A virology laboratory receives an acute phase serum sample and a convalescent serum sample collected 12 days later from a patient with suspected arboviral fever. Demonstration of a four-fold rise in specific IgG antibody titer between the two samples indicates:
- A Recent vaccination rather than natural infection
- B Recent primary infection with the tested virus ✓
- C Current active infection with high viremia
- D Remote past exposure with lifelong immunity
Correct answer: B. Recent primary infection with the tested virus
Explanation
A significant rise in antibody titer between paired acute and convalescent sera is the classical serological confirmation of recent infection, because antibody levels rise during the course of an acute illness. A single high IgG titer cannot distinguish current infection from past exposure, which is why two time-spaced samples are required. Vaccination would give a similar rise, so the history must exclude recent immunization, but among these options a rising titer signifies recent primary infection.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
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