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Difficulty: HardOrdering Laboratory Tests

A 26-year-old man presents to the ambulatory care center for routine sexually transmitted infection screening. He has no physical complaints but reports an episode of condomless receptive anal intercourse with a new male partner approximately two weeks ago. Vital signs are within normal limits, and the physical examination is unremarkable. A fourth-generation HIV-1/2 antigen/antibody combination immunoassay is drawn and returns a reactive result. Reflex testing with an HIV-1/HIV-2 antibody differentiation immunoassay is subsequently performed, yielding an indeterminate result for HIV-1 and a negative result for HIV-2. Which of the following is the most appropriate next step in laboratory evaluation?

  1. HIV-1 RNA nucleic acid amplification testAnswer
  2. B
    HIV-1 Western blot
  3. C
    CD4+ T-lymphocyte count
  4. D
    Genotypic antiretroviral resistance testing
  5. E
    Repeat the fourth-generation immunoassay in 4 weeks

Answer

The correct action is to perform an HIV-1 RNA nucleic acid amplification test (NAAT).
The standard diagnostic algorithm for HIV infection begins with a fourth-generation HIV-1/2 antigen/antibody combination immunoassay. If this initial test is reactive, the laboratory automatically reflexes to an HIV-1/HIV-2 antibody differentiation immunoassay. In cases of acute HIV infection, the initial test may be reactive solely because it detects the viral p24 antigen, which appears early. Because the patient has not yet mounted a mature antibody response (which takes several weeks), the subsequent antibody-only differentiation assay will return negative or indeterminate. To resolve this discrepancy and confirm the diagnosis of acute HIV-1 infection, the established next step is to order an HIV-1 RNA nucleic acid amplification test (NAAT).

Step-by-Step Solution

1
Analyze the patient's clinical history and initial test results.
The patient had a high-risk exposure 2 weeks ago and presents with a reactive 4th-generation HIV-1/2 antigen/antibody immunoassay followed by an indeterminate differentiation assay.
Establishing the timeline of exposure is critical for interpreting the specific combination of reactive and indeterminate lab results.
2
Interpret the mechanism and meaning of the discordant testing sequence.
A 4th-generation assay detects both p24 antigen and HIV antibodies, while the differentiation assay detects only established antibodies. The discrepancy strongly suggests the presence of p24 antigen before full antibody seroconversion.
This specific pattern—reactive screening test with a negative or indeterminate differentiation test—is the classic laboratory signature of acute HIV infection.
3
Determine the protocol-driven next step in laboratory evaluation.
The CDC diagnostic algorithm mandates resolving this specific discrepancy with an HIV-1 RNA test.
Detecting viral RNA confirms acute HIV-1 infection, allowing for immediate counseling and initiation of antiretroviral therapy.

Key Concept

The CDC HIV testing algorithm requires an HIV-1 RNA nucleic acid amplification test (NAAT) to resolve a reactive antigen/antibody combination immunoassay that is followed by an indeterminate or negative antibody differentiation assay, as this pattern indicates acute HIV infection.
Estimated Time:1m 0s
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