Fluctuations of anti-Xa concentrations during maintenance enoxaparin therapy for neonatal thrombosis

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To evaluate fluctuations in anti-Xa concentrations in infants treated with enoxaparin for thrombosis and describe clinical outcomes.


A retrospective chart review was performed on infants treated with enoxaparin in the Neonatal Intensive Care Unit, and data on enoxaparin doses, anti-Xa concentrations, clinical characteristics and outcomes were abstracted.


Our cohort (n = 26) had a median gestation of 36 (range, 23–41) weeks, birthweight of 2522 (510–3912) grams and 5-min Apgar score of 8(4–9). Fifteen (57.7%) infants were males. Thromboses was diagnosed at a median age of 22 (range, 1–97) days; enoxaparin was initiated at 27.5 (range, 4–98) days at a mean (SD) dose of 1.4 (0.3) mg/kg every 12 h. Therapeutic anti-Xa concentrations (0.5–1 U/mL) were achieved at a mean (SD) dose of 2.1 (0.6) mg/kg at 12.5 (12.2) days of treatment. Of the 143 anti-Xa concentrations, 39 (27%) were within the therapeutic range. During maintenance therapy following initial therapeutic anti-Xa concentration, 40% concentrations were therapeutic. Minor bleeding was noted in four infants and intracranial bleed in one infant; four infants died. During treatment, thrombocytopenia, renal and hepatic impairment during treatment were noted in 7, 2 and 4 infants, respectively. Clot resolution was observed in 21 (81%) infants.


Anti-Xa concentrations fluctuate during maintenance enoxaparin therapy, with therapeutic levels being achieved only sporadically in young infants. Despite this, enoxaparin appears efficacious in thrombosis resolution. Further studies on the impact of stringent control of concentrations on outcomes in this population are warranted.

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