You are preparing a 4‑well control matrix for a diagnostic indirect agglutination assay: Well 1 = patient sample + test beads; Well 2 = patient sample + negative control beads; Well 3 = known positive antigen + test beads; Well 4 = buffer + test beads. Which interpretation of results (agglutination present/absent in each well) validates that a positive result in Well 1 is a true positive?

• A. Valid pattern: Well 1 agglutinates (patient positive), Well 2 no agglutination (rules out nonspecific sample‑driven aggregation), Well 3 agglutinates (reagent functional positive control), Well 4 no agglutination (no background bead clumping). This combination supports that Well 1 is a true positive.
• B. Valid pattern: Wells 1 and 2 agglutinate and Wells 3 and 4 do not, because if patient sample drives clumping in both beads and positive control fails, this proves the patient has antigen while the reagent is unreliable.
• C. Valid pattern: Well 1 no agglutination, Well 2 agglutinates, Well 3 no agglutination, Well 4 agglutinates; this random pattern indicates assay specificity and confirms Well 1 is positive by exclusion.
• D. Valid pattern: All wells agglutinate, because universal clumping indicates the test is sensitive and a positive in Well 1 can therefore be trusted despite any negative controls.

Answer: (A)