Given a heterophile antibody titer of 224, which of the following results indicate serum sickness?

A heterophile antibody titer of 224 suggests the presence of heterophile antibodies, commonly associated with infections such as infectious mononucleosis. In the context of serum sickness, the expected laboratory findings involve an assessment of how the titer changes after dilution.

When evaluating the outcomes based on titer reductions, a five-tube titer reduction indicates a significant decline in the concentration of heterophile antibodies upon serial dilution, which is consistent with serum sickness. This condition typically arises after exposure to foreign proteins, leading to the production of antibodies against those proteins. In serum sickness, titers usually decrease as the immune system clears these antibodies.

In the case provided, two five-tube reductions suggest that the immune response is actively diminishing the titer levels of heterophile antibodies, supporting the diagnosis of serum sickness. The significant reduction in titers reflects a typical response where the antibodies are being sufficiently neutralized or cleared from circulation over multiple dilutions.

This reasoning highlights why option C is the correct choice—it effectively demonstrates the expected laboratory response associated with serum sickness.