Heparin anti Factor Xa

Heparin is a negatively charged mucopolysaccharide extracted from either bovine lung or porcine intestinal mucosa for pharmacological use as an anticoagulant. Unfractionated heparin (UFH) therapy must be monitored because the response can vary considerably from patient to patient. Traditionally, UFH therapy has been monitored using the activated partial thromboplastin time (aPTT) and dose adjustments were made based on the aPTT according to a heparin dosing protocol. However, the correlation between aPTT and heparin levels was imperfect. Heparin assays based on either protamine titration or anti-factor Xa activity are more accurate, but are not widely used in the US due to perceived lack of availability and cost. A study of 268 patients comparing UFH therapy monitoring by either the aPTT or by an anti-factor Xa heparin assays showed that the number of monitoring tests and dosage changes were significantly less in the heparin assay group as compared to the aPTT group (Rosborough TK. Pharmaco-therapy 1999;19:760-766). Another study showed a marked decrease in the time to achieving therapeutic anticoagulation by using anti-factor Xa heparin for monitoring, as compared to historical controls that used aPTT for monitoring, from a mean of 63 h to a mean of 34 h (Baird RW. BUMC Proceedings 2001;14:294-296).

Therapeutic monitoring for low molecular weight heparin (LMWH) is not routinely recommended. However, under certain circumstances, such as impaired renal function and extreme obesity, monitoring LMWH therapy may be appropriate. In these patients, specimens for monitoring should be drawn 4 hours after the last dose, if a standard twice daily dosing regimen is being used.

Historically, heparin order sets have used aPTT for monitoring and dose adjustments. Revised heparin order sets with dosing guidelines based on heparin levels were introduced in July 2008. The new order sets specify the use of heparin level (heparin anti-factor Xa assay)for monitoring and provide the recommended schedule of testing. The heparin anti-factor Xa assay can be ordered STAT and has a turnaround time similar to the aPTT.

Results of the heparin anti-factor Xa assay will be reported in IU/mL with the following interpretive comments:

  • Expected therapeutic range for unfractionated heparin: 0.3 - 0.7 IU/ml (or as specified by order set or physician)
  • Expected therapeutic range for low molecular weight heparin: 0.5 - 1.0 IU/ml (or as specified by order set or physician)
  • Critical value is 1.1 IU/ml for UFH (for non-cardiac patients).

The anti-factor Xa assay is not valid for grossly lipemic specimens. In this situation, an aPTT should be used to monitor and adjust heparin doses. The following table can be used to convert aPTT to heparin levels.

aPTT (seconds)

Heparin Assay (U/mL)

IV Heparin Adjustment

35 or less


Give full IV dose & increase rate by 300 units/hour

36 – 59

0.06 – 0.29

Give half of IV bolus and increase rated by 100 units/hour

60 – 100

0.30 – 0.69

No change

101 – 120

0.70 – 0.90

Decrease rate by 100 units/hour



Hold infusion for one hour, then decrease rate by 200 units/hour

Specimen requirement is one light blue top (citrate) tube. To avoid tissue fluid contamination a 5mL plain red top tube should be drawn first and discarded. The specimen should be delivered to the laboratory within two hours of collection. If transportation will be delayed the tube should be centrifuged for 10 minutes at 2500 g and the plasma transferred to a plastic tube. The plasma should be centrifuged again and the supernatant transferred to a screw capped plastic vial and frozen.

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