Test Overview
Test Methodology

Chromogenic Substrate Assay

Test Usage

Used to monitor treatment with low-molecular weight heparins: danaproid, enoxaparin, dalteparin, fragmin, lovenox

Reference Range *

Normal individuals have no low molecular weight heparin activity.
Target Therapeutic Range:

  • Enoxaparin 1.0 mg/kg SC twice daily, 5 hours after dose 0.5 - 1.0 IU/mL;
    Enoxaparin 1.5 mg/kg SC once per day, 5 hours after dose 1.0 - 2.0 IU/mL;
    Enoxaparin 1.0 mg/kg SC once per day
    Trough <0.4 IU/mL drawn just prior to next dose.

Critical value is >2.0 IU/mL.

* Reference ranges may change over time. Please refer to the original patient report when evaluating results.

Test Details
Days Set Up
Daily, 24 hours
Analytic Time

2 hours

Soft Order Code
MiChart Code
Anti-Xa, (Enoxaparin, Lovenox) Low Molecular Weight Heparin
  • Coagulation Factor Xa Inhibition
  • Factor Xa Inhibition
  • Heparin (LMWH) Anti-Xa Assay
  • Heparin, Low Molecular Weight
  • Low Molecular Weight Heparin
  • Enoxaparin
  • Lovenox
  • Dalteparin
  • Fragmin
  • LMWH Anti-Xa
  • Anti-Xa, LMWH
  • Anti-Xa LMW Heparin
  • Chromogenic Xa
  • Xa Chromogenic
  • LMWH
  • Factor 10a Inhibition
  • Anti-10a LMWH
STAT Availability

STAT requests for this test will be performed on a STAT basis (supervisory staff approval is not required).

Specimen Requirements
Collection Instructions

Collect specimen in a blue top 3.2% sodium citrate Vacutainer® tube. Mix by inversion. Specimen should arrive at lab within 23 hours of collection; transport at room temperature. Alternatively, centrifuge, aliquot plasma into a plastic tube, and freeze the specimen within 24 hours of collection. Transport frozen specimen on dry ice. Collection of the blood through lines that have been previously flushed with heparin should be avoided. If the blood must be drawn through a VAD (vascular access device), the line should be flushed with 5 mL of saline and the first 5 mL of blood or six dead space volumes of the VAD discarded.

Special Handling

Specimen must be received in lab within 23 hours of collection to allow for adequate processing and testing time. Alternatively, aliquot and freeze within 24 hours of collection.

Test is not indicated if the patient is receiving unfractionated heparin or other Anti-Xa drugs.
Normal Volume
Full 2.7mL blue top 3.2% sodium citrate Vacutainer® tube or 1mL plasma aliquot
Minimum Volume
Full 1.8mL blue top 3.2% sodium citrate Vacutainer® tube or 1mL plasma aliquot
Storage Temperature
Room temperature for whole blood or frozen aliquot of citrated plasma.
Rejection Criteria
Specimens will be rejected if not properly filled, clotted, grossly hemolyzed, contaminated with heparin or received outside acceptable time limits.
Additional Information

Sample should be drawn after 3 doses of enoxaparin, at steady state.
The peak level is recommended and should be drawn 4-6 hours post dose.
Trough level is preferred in renally impaired patients.

CPT Code
Fee Code