In a particle-counting immunoassay using reagent antibody attached to latex particles, if the particle count in solution is very low, what does this mean about the presence of patient antigen?

In a particle-counting immunoassay using reagent antibody attached to latex particles, if the particle count in solution is very low, what does this mean about the presence of patient antigen?

• A. The patient has no antigen present.
• B. The patient has a very small amount of antigen.
• C. The patient has a large amount of antigen present.
• D. The test is invalid.

Answer: (C)

In this latex-particle immunoassay, the particles are coated with antibody and will form cross-linked aggregates when they capture antigen from the patient sample. The more antigen present, the more cross-linking occurs, so more particles end up in large complexes and are no longer counted as single, free particles. Therefore, a very low particle count in the solution indicates extensive aggregation due to a high amount of patient antigen being present. If antigen were scarce, few cross-links would form and most particles would remain separate, giving a higher particle count.

In this latex-particle immunoassay, the particles are coated with antibody and will form cross-linked aggregates when they capture antigen from the patient sample. The more antigen present, the more cross-linking occurs, so more particles end up in large complexes and are no longer counted as single, free particles. Therefore, a very low particle count in the solution indicates extensive aggregation due to a high amount of patient antigen being present. If antigen were scarce, few cross-links would form and most particles would remain separate, giving a higher particle count.