Which description correctly contrasts sample source and assay setup for direct versus indirect FA tests?

• A. Direct FA uses labeled secondary antibodies applied to a slide with known antigens to amplify weak signals from archived serum, whereas indirect FA uses non-labeled primary antibodies detected by enzyme substrates on a plate reader.
• B. Both direct and indirect FA always use live cultured pathogens grown from the patient sample mounted on a slide before any antibodies are applied, because fixed specimens cannot be used with fluorescent labels.
• C. Direct FA requires purified lab antigen sequestered on beads and patient serum is mixed with fluorescent secondary antibody, while indirect FA directly smears patient tissue on a slide and uses labeled primary antibodies to detect antibodies present.
• D. Direct FA uses patient material fixed on a slide as the antigen source and adds labeled primary antibody, whereas indirect FA uses known laboratory antigen on the slide and incubates it with patient serum followed by labeled secondary antibody.

Answer: (D)