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Difficulty: EasyOrdering Laboratory Tests

A 28-year-old man comes to the clinic requesting sexually transmitted infection screening. He is currently asymptomatic but reports a new male sexual partner. He states that he does not use condoms consistently. The physician decides to screen for human immunodeficiency virus (HIV) using the standard diagnostic testing algorithm. Arrange the following laboratory assays in the correct sequential order as recommended by current diagnostic guidelines.

  1. 1HIV-1/2 antigen/antibody combination immunoassay
  2. 2HIV-1/HIV-2 antibody differentiation immunoassay
  3. 3HIV-1 Nucleic Acid Test (NAT)

Answer

The correct sequence is: (1) HIV-1/2 antigen/antibody combination immunoassay, (2) HIV-1/HIV-2 antibody differentiation immunoassay, (3) HIV-1 Nucleic Acid Test (NAT).
The Centers for Disease Control and Prevention (CDC) recommends a specific stepwise algorithm for HIV testing. Testing begins with a highly sensitive fourth-generation HIV-1/2 antigen/antibody combination immunoassay. If this initial test is reactive, it must be followed by an HIV-1/HIV-2 antibody differentiation immunoassay to confirm the diagnosis and type the infection. If the differentiation assay is indeterminate or negative (which suggests an acute infection where the p24 antigen is present but antibodies are not yet fully detectable), the final step is an HIV-1 Nucleic Acid Test (NAT) to detect viral RNA.

Step-by-Step Solution

1
Identify the most sensitive initial screening test for HIV.
HIV-1/2 antigen/antibody combination immunoassay.
This fourth-generation test is recommended as the first step because it detects both HIV antibodies and the p24 antigen, allowing for earlier detection of acute infection.
2
Determine the next step if the initial combination immunoassay is reactive.
HIV-1/HIV-2 antibody differentiation immunoassay.
A reactive initial screen must be followed by a supplemental test that differentiates between HIV-1 and HIV-2 antibodies to confirm the diagnosis and guide therapy.
3
Determine the final step if the differentiation assay is negative or indeterminate in the setting of a reactive initial screen.
HIV-1 Nucleic Acid Test (NAT).
This discrepancy typically occurs during acute HIV infection when p24 antigen is present but antibodies have not yet fully formed; a NAT confirms the presence of HIV RNA.

Key Concept

Standard diagnostic algorithm for HIV infection
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