Which protein of HIV is detected in ELISA testing?

AIIMS CRE 2025

Applied Microbiology & Infection control

Easy

Appeared in: AIIMS CRE 2025

Explanation

p24 is the major capsid protein that forms the structural core of the Human Immunodeficiency Virus (HIV).
Fourth-generation ELISA tests are combination assays that simultaneously detect both antibodies to HIV and the p24 antigen.
The p24 antigen appears in the blood earlier than antibodies, typically within 1-3 weeks of infection.
Detecting the p24 antigen allows for the diagnosis of acute HIV infection during the 'window period' before seroconversion (development of antibodies).

Option A: p17 is a matrix protein that lines the inner viral envelope. It is not the primary antigen targeted in standard screening ELISA tests.
Option C: gp41 is a transmembrane glycoprotein essential for the virus to fuse with a host cell. While the body produces antibodies against gp41 (which can be detected), the gp41 antigen itself is not the target of the antigen-detection component of screening ELISAs.
Option D: gp120 is the surface glycoprotein that binds to the CD4 receptor on T-cells. Similar to gp41, it is a major target for the body's antibody response, but the gp120 antigen is not what is measured in the antigen portion of a 4th-generation ELISA.

Visual 1: Diagram - A labeled diagram of the HIV virion structure. This visual should clearly show the location of the p24 capsid (core), p17 matrix, and the gp120/gp41 envelope glycoproteins to help differentiate them.
Visual 2: Graph - A timeline of HIV infection markers. This graph should plot the levels of p24 antigen, HIV RNA, and HIV antibodies over time, illustrating that p24 antigen peaks early in the infection before antibodies become detectable.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain HIV diagnostic testing, specifically the role of viral proteins in ELISA assays as background academic context rather than a clinical decision trigger.
Early diagnosis through p24 antigen detection allows for prompt initiation of Antiretroviral Therapy (ART), which improves patient prognosis and reduces the risk of transmission to others.
A positive 4th-generation ELISA result is considered preliminary and must be confirmed with a more specific test, such as an HIV-1/HIV-2 antibody differentiation immunoassay or a Nucleic Acid Test (NAT), to provide a definitive diagnosis.
What if? If a 3rd generation ELISA test was used instead of a 4th generation test, it would only detect HIV antibodies. This would result in a longer window period (3-12 weeks), and an acute infection where p24 antigen is present but antibodies have not yet formed would be missed, leading to a false-negative result.

How to Approach the Question

First, identify the core question: It asks for the specific HIV protein used for detection in an ELISA test.
Recognize that 'ELISA' in modern diagnostics often refers to 4th-generation combination tests.
Differentiate between the viral components (antigens) and the body's response (antibodies). The question is about the antigen.
Recall the structure of the HIV virus. Associate p24 with the inner core (capsid) and gp120/gp41 with the outer envelope.
Connect the concept of the 'window period' with the need to detect something that appears before antibodies. This 'something' is the p24 antigen.

Concept Tested & Keywords

Concept Tested: HIV diagnostic testing, specifically the role of viral proteins in ELISA assays.
Stem keywords: HIV, protein, ELISA testing
Lead-in keywords: Which

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