HIV Ag/Ab Combo ECLIA Test
The HIV Ag/Ab Combo ECLIA (Electrochemiluminescence Immunoassay) is a fourth-generation screening test that simultaneously detects both HIV-1/2 antibodies and HIV-1 p24 antigen in blood, allowing for earlier diagnosis of HIV infection compared to antibody-only tests. 1, 2
What This Test Detects
The combination assay identifies two distinct markers of HIV infection:
- HIV-1 p24 antigen: A viral core protein that appears in blood 14-22 days after infection, before antibodies develop 1
- HIV-1 and HIV-2 antibodies: Both IgM and IgG antibodies that develop later in the infection course 1
Clinical Advantages
Fourth-generation assays like the ECLIA reduce the diagnostic window period significantly, becoming positive 4-7 days after detectable virus by nucleic acid testing, compared to 7-14 days for third-generation antibody-only tests. 1
The CDC and Association of Public Health Laboratories recommend fourth-generation antigen/antibody combination assays as the initial screening test for HIV diagnosis, with sensitivity and specificity exceeding 99.5%. 2
How Results Are Interpreted
- Reactive (positive) result: Requires confirmatory testing with an HIV-1/HIV-2 antibody differentiation immunoassay to distinguish between HIV-1 and HIV-2 infections 1
- If differentiation assay is negative: Nucleic acid amplification testing (NAAT) should be performed to rule out acute HIV-1 infection 1
- If differentiation assay is positive: Proceed with viral load testing and CD4 count determination to guide management 1
Important Clinical Caveats
The test has a 10-14 day "eclipse period" after infection when no markers are detectable. If initial testing is negative but exposure occurred recently, repeat testing 2-4 weeks later is essential. 1
Antiretroviral drugs can suppress viral load and delay seroconversion, potentially causing false-negative results in persons taking PrEP or PEP. 1 In these situations, both a laboratory-based Ag/Ab test and diagnostic NAAT are recommended for follow-up. 1
In neonates, antibody testing is unreliable due to maternal antibodies; HIV-1 RNA viral load testing should be used instead for diagnosis. 1
Specimen Requirements
The test uses serum or plasma collected in standard tubes (clot tube for serum, EDTA tube for plasma), with optimal transport at room temperature within 2 hours. 1