A specimen tested for varicella antibodies yields titers of 320, then 640 two weeks later, then 320 four weeks after the first test. What is the disease status?

A specimen tested for varicella antibodies yields titers of 320, then 640 two weeks later, then 320 four weeks after the first test. What is the disease status?

• A. Active infection
• B. Vaccination response
• C. Past infection with varicella
• D. Indeterminate/unclear status

Answer: (C)

The key idea is how to read serial serology results for varicella. A true acute infection usually shows a large, fourfold rise in IgG between the acute and convalescent samples, often with IgM present early. Here, the titer goes from 320 to 640 (only a twofold rise) and then returns to 320 four weeks later. This pattern does not fit an active infection, which would push the titer higher or remain elevated, and it doesn’t indicate a recent vaccination event. Instead, the presence of detectable IgG that shows only a temporary fluctuation and then returns toward baseline points to immunity from a prior exposure rather than current disease. Therefore, the most consistent interpretation is past infection with varicella.

The key idea is how to read serial serology results for varicella. A true acute infection usually shows a large, fourfold rise in IgG between the acute and convalescent samples, often with IgM present early. Here, the titer goes from 320 to 640 (only a twofold rise) and then returns to 320 four weeks later. This pattern does not fit an active infection, which would push the titer higher or remain elevated, and it doesn’t indicate a recent vaccination event. Instead, the presence of detectable IgG that shows only a temporary fluctuation and then returns toward baseline points to immunity from a prior exposure rather than current disease. Therefore, the most consistent interpretation is past infection with varicella.