Heparin Anti-Xa
| Heparin Anti-Xa | | | |
| Number | | 117101 |
| CPT | | 85520 |
| Related Information | | Hemostasis and Thrombosis Appendix |
| Synonyms | | Heparin Assay |
| Special Instructions | | If the patient's hematocrit exceeds 55%, the volume of citrate in the collection tube must be adjusted. Refer to Coagulation Collection Procedures for directions. |
| Specimen | | Plasma, frozen |
| Volume | | 2 mL |
| Minimum Volume | | 1 mL |
| Container | | Blue-top (sodium citrate) tube |
Collection | | Blood should be collected in a blue-top tube containing
3.2% buffered sodium citrate.1 Evacuated
collection tubes must be filled to completion to ensure a
proper blood to anticoagulant ratio.2,3 The
sample should be mixed immediately by gentle inversion at
least six times to ensure adequate mixing of the
anticoagulant with the blood. A discard tube is not
required prior to collection of coagulation
samples.4,5 When noncitrate tubes are collected
for other tests, collect sterile and nonadditive (red top)
tubes prior to citrate (blue top) tubes. Any tube
containing an alternate anticoagulant should be collected
after the blue-top tube. Gel-barrier tubes and serum tubes
with clot initiators should also be collected after the
citrate tubes. Centrifuge and carefully remove the plasma
using a plastic transfer pipette, being careful not to
disturb the cells. Transfer the plasma into a LabCorp PP
transpak frozen purple tube with screw cap (LabCorp
No 49482). Freeze immediately and maintain
frozen until tested. To avoid delays in turnaround time
when requesting multiple tests on frozen samples, please
submit separate frozen specimens for each test requested.
Please print and use the Specimen
Collection Bulletin as a tube-filling guide. |
| Storage Instructions | | Freeze |
| Causes for Rejection | | Gross hemolysis; clotted specimen; frozen specimen thawed in transit; improper labeling |
| Reference Interval | | Reference intervals indicate therapeutic levels. - Unfractionated heparin (UFH): 0.30-0.70 IU/mL
- Low molecular weight heparin (LMWH): 0.50-1.00 IU/mL
|
| Use | | Determine the plasma level of unfractionated heparin (UFH) or low molecular weight heparin (LMWH) |
| Methodology | | The heparin anti-Xa assay is based on the ability of heparin to inhibit the activity of activated factor X (Xa) in the reagent. The reagent includes excess antithrombin, making the heparin in the sample the rate-limiting reagent for Xa inhibition. Heparin in the patient sample inhibits the enzymatic conversion of a Xa-specific chromogenic substrate to colored product by factor Xa. Standard curves are created using multiple concentrations of UFH and LMWH and are used to calculate concentration in the patient plasma. |
| Additional Information | | UFH and LMWH are anticoagulants commonly used to decrease the risk of venous or arterial thrombosis.6,7,8,9 Overdosing with these medications can increase the risk of hemorrhage and inadequate dosing decreases the efficacy of anticoagulation. These drugs work as anticoagulants by enhancing the ability of plasma antithrombin to bind and inactivate the serine proteases XIIa, XIa, IXa, Xa, and thrombin. Therapeutic monitoring is commonly performed because of the wide interindividual variation in response to this therapy. Historically, many laboratories have monitored heparin levels using the activated partial thromboplastin time (aPTT) test. A large number of conditions can complicate the use of the aPTT in monitoring UFH therapy.6 The College of American Pathologists (CAP) divided these factors into three groups.6 The first group of complicating factors encompasses those that affect the bioavailability of heparin. Aging, obesity, changes in heparin binding proteins, hepatic disease, renal disease, and heparin resistance fall under this heading. The second group of complicating factors includes those that alter the aPTT dose response to heparin. Increased factor VIII or fibrinogen levels, decreased antithrombin levels, or a mild reduction in multiple factors (as might be seen in the early stages of a consumptive coagulopathy or in oral anticoagulant therapy) are examples of this type of complicating factor. Lastly, those factors that tend to cause a prolonged aPTT in the absence of heparin therapy should be considered. Lupus anticoagulants or deficiency in contact factors can produce an extended baseline aPTT. In all of these cases, the heparin anti-Xa assay may be more appropriate for monitoring heparin therapy. The aPTT should not be used for therapeutic monitoring of low molecular weight heparin because this drug typically does not affect the aPTT significantly.6,7 |
| Footnotes | | - Adcock DM, Kressin DC, and Marlar RA, “Effect of 3.2% vs 3.8% Sodium Citrate Concentration on Routine Coagulation Testing,” Am J Clin Pathol, 1997, 107(1):105-10.
- Reneke J, Etzell J, Leslie S, et al, “Prolonged Prothrombin Time and Activated Partial Thromboplastin Time Due to Underfilled Specimen Tubes With 109 mmol/L (3.2%) Citrate Anticoagulant,” Am J Clin Pathol, 1998, 109(6):754-7.
- “National Committee for Clinical Laboratory Standardization: Collection, Transport, and Processing of Blood Specimens for Coagulation Testing and General Performance of Coagulation Assays; Approved Guideline,” Third Edition, Villanova: NCCLS Document H21-A3:11(23), 1999.
- Gottfried EL and Adachi MM, “Prothrombin Time and Activated Partial Thromboplastin Time Can Be Performed on the First Tube,” Am J Clin Pathol, 1997, 107(6):681-3.
- McGlasson DL, More L, Best HA, et al, “Drawing Specimens for Coagulation Testing: Is a Second Tube Necessary?” Clin Lab Sci, 1999, 12(3):137-9.
- Olson JD, Arkin CF, Brandt JT, et al, “College of American Pathologists Conference XXXI on Laboratory Monitoring of Anticoagulant Therapy: Laboratory Monitoring of Unfractionated Heparin Therapy,” Arch Pathol Lab Med, 1998, 122(9):782-98.
- Hirsh J, Anand SS, Halperin JL, et al, “Guide to Anticoagulant Therapy: Heparin: A Statement for Healthcare Professionals From the American Heart Association,” Circulation, 2001, 103(24):2994-3018.
- Hirsh J, Warkentin TE, Shaughnessy SG, et al, “Heparin and Low-Molecular-Weight Heparin: Mechanisms of Action, Pharmacokinetics, Dosing, Monitoring, Efficacy, and Safety,” Chest, 2001, 119(1 Suppl):64S-94S.
- Francis CW and Berkowitz SD, “Antithrombotic and Thrombolytic Agents,” Consultative Hemostasis and Thrombosis, Kitchens CS, Alving BM, and Kessler CM, eds, Philadelphia, PA: WB Saunders Co, 2002, 375-91
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