From the Guidelines

A lumbar puncture (LP) is generally considered safe to perform at a platelet count of 50 × 10^9 cells/L or higher.

The evidence from studies 1 suggests that bleeding complications after LP are rare, and the risk of serious complications is low even at lower platelet counts.
The AABB recommends prophylactic platelet transfusion for patients having elective diagnostic lumbar puncture with a platelet count less than 50 × 10^9 cells/L, although this is a weak recommendation based on very-low-quality evidence 1.
Other studies suggest that a platelet count of >40 × 10^9/L may be safe, and even lower counts may be acceptable depending on the individual case, considering factors such as the trend and cause of thrombocytopenia 1.

The risk of bleeding complications must be balanced against the benefits of having a definitive diagnosis.
The trend and cause of thrombocytopenia, as well as the presence of other risk factors, should be taken into consideration when deciding whether to perform an LP.
In patients with a rapidly falling platelet count or other coagulopathy, the risk of bleeding complications may be higher, and alternative diagnostic approaches may be considered.

From the Research

The safety of performing a lumbar puncture (LP) at various platelet counts has been investigated in several studies.

A study published in 2023 2 found that the incidence of post-LP spinal bleeding was not associated with platelet counts below the guideline threshold of 50,000 platelets/μL.
Another study from 2024 3 reported that lowering the platelet threshold to 20,000/μL for fluoroscopy-guided lumbar puncture did not result in observed clinical adverse outcomes.
A 2020 study 4 found that the risk of spinal hematoma after lumbar puncture was 0.20% among patients without coagulopathy and 0.23% among those with coagulopathy.
A 2015 study 5 on thrombocytopenic children with cancer found no complications after lumbar punctures, even with platelet counts below 50,000 platelets/μL.

The risk of spinal hematoma after lumbar puncture is low, even in patients with thrombocytopenia.
Platelet counts below 50,000 platelets/μL may not be associated with an increased risk of spinal bleeding.
Lowering the platelet threshold to 20,000/μL for fluoroscopy-guided lumbar puncture may be safe.
The incidence of bloody tap is relatively high, but it is not necessarily associated with serious complications.

50,000 platelets/μL: traditionally considered the minimum safe threshold for lumbar puncture.
20,000/μL: a potentially safe threshold for fluoroscopy-guided lumbar puncture, according to a 2024 study 3.
Below 20,000/μL: the safety of lumbar puncture at these platelet counts is not well established, but a 2023 study 2 found no significant increase in spinal bleeding risk.