Anti Xa for Heparin Infusion

Interpretation

Method

This method determines a factor Xa inhibitor level by measuring the inhibition of factor Xa cleavage of a chromogenic substrate. Because no exogenous antithrombin is added in this assay, and because antithrombin plays a significant role in factor Xa inhibition with heparin, low molecular weight heparin or fondaparinux, patients with lower antithrombin levels (<70%) may demonstrate lower anti-Xa activity.

Reference Range

Units: IU/mL

Ref. Range Notes

Regular Intensity Heparin Therapy: 0.3-0.7 IU/mL

Low Intensity Heparin Therapy: 0.3-0.5 IU/mL

For more information, visit UW Medicine Anticoagulation Services

Interferences and Limitations

HIXA results will be falsely elevated in the presence of direct factor Xa inhibitors (such as apixaban, rivaroxaban, edoxaban) or low molecular weight heparin.

Low antithrombin level (<70%) may cause an underestimation of the heparin level.

This assay is insensitive to hemoglobin (up to 0.15 g/dL), conjugated bilirubin (up to 28.8 mg/dL), unconjugated bilirubin (up to 13.8 mg/dL), and triglycerides (up to 690 mg/dL).

Ordering & Collection

Specimen Type

Blood

Collection

4.5 mL BLUE TOP (CITRATE) tube or 2.7 mL BLUE TOP (CITRATE) Tube or 1.8mL "Light" BLUE TOP (Citrate) tube

Handling Instructions

Specimen must be processed/centrifuged within one hour of collection. After initial centrifugation the test is viable on samples up to 4 hours old. If testing cannot be done within 4 hours, remove and re-spin plasma, decant upper layer of plasma and freeze until testing can be performed.

Sodium citrate samples at UW-NW must be centrifuged within one hour and are stable at room temperature up to 2 hours. If there is a delay greater than 1 hour, re-spin plasma, remove upper layer of plasma and freeze until testing can be performed.

Outside Laboratories: Within one hour of collection, centrifuge sample for 10 minutes, remove plasma & re-spin for another 10 minutes. Remove upper layer of plasma and freeze (1 mL) @ -20 ˚C to - 80 ˚C. Transport frozen on dry ice.

Note: Samples that are drawn through a heparinized line may be contaminated by the heparin in the line. HIXA results on this sample would reflect the anti-Xa activity of the heparin in the patient as well as the heparin from the line and may be erroneously increased.

Samples containing platelet clumps or hemolysis due to difficult blood draws may show decreased heparin activity due to in vitro platelet activation. If results are unexpectedly low, redraw the sample and repeat the test.

Quantity

Requested: 1.0 mL citrated plasma
Minimum: 0.5 mL citrated plasma

Processing

Receiving Instructions

Take whole blood specimen to COAG Lab

For HIXA as add-on test to previous sample, check with coag testing bench personnel to determine if testing can be performed due to specimen stability or volume issues

Misc Sendout

Performance

Lab Department

Coagulation(COAG)

Frequency

Daily, performed as received

Available STAT?

Yes

Performing Location(s)

HMC	Coagulation 206-520-4600 325 9th Ave, Rm # GWH-47, Seattle, WA 98104-2420
UW-NW	Main Lab 206-668-1344 UW Medical Center – Northwest 1550 N 115th Street, A200 Seattle, WA 98133
UW-MT	Coagulation 206-520-4600 Clinical Lab, Room NW220, University of Washington Medical Center, 1959 NE Pacific street, Seattle, WA 98195

Billing & Coding

CPT Codes

85520

LOINC

66483-9

Interfaced Order Code

UOW2729

Interfaced Result Code

UOW2729