After exposure to measles, a patient tests with a titer of 1:20. This result indicates which of the following actions?

After exposure to measles, a patient tests with a titer of 1:20. This result indicates which of the following actions?

• A. The test should be repeated in 10 days to 2 weeks
• B. Immunization should be started immediately
• C. No further testing is needed
• D. Active measles infection is confirmed

Answer: (A)

When evaluating exposure to measles, a single antibody titer taken early after exposure is not diagnostic. Antibody levels can be undetectable or only weakly detectable during the window period before the immune system has mounted a response, so a value like 1:20 may not indicate current infection or immunity. The correct approach is to obtain a convalescent sample about 10 to 14 days later to look for seroconversion—a fourfold rise in titer—or the appearance of IgM, which would indicate a recent or current infection. If nothing changes on the second test, it argues against acute infection. Because the initial result is not definitive, repeating the test in that time frame is the appropriate next step.

When evaluating exposure to measles, a single antibody titer taken early after exposure is not diagnostic. Antibody levels can be undetectable or only weakly detectable during the window period before the immune system has mounted a response, so a value like 1:20 may not indicate current infection or immunity. The correct approach is to obtain a convalescent sample about 10 to 14 days later to look for seroconversion—a fourfold rise in titer—or the appearance of IgM, which would indicate a recent or current infection. If nothing changes on the second test, it argues against acute infection. Because the initial result is not definitive, repeating the test in that time frame is the appropriate next step.