A clinician wants to know if a patient is actively shedding a respiratory virus right now. Which FA format is appropriate and why?

• A. Use a direct FA test because fluorescently labeled primary antibodies will bind viral antigen present in the patient sample and indicate current infection.
• B. Use an indirect FA test because fluorescent secondary antibodies bind host complement on the slide and provide a sensitive readout of viral shedding over weeks after infection.
• C. Use a serological ELISA that detects IgG with fluorescent tertiary antibodies because IgG levels rise immediately at the onset of infection and are the best marker for current viral shedding.
• D. Use Western blotting instead because it uses gel separation which is the only method capable of distinguishing active viral replication from past exposure reliably in a single step.

Answer: (A)