HIV viral load testing is routinely used to monitor response to antiretroviral therapy (ART).1 Yet, reverse transcriptase by polymerase chain reaction (RT-PCR), which measures viral RNA, requires sophisticated infrastructure, technical expertise, and rigorous quality control. With simpler technology and lower cost, the ultrasensitive p24 antigen (Up24) assay uses a standard ELISA (enzyme-linked immunosorbent assay) format to detect p24 antigen.2 Two modifications increase sensitivity: heat denaturation causing dissociation of antigen-antibody complexes and tyramide-based amplification. Schupbach et al. further enhanced sensitivity with an improved virus lysis buffer.3
Schupbach et al. have shown that Up24 is a reliable marker for HIV-1 subtype B infection, disease progression, survival in advanced disease,4 and disease progression in early-stage disease.5 Additional studies have demonstrated the utility of Up24 to monitor response to ART.6–9 Evaluations of the assay in populations with predominantly non-B subtypes, have predicted mortality in Zimbabwe (subtype C)10 and Uganda (subtypes A, D, A/D recombinant).11 Investigators have found the Up24 assay accurate in samples from South Africa (subtype C)12 and Malawi (subtype C),13 but inferior to HIV RNA in Cote d’Ivoire (CRF02_AG),14 Burkina Faso (CRF06_cpx, CRF02_AG, subtypes A and G),15 and Senegal (CRF02_AG and subtype A).16 Of primary concern is limited sensitivity at lower viral levels.
Applications of special interest for use of a p24 assay in resource-limited settings have included early diagnosis in paediatric populations and recognition of acute HIV infection. Investigators using acid rather than heat for immune complex dissociation found that p24 antigenemia was associated with poor survival in HIV-1 infected Ugandan children, yet sensitivity to detect infection especially in the first weeks of life was low.17 Heat dissociation increased sensitivity. Subsequent studies of infants in Tanzania18 and Democratic Republic of Congo,19 incorporated heat dissociation and showed sensitivity of 98.7% and 100%, respectively. Recently, Fiscus et al. demonstrated sensitivity of 88% for the Up24 assay to diagnose acute HIV-1 infection in Malawi.20
Due to the potential cost savings (Up24, US$19 versus RT-PCR, US$60)14 and ongoing improvements to the Up24 assay, investigators continue to examine its accuracy, especially in populations infected with non-B subtypes and during ART. Development of a new virus dissociation buffer by the Swiss National Center for Retroviruses (SNCR) has optimized virus disruption and improved the sensitivity of the assay.3 The new buffer improves detection of particle-associated p24 from 250,000 copies/ml to 50,000 copies/ml, and detects continued presence of p24 reactivity in patients with HIV-1 RNA < 50 copies/ml in subtype B virus. Because the assay measures viral particles rather than nucleic acid, and mechanisms of viral pathogenicity involve protein mediators, Up24 may even have an advantage over conventional methods for ART monitoring.
In this population of Ugandans infected with predominantly HIV-1 A, D, and A/D recombinant subtypes,21 we compared the Up24 assay to RT-PCR. This study reports the agreement of Up24 with RT-PCR prior to ART and during ART in patients in Kampala, Uganda.