Learning Objectives

• Distinguish between Direct and Indirect ELISA.
• Understand the role of the enzyme-linked antibody-substrate reaction.
• Evaluate the sensitivity vs. specificity of ELISA compared to Western blot.
• Identify clinical screening uses, specifically for HIV.

1. Overview & Mechanics

The Enzyme-Linked Immunosorbent Assay (ELISA) is an immunologic test used to detect the presence of a target antigen or antibody in a patient’s blood sample. It relies on a colorimetric signal produced by an enzyme-substrate reaction.

• The Detection: An antibody is linked to an enzyme.
• The Signal: When a substrate is added, it reacts with the enzyme to produce a detectable fluorescent or color signal.

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2. Direct vs. Indirect ELISA

The primary difference lies in what is being measured and the number of antibodies used.

[Image comparing direct ELISA with a labeled primary antibody vs indirect ELISA with a labeled secondary antibody]

• Direct ELISA: Detects a specific antigen. A labeled primary antibody binds directly to the antigen fixed on the plate.
• Indirect ELISA: Detects a specific antibody. A patient’s antibody binds to a fixed antigen, and a labeled secondary antibody is then added to bind to the patient’s antibody.

3. Clinical High-Yields

• HIV Screening: ELISA is frequently used as the initial screen for HIV infection due to its high sensitivity.
• Comparison: While ELISA has high sensitivity and specificity, it is less specific than Western blot. For this reason, a positive ELISA result was traditionally confirmed by Western blot (though modern algorithms have since shifted).

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