Factors influencing enoxaparin anti-Xa activity in surgical critically ill patients

The objectives of the present study were to describe the incidence of low anti-Xa levels defined as below 0.1 IU/mL in a general surgical intensive care unit population and to evaluate factors independently influencing anti-Xa activity.

A prospective study was undertaken.

Thirty-six patients admitted to a general intensive care unit and receiving subcutaneous (SC) enoxaparin 30 mg twice daily for thromboprophylaxis between November 2003 and August 2005 were included in the study.

After reaching steady state, anti-Xa activity was determined by chromogenic assay at 0, 3, 6, and 9 hours after injection. Anti-Xa levels below 0.1 IU/mL at any time were considered subtherapeutic. Areas under the curve (AUCs) for a 12-hour dosing interval were estimated. Factors influencing anti-Xa AUC were evaluated using linear regression. Two patients (5.6%) did not attain therapeutic levels defined as anti-Xa more than 0.1 IU/mL at 3 hours post dose. Median AUC was 1.84 IU·h/mL (interquartile range, 1.47 IU·h/mL). In the linear regression analysis, sex and creatinine clearance were significant predictors of anti-Xa AUC(0-12h) levels.

In the study, prophylactic SC enoxaparin in critically ill patients at the current 30 mg SC twice daily dosage attained an anti-Xa level more than 0.1 U/mL in nearly all patients. In addition, low creatinine clearances and female sex are associated with higher anti-Xa activity AUC(0-12h).