After rubella exposure, when is a fourfold rise in titer most likely to be detected?

• A. 1 week
• B. 3 weeks
• C. 6 weeks
• D. 3 months

Answer: (B)

When rubella exposure occurs, the antibody response takes time to develop. A fourfold rise in rubella-specific antibody between acute and convalescent sera reflects a recent infection, and this rise typically becomes detectable about three weeks after exposure. Testing too early (around one week) often shows little change, because the antibody levels haven’t risen enough yet. Waiting much longer (six weeks or three months) isn’t as reliable for capturing that paired rise, since the major increase has already occurred and the titer may plateau. Therefore, about three weeks after exposure is the window when a fourfold rise is most likely to be observed.