Assessment of Anti-Factor Xa Levels in Children Receiving Low-Molecular-Weight Heparin for Treatment and Prophylaxis

Background: The use of low-molecular-weight heparins (LMWH) for the treatment and prevention of thromboembolic diseases in pediatric patients is increasing, although optimal dosing and monitoring strategies remain insufficiently defined. Methods: This study was conducted at the Clinic of Pediatrics, University Hospital “St. George”, and included 26 hospitalized patients aged 0–18 years with confirmed arterial or venous thrombosis receiving treatment or prophylaxis with enoxaparin. A total of 42 samples were analyzed. Anti-factor Xa activity was measured using an LMWH-calibrated anti-FXa assay (Innovance Heparin, Siemens Healthineers) on a Sysmex CS-2500 analyzer. Therapeutic and prophylactic dosing followed CHEST 2012 guidelines. The study evaluated age- and weight-based dosing, the number of dose adjustments required to achieve target Anti-Xa levels, and the relationship between Anti-Xa levels and anticoagulant response. Results: The mean baseline Anti-Xa level achieved with the initial weight-based dose (1.0 mg/kg/12 h) was significantly lower in children aged 1–2 years compared with older age groups. Older children, as well as patients with oncological and nephrological diseases, achieved higher anticoagulant levels with standard prophylactic dosing. Age under 1 year, oncological disease, and baseline Anti-Xa level < 0.3 IU/mL were identified as independent predictors of the need for more frequent dose adjustments to achieve a therapeutic response. Conclusions: Monitoring of Anti-factor Xa levels is essential in pediatric patients receiving LMWH for both treatment and prophylaxis. Standard dosing regimens may be insufficient in younger children and specific clinical subgroups, supporting the need for individualized dosing strategies.