A pregnant woman exposed to a child with rubella shows a rise from 1:8 to 1:128 three weeks later with no symptoms. The change is most indicative of which condition?

• A. No infection
• B. A four-fold rise in titer indicates early infection
• C. Infection only if clinical signs are present
• D. Clinical symptoms must accompany laboratory findings

Answer: (B)

In serology, a significant rise in a disease-specific antibody titer between the acute and convalescent samples indicates a recent infection. The titer here climbs from 1:8 to 1:128, a sixteenfold increase, which clearly meets and exceeds the fourfold rise used to define an acute infection. This means rubella infection has occurred recently, even if no clinical symptoms are present—maternal rubella can be asymptomatic but still carry fetal risk.