BackgroundSubcutaneously administered low-molecular-weight heparins are widely used for prevention of venous thromboembolism. The appropriateness of the subcutaneous route in critically ill patients has never been established.ObjectiveTo determine anti-Xa activities in critically ill patients and in noncritically ill patients receiving prophylactic doses of subcutaneous enoxaparin.DesignProspective, controlled, open-labeled study.SettingTertiary medical-cardiologic-postoperative intensive care unit and a general medical ward at a university hospital.PatientsA total of 16 intensive care unit patients (group 1; age, 61.1 ± 16 yrs; male/female ratio, 7/9; Acute Physiology and Chronic Health Evaluation II score, 20.9 ± 7; mechanical ventilation, n = 15; vasopressors, n = 13) and 13 noncritically ill medical patients (group 2; age, 61.7 ± 9 yrs; male/female ratio, 7/6) were studied. Body mass index (25.7 ± 5 vs. 24 ± 6 kg/m2,p= not significant) was comparable and serum creatinine levels (0.83 ± 0.25 vs. 1.07 ± 0.3 mg/dL, group 1 vs. 2) were within the normal range in both groups. Patients with impaired renal function, receiving hemofiltration, or requiring therapeutic anticoagulation were not eligible.InterventionsNone.Measurements and Main ResultsAnti-Xa activities were determined at 0, 1, 3, 6, and 12 hrs after a single daily subcutaneous dose of 40 mg enoxaparin on day 1 and at 3 hrs after 40 mg of enoxaparin on days 2–5. Mean anti-Xa levels at 0 to 12 hrs were consistently lower in group 1 compared with group 2 by analysis of variance (p= .001 between groups and over time), as was the area under the curve at 0 to 12 hrs (2.6 ± 1 vs. 4.2 ± 1.7 units·mL−1·hr−1, group 1 vs. 2,p= .008). Significant differences in anti-Xa activity were also found on days 2–5 (p= .001). Peak anti-Xa activities at 3 hrs after administration were negatively correlated with the body mass index (r= −.41,p< .03). No correlation was found between the anti-Xa activity at 3 hrs and the dose of norepinephrine (r= .12,p= .7).ConclusionCritically ill patients with normal renal function demonstrated significantly lower anti-Xa levels in response to a single daily dose of subcutaneous enoxaparin when compared with medical patients in the normal ward.