Which of the following is required for a positive result in an indirect ELISA assay?

• A. The use of a radioactive label instead of an enzyme
• B. The presence of specific antigens in the sample
• C. The absence of enzyme-linked secondary antibody
• D. The presence of specific antibodies in the sample

Answer: (D)

Verified step by step guidance

1. Understand the principle of an indirect ELISA assay: it is designed to detect the presence of specific antibodies in a sample by using antigen-coated wells.
2. Recognize that in an indirect ELISA, the antigen is fixed to the surface, and the sample is added to see if it contains antibodies that specifically bind to that antigen.
3. Note that after the primary antibody (from the sample) binds to the antigen, an enzyme-linked secondary antibody that recognizes the primary antibody is added to produce a detectable signal.
4. Identify that a positive result requires the presence of specific antibodies in the sample, because without these antibodies, the secondary antibody has nothing to bind to, and no signal will be generated.
5. Understand why the other options are incorrect: a radioactive label is not required (enzymes are typically used), the presence of antigens in the sample is not necessary (antigens are coated on the plate), and the absence of enzyme-linked secondary antibody would prevent detection.