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Difficulty: MediumOrdering Laboratory Tests

A 25-year-old man comes to the clinic reporting a 10-day history of fever, sore throat, severe fatigue, and a generalized rash. He reports unprotected sexual intercourse with a new male partner approximately 3 weeks ago. His temperature is 38.2°C (100.8°F). Physical examination reveals bilateral cervical lymphadenopathy and a faint, nonpruritic, erythematous macular rash on the trunk. The physician suspects acute human immunodeficiency virus (HIV) infection. Arrange the following laboratory tests in the most appropriate sequential order to establish the diagnosis, according to the standard evidence-based diagnostic testing algorithm.

  1. 1HIV-1/2 antigen/antibody combination immunoassay
  2. 2HIV-1/HIV-2 antibody differentiation immunoassay
  3. 3HIV-1 nucleic acid amplification test (NAT)

Answer

The correct diagnostic sequence is: 1) HIV-1/2 antigen/antibody combination immunoassay, 2) HIV-1/HIV-2 antibody differentiation immunoassay, and 3) HIV-1 nucleic acid amplification test (NAT).
The Centers for Disease Control and Prevention (CDC) recommends a specific sequential algorithm for establishing an HIV diagnosis. The first step is an HIV-1/2 antigen/antibody combination immunoassay, which detects both HIV antibodies and the p24 viral antigen. If this initial screen is reactive, a reflex HIV-1/HIV-2 antibody differentiation immunoassay is performed to confirm the diagnosis and determine the viral type. In a patient with acute retroviral syndrome, the initial combination screen is often positive due to the early presence of the p24 antigen, but the differentiation assay may be negative or indeterminate because targeted antibodies have not yet fully formed. In this scenario, the third required step is an HIV-1 nucleic acid amplification test (NAT) to detect viral RNA and definitively diagnose acute HIV-1 infection.

Step-by-Step Solution

1
Order an HIV-1/2 antigen/antibody combination immunoassay (4th-generation test).
The test will likely be reactive due to the presence of the p24 antigen, which appears before HIV antibodies during the acute phase of infection.
This is the most sensitive and cost-effective initial screening test for both acute and chronic HIV infections.
2
Proceed to an HIV-1/HIV-2 antibody differentiation immunoassay.
In acute HIV infection, this test may be negative or indeterminate because the patient has not yet mounted a full antibody response (the 'window period').
This step is required to confirm a reactive initial screening test and to differentiate between HIV-1 and HIV-2, which affects treatment.
3
Perform an HIV-1 nucleic acid amplification test (NAT).
The NAT will detect HIV-1 viral RNA, establishing the definitive diagnosis of acute HIV infection.
When the initial antigen/antibody screen is reactive but the differentiation assay is negative or indeterminate, NAT is necessary to rule in acute HIV-1 infection (where only the p24 antigen and viral RNA are present).

Key Concept

The standard diagnostic algorithm for HIV testing requires a sequence of a 4th-generation antigen/antibody combination assay, followed by an antibody differentiation assay, and finally a nucleic acid amplification test (NAT) if the differentiation is indeterminate or negative (as seen in acute HIV infection).
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