American Board of Internal Medicine (ABIM) Certification Practice Exam

In the context of diagnosing hematologic malignancies, a lumbar puncture is often necessary to evaluate for central nervous system involvement, particularly when leukemia or lymphoma is suspected. The threshold for safely performing a lumbar puncture in patients with thrombocytopenia, which is a low platelet count, is critical to ensure that the procedure can be performed with minimal risk of complications, particularly bleeding.

A platelet count of 20,000/microL is widely accepted in clinical practice as the minimum threshold for performing a lumbar puncture. At this level, the risk of significant bleeding is reduced to an acceptable range, thus allowing physicians to safely obtain cerebrospinal fluid (CSF) for diagnostic analysis. If the platelet count falls below this threshold, the risks of a lumbar puncture may outweigh the potential benefits, leading clinicians to consider alternative diagnostic approaches or to take precautions to mitigate bleeding risk.

The other options provided either propose a higher or lower platelet threshold, which may not align with the established guidelines for this procedure. Therefore, the correct threshold that enables safe and effective lumbar puncture in cases of suspected hematologic malignancies is indeed 20,000/microL.