In April 2026, NASA’s Office of the Chief Medical and Health Officer (OCHMO) initiated a working group to review updated VTE case information, additional data collected revealing altered blood flow status within a cohort of astronauts, and discuss the progress of research and clinical activities aimed at mitigating the risk of VTE during spaceflight with new evidence-based clinical practice recommendations.

Below is a summary of the working group’s recommendations:

• The working group’s conclusions emphasized that stasis in the left internal jugular vein (LIV) is consistently considered a dominant risk factor for VTE in microgravity, although there is active debate about the relative contributions of slow qualitative flow, endothelial factors, and/or retrograde flow.
• Limitations of current in-flight ultrasound capabilities to accurately measure stasis/slow flow were cited as a concern, especially if stasis alone is used as a factor to indicate the need for prophylaxis.
• Further review of stasis assessment with in-flight ultrasound was recommended.
• After reviewing the risk factors, additional post-workgroup discussions led to the majority of the panel agreeing that stasis and retrograde flow justified the use of prophylaxis.
• The task force also recommended evaluating all other risk factors other than stasis to also determine when prophylaxis is warranted.
• Based on a review of the literature and a summary of the panel’s contributions, it is VTE Risk Scoring Algorithm for Astronauts was developed, which includes providing anticoagulant prophylaxis only for stasis or a combination of other thrombosis risk factors weighed by the terrestrial literature.

NASA initially formed a task force in October 2024 after diagnosing venous thromboembolisms (VTE) in astronauts during missions to the ISS. Experts reviewed case data, updated clinical practice guidelines and examined possible causes.