Fondaparinux monitoring is not required among noncritically ill patients due to a predictable dose-response effect. However, this is debatable among critically ill patients, because fondaparinux bioavailability can be influenced by complicated medical conditions.


To investigate fondaparinux monitoring among the critically ill.


Retrospective analysis of patients admitted in intensive care unit from February 2021 to December 2021, who received prophylactic fondaparinux and had anti-Xa activity tests.


Of 156 anti-Xa values, 86 (55.1%) were within 0.10–0.50 μg/mL (the recommended prophylactic range), 38 (24.4%) were less than 0.10 μg/mL, 32 (20.5%) were greater than 0.50 μg/mL, demonstrating an unpredictable dose-response effect. Among 70 patients, thrombotic tendency was controlled in 32 (45.7%), thrombosis progressed in 22 (31.4%), bleeding events occurred in 16 (22.9%). Patients with progressed thrombosis had 17 of 54 (31.5%) anti-Xa less than 0.10 μg/mL, even though this proportion was greater than that of patients with controlled thrombotic tendency (11 of 72, 15.3%), it was similar to that of patients with bleeding (10 of 30, 33.3%), indicating a weak practicability of anti-Xa for monitoring fondaparinux efficacy. Thrombin-antithrombin complex showed a gradual decline among patients with controlled thrombotic tendency, but a bounce-back effect among patients with progressed thrombosis. Thrombelastography R value above the upper reference value occurred more frequently among patients with bleeding (4 of 6, 66.7%) compared to patients without bleeding (4 of 22, 18.2%) (P = .01).


The fondaparinux dose-response effect was unpredictable among the critically ill; anti-Xa activity combined with thrombin-antithrombin complex and thrombelastography can be helpful to guide a precise fondaparinux therapy in this population.

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Author notes

This work was supported by the National Natural Science Foundation of China (82302601 to Liqin Ling, and 82002215 to Yang Fu).

The authors have no relevant financial interest in the products or companies described in this article.