Anti-Xa Activity Test Is Needed but Is Not Enough for Monitoring Fondaparinux Therapy Among Critically Ill Patients.

Context.—: 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.

Objective.—: To investigate fondaparinux monitoring among the critically ill.

Design.—: 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.

Results.—: 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, and 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%), and bleeding events occurred in 16 (22.9%). Patients with progressed thrombosis had 17 of 54 (31.5%) anti-Xa values 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).

Conclusions.—: 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.