Ab Combo assay and VIDAS quantitative HIV p24 11 assay were identified in the laboratory with the following results:

  1. p24 antigen-positive serum (n=35)

  2. p24 antigen-negative, HIV antibody-positive: plasma (n=30) and EDTA whole blood (n=20)

  3. p24 antigen-negative, HIV antibody-negative serum (n=35), that is, HIV-negative

Sample size was determined according to number of stored p24 antigen samples in the laboratory and formed a random series.

Samples were tested with the Alere HIV Combo POCT assay, which uses lateral flow and has separate reading windows for antigen and antibody. We used pipetted samples of the recommended size of 50 μL. Alere states that serum/plasma or whole blood from finger prick or venepuncture can be used. Results were read at both 20 min (recommended) and 40 min.

Ethical approval was not required because all samples used were residual samples from individuals who had consented for an HIV test. Samples were identified in the laboratory and assigned a number with no correlation with patient or clinical details. Patient and clinical information was removed prior to samples being made available for testing and could not be traced back. The only information retained for samples was HIV antibody and p24 antigen status. Results from testing using the study assay did not impact on clinical care.

Sensitivity and specificity were calculated with 95% CIs.

Results

One hundred and twenty samples were analysed, one of which produced an invalid result (0.8%) and was excluded from analysis. Results are summarised in table 1.

Table 1

Alere HIV-Combo point-of-care testing (POCT) results for detection of p24 antigen

The Alere HIV Combo POCT detected p24 antigen in 30 of 34 (88%) p24 antigen-positive serum samples at 20 min. One further sample was positive for p24 antigen at 40 min (delayed detection). The mean p24 antigen level, as detected by the VIDAS quantitative HIV p24 11 assay, was 279.6 pg/mL (range 8.3– >400 pg/mL). The p24 antigen levels for the three false-negative cases were 8.3 pg/mL, 27.6 pg/mL and >400 pg/mL and for the delayed detection case 13.6 pg/mL. The p24 antigen false-negative results were confirmed by repeat testing. All three samples were antibody-positive on both Alere HIV Combo POCT and laboratory testing.

In cases of chronic HIV infection (HIV antibody-positive, p24 antigen-negative), the Alere HIV Combo POCT detected antibody in 100% (30/30) of plasma and 100% (20/20) of whole blood sample cases with no false-positive p24 Ag results. All 35 HIV-negative controls tested negative for both antigen and antibody.

Overall the Alere HIV Combo POCT test demonstrated 88% sensitivity 95% CI (78% to 98%) and 100% specificity CI 95% (99.7% to 100%) for detection of p24 antigen when read at 20 min and 91% sensitivity 95% CI (80% to 100%) at 40 min. The antibody portion of the test showed 100% sensitivity and specificity.

Discussion

The detection of acute HIV infection remains critical for increasing early HIV diagnosis to reduce mortality and limit onward HIV transmission. In a laboratory setting, the Alere HIV Combo fourth-generation POCT was 88% sensitive 95% CI (78% to 98%) and 100% specific for detection of p24 antigen following the manufacturer's instructions. Of the four false-negative p24 antigen results, one had a p24 antigen level (8.3 pg/mL) close to the positive detection threshold using the VIDAS assay (5 pg/mL) but the others were higher (13.6 pg/mL, 27.6 pg/mL, >400 pg/mL). Therefore, the reason for these results remains unclear.

These results demonstrate improved detection of p24 antigen compared with an earlier version of the Alere fourth-generation POCT. However, POCT remains less sensitive than fourth-generation laboratory based serological testing.9 ,10 Sensitivity was slightly improved following extension of the reading frame from 20 min to 40 min. The low number of invalid tests is encouraging for clinical use9 ,10 Results from whole blood were also reassuring as this is the closest comparison to a clinical POCT setting. However, testing whole blood samples from individuals with acute HIV infection and evaluating the impact of this test on clinic workflow needs to be explored.

References

View Abstract

Footnotes

  • This information has previously been presented at the British HIV Association Spring Conference, 21–24 April 2015, Brighton, UK and at the European AIDS Clinical Society Conference, 21–24 October, Barcelona, Spain in poster format.

  • Handling editor Jackie A Cassell

  • Contributors We confirm that all authors have seen and approved the content and have contributed significantly to this work. MC was responsible for locating and storing all samples used in this study. SO was responsible for overseeing the correct identification of samples and for teaching laboratory based testing techniques and safety. She also provided information about the standard laboratory based assays currently used and assisted with manuscript editing. JF oversaw the study and assisted with manuscript editing.

  • Funding The point-of-care tests were provided by the manufacturer, Alere.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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