What is the correct lab order to test for Human Immunodeficiency Virus (HIV)?

From the Guidelines

The correct lab order to test for HIV is a two-step process starting with an HIV antibody/antigen combination test, followed by a confirmatory test if the initial screening is reactive. The initial test (often called a 4th generation test) detects both HIV antibodies and the p24 antigen, allowing for earlier detection of infection compared to antibody-only tests 1. If this screening test is reactive, a confirmatory test such as the HIV-1/HIV-2 differentiation immunoassay or HIV RNA test (viral load) should be performed to verify the diagnosis. For patients with suspected acute HIV infection who may be in the window period before antibody development, direct viral detection through HIV RNA testing may be appropriate.

Key Considerations

• The CDC recommends that all adults be tested for HIV at least once in their lifetime, with more frequent testing for those at higher risk 1.
• Testing can be ordered as "HIV screening" or "HIV Ag/Ab combo with reflex to confirmation," which automatically triggers the confirmatory test if needed.
• Results typically return within 1-3 days, and patients should be counseled about the meaning of both positive and negative results, including the possibility of false negatives during the window period of approximately 10-33 days after infection.
• The use of fourth-generation assays, which incorporate HIV p24 antigen detection, allows for even earlier diagnosis of infection compared to third-generation assays that only detect antibodies 1.

Testing Algorithm

• Initial screening with an HIV antibody/antigen combination test
• If reactive, perform a confirmatory test such as HIV-1/HIV-2 differentiation immunoassay or HIV RNA test (viral load)
• For patients with suspected acute HIV infection, consider direct viral detection through HIV RNA testing
• Counsel patients on the meaning of test results and the possibility of false negatives during the window period.

From the Research

Lab Order for HIV Testing

The correct lab order to test for Human Immunodeficiency Virus (HIV) involves a combination of assays that detect both HIV antibodies and p24 antigen. This approach reduces the diagnostic window period of HIV testing 2, 3, 4, 5.

Recommended Assays

• Fourth-generation combination assays that detect both HIV antibodies and p24 antigen are recommended for HIV testing 3, 4, 5.
• These assays have been shown to have high sensitivity and specificity for detecting HIV-1 and HIV-2 infections 3, 4, 5.
• Examples of fourth-generation combination assays include the Enzymun-Test HIV Combi, AxSYM, ARCHITECT, and PRISM 3, 5.

Key Considerations

• The choice of assay may depend on the specific clinical setting and the population being tested 2, 6.
• It is essential to follow the manufacturer's instructions for each assay and to use validated testing protocols 2, 6, 3, 4, 5.
• A positive result should be confirmed with additional testing, such as Western blot or viral load measurement 3, 4, 5.

Benefits of Combination Assays

• Combination assays can detect HIV infection earlier than antibody-only tests 3, 4.
• They can reduce the diagnostic window period, which is the time between HIV infection and laboratory diagnosis 3, 4.
• Combination assays can also improve the detection of acute HIV infection, which is critical for HIV public health and diagnostics 6.