A bloody tap on lumbar puncture (LP) is most commonly associated with which condition?

A bloody tap during a lumbar puncture (LP) is most commonly associated with subarachnoid hemorrhage. This condition occurs when there is bleeding into the subarachnoid space, often due to a ruptured cerebral aneurysm or trauma. When performing an LP in a patient with subarachnoid hemorrhage, blood can enter the cerebrospinal fluid (CSF) directly, resulting in a bloody tap. The presence of xanthochromia (yellow discoloration of the CSF) due to the breakdown of bilirubin from the hemoglobin released in the CSF can be observed if the LP is performed a few hours to days after the hemorrhage.

While other conditions might cause a bloody tap to some extent, such as bacterial meningitis or encephalitis, they are less commonly associated with a clearly bloody tap compared to subarachnoid hemorrhage. In bacterial meningitis, pleocytosis and the presence of pathogens are more typically noted, and any blood present would often come from a traumatic tap rather than from the condition itself. Therefore, subarachnoid hemorrhage is the primary condition linked to a definitive bloody tap during an LP.