Anti-factor Xa Levels (Enoxaparin and Heparin)

*Standard dosing (enoxaparin 30 q12h) may be inadequate in high risk trauma; lead to inc rates of DVT

Monitor antifactor Xa level

  • Normal value = <0.1
  • >0.5 considered therapeutically anticoagulated
  • 0.2 to 0.5 considered target level for prophylaxis

–Send trough before 4th dose

  • If normal –>increase to 40mg q12h; then 50 sq q12h or even 60 q12h

–With this approach, majority of patients require enoxaparin in 40 q12h dose



Patients with morbid obesity or renal failure may require monitoring of Anti-Xa levels.

  • anti-Xa levels in plasma should be measured 4 hours after LMWH administration
  • desired anti-Xa level = 0.6-1.0 units/mL for BID dosing and >1 unit/mL for OD dosing


Early RCTs on LMWS excluded the following patients:

1. BMI>50 kg//m2

2. Pregnant

3. Renally impaired (Cr clearance <30 ml/min)


UFH affects Factor II and Factor Xa, LMWH affects predominantly Factor Xa.

Peak anti-factor Xa level is reached 3-5 hours after administration.


Suggested peak anti-factor Xa levels for enoxaparin (therapeutic):

1. BID dosing – 0.6 to 1.0 IU/mL

2. OD dosing – 1.0-2.0 IU/mL


Table. Target anti-factor Xa ranges for therapeutic anticoagulation with LMWH.


Target range for prophylactic doses of LMWH not well defined.

Table. Target anti-factor Xa ranges for prophylactic doses of LMWH.


Limited evidence available, but guidelines by ACCP suggests use of increased doses of LMWH perioperative for bariatric patients. Anti-factor Xa monitoring is recommended in patients with high-risk trauma and burns. Critically ill patients on inotropes may also be subtherapeutic on LMWH due to impaired peripheral circulation.


Table.  Target anti-factor Xa ranges for thromboprophylaxis in bariatric patients.


BOTTOM LINE:  Reasonable anti-factor Xa target range is 0.2-0.5 IU/mL. Prospective studies are required to validate this recommendation.




Marino, P. and Sutin, K. (2007). The ICU book. Philadelphia: Lippincott Williams & Wilkins.

Wei, M. and Ward, S. (2015). The anti-factor Xa range for low molecular weight heparin thromboprophylaxis. Hematology Reports, 7(4).

Click to access Therapeutic_Unfractionated_Heparin_Infusion_Guideline.pdf



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