American Board of Internal Medicine (ABIM) Certification Practice Exam

What CD4 count is associated with a concern for primary CNS lymphoma?

1. 100/µL

2. 75/µL

3. 50/µL

4. 25/µL

The correct answer is: 50/µL

A CD4 count of 50/µL is particularly concerning for the development of primary central nervous system (CNS) lymphoma, especially in the context of HIV infection. This threshold reflects a significant level of immunosuppression, which increases the risk of opportunistic infections and malignancies, including CNS lymphoma. In patients with advanced HIV disease, such low CD4 counts correlate with higher incidence rates of various neurological complications, including CNS lymphoma. This type of lymphoma is more common in individuals with severe immunosuppression, as their compromised immune systems are less capable of controlling abnormal cell growth, leading to an increased risk of malignancies. While higher CD4 counts (like 75/µL or 100/µL) indicate some level of immune function, they are still low enough that patients may be at risk for opportunistic infections but typically not at the same elevated risk for CNS lymphoma as those with counts at the 50/µL mark. A CD4 count of 25/µL reflects extreme immunosuppression and could certainly indicate various opportunistic processes, including the potential for CNS lymphoma, but the noted correlation and risk is strongest at the 50/µL threshold.