Given a heterophile antibody titer of 224, which of the following results indicate infectious mononucleosis (IM)?

A heterophile antibody titer is primarily used in diagnosing infectious mononucleosis, a condition commonly caused by Epstein-Barr virus (EBV). In the context of this question, the interpretation of the titer reduction is essential for confirming the diagnosis.

When a heterophile antibody titer is performed, a decrease in titer across multiple dilutions indicates that the antibodies are present at high levels initially, and their concentration is reducing as the serum is diluted. This reduction is indicative of active disease, as the immune response to EBV is characterized by the presence of these heterophile antibodies.

The option indicating a "Two-tube titer reduction" and "Five-tube titer reduction" confirms that the heterophile antibodies are still detectable at dilutions, suggesting that the patient is likely still experiencing the symptoms of infectious mononucleosis, reinforcing the diagnosis. A reduction in titers across multiple dilutions is a classic hallmark of the body's adaptive immune response to an infection, particularly in the case of infectious mononucleosis.

In contrast, the other options may suggest that there is either no ongoing infection or that the heterophile antibodies are not active or present, which would make them less reliable indicators of infectious mononucleosis. Thus,