What is the initial test for Human Immunodeficiency Virus (HIV)?

From the Guidelines

The initial test for HIV is typically a combination HIV antigen/antibody test, which detects both HIV antibodies and the p24 antigen. This test is recommended as the first step in HIV screening because it can detect infection earlier than antibody-only tests, potentially as soon as 2-3 weeks after exposure 1. The test is usually performed on a blood sample, though some versions can use oral fluid. If this initial test is reactive (positive), a confirmatory test is needed, typically an HIV-1/HIV-2 antibody differentiation immunoassay or HIV RNA test. Most healthcare settings use 4th generation tests that can detect both antibodies and antigens, improving early detection 1. Testing is recommended for everyone between ages 13-64 at least once, and more frequently for those with risk factors. Early detection is crucial because it allows for prompt treatment, which improves long-term health outcomes and reduces transmission risk. The test is highly accurate, with sensitivity and specificity exceeding 99% when the confirmatory algorithm is followed. Key benefits of this approach include:

• Early detection of HIV infection
• Improved long-term health outcomes
• Reduced transmission risk
• High accuracy with sensitivity and specificity exceeding 99% Some important considerations when interpreting test results include:
• False positive results can occur, especially with low copy number results
• Retesting may be necessary to confirm diagnosis
• NAAT is not 100% sensitive in individuals with established HIV infection due to viral suppression or improper specimen collection/handling 1.

From the Research

Initial Test for HIV

The initial test for HIV typically involves the use of rapid diagnostic tests that detect both HIV antibodies and p24 antigen.

• The Alere™ HIV Combo (now rebranded to Determine™ HIV Ultra, from Abbott) is an example of a fourth-generation rapid diagnostic test used to detect p24 antigen and/or immunoglobulin M (IgM) and G (IgG) antibodies to HIV-1/HIV-2 (HIV Ab) 2.
• Another example is the Determine™ HIV-1/2 Ag/Ab Combo fourth generation rapid strip, which detects both HIV-1/2 antibodies and p24 antigen 3.
• The Architect HIV Ag/Ab Combo is a fourth-generation immunoassay that simultaneously detects both antibody to human immunodeficiency virus (HIV) and HIV p24 antigen, and has been shown to detect the majority of infections, including acute infections 4.

Diagnostic Window

The use of fourth-generation HIV screening immunoassays reduces the diagnostic window between infection and diagnosis by including HIV p24 antigen detection together with HIV antibody detection in the same test.

• This can open a second diagnostic window, allowing for earlier detection of HIV infection 5.
• The analytical sensitivity of combined antigen/antibody assays has been evaluated using the p24 WHO standard, with some assays having a lower limit of detection below 2 IU/ml 6.

Test Characteristics

The characteristics of the initial test for HIV include:

• Sensitivity: The ability of the test to detect true positives, with some assays having a sensitivity of 90.7% for the detection of HIV-1/2 antibodies 3.
• Specificity: The ability of the test to detect true negatives, with some assays having a specificity of 100% for the detection of HIV-1/2 antibodies 3.
• False-positive results: The occurrence of false-positive results, which can be as low as 0.07% of total tests 2.