David L. McGlasson, MS, MLS(ASCP)

Posted: Friday 22, 2019

Since its introduction in 1953, the activated partial thromboplastin time (aPTT) has been used to monitor the treatment effect that heparin is having on a sample of blood. And for about as long, a litany of factors that interfere with the aPTT test’s ability to accurately measure heparin levels has also been identified. In addition to standard analytical and pre-analytical variables that may affect any lab assay, additional interfering factors ranging from the physiological to the pharmaceutical have been identified.  Further, the aPTT test cannot be used to monitor levels of the low molecular weight heparin (LMWH), such as Enoxaparin, Dalteparin, or Fondaparinux, at all. In 1976 and 1977, Teien introduced an assay to measure levels of heparin by monitoring the release of a chromogenic substance by residual factor Xa in the presence of heparin. This Anti-factor Xa assay can monitor not only, heparin and its LMWH analogues, but also the newer generation of direct oral anti-coagulants (DOACs) as well.


What is heparin, and why do we need to measure it?

Heparin is a naturally occurring oligosaccharide polymer of varying length. Heparin acts as an anticoagulant (blood thinner) by its ability to act of a cofactor. Upon binding to the natural anticoagulant antithrombin, heparin greatly increases the activity of antithrombin speeding up the process of thrombin inactivation. Interestingly, it slows the coagulation primarily by binding to both thrombin (Factor IIa) and Factor Xa is approximately equal proportions.

There are several different types of heparins used in a variety of settings. The aPTT used to be the most commonly used method to monitor the effect of UFH therapy. UFH potentiates the activity of antithrombin and covalently neutralizes thrombin and activated factor X (anti-FXa). The LMWH compounds such as enoxaparin, nadroparin, tinzaparin and dalteparin, selectively catalyze the neutralization of FXa over thrombin and cannot be measured using the aPTT although they may cause a slight prolongation.

Though its method of action does not directly lead to dissolution of the clot, it does allow the body additional time to resolve the clot on its own. Heparin is primarily used for the prevention and/or treatment of spontaneous or iatrogenic venous or arterial thromboembolisms, such as a deep vein thrombus or myocardial infarct, unstable angina, and stroke or transient ischemic attack (TIA, or mini-stroke) to decrease morbidity and improve associated mortality rates. Heparin is frequently used to coat the inner linings of blood contacting tubes used in coronary angioplasty, bypass surgery, and Extra Corporeal Membrane Oxygenation (ECMO) systems.

So, why do we monitor heparin levels? Simply, heparin shows a wide range of activity from patient-to-patient. Heparin may bind nonspecifically to plasma proteins as well as blood and endothelial cells. Binding to the acute phase proteins, such as fibrinogen or Factor VIII, leads to decreases in the effectiveness of heparin and reported aPTT times. Different diseases as well as extremes on either end of the body-mass index spectrum also lead to its wide range of dose responses.

The aPTT has historically been the most commonly used assay to monitor heparin in patients. Clinicians target a heparin dosage that provides an aPTT of 1.5 to 2.5 of a normal control value within 24 hours of start of therapy. Sub-therapeutic heparin levels during these first 24 hours are associated with a high incidence of venous thromboembolism (VTE) recurrence. Too high a dose of heparin may lead to uncontrolled bleeding and hemorrhage; so clinicians need to balance the risk of clotting and bleeding. Unfortunately, a direct relationship between super-therapeutic aPTT times and bleeding is less clear. Recent trauma or surgery, the presence of peptic ulcers, occult malignancy, hemostatic defects of liver disease, low hemoglobin levels, the female gender, and age (older than 65) are all underlying clinical factors with a closer relation to hemorrhagic complications than an elevated aPTT.

In addition to the obvious effect that heparin has on the aPTT test there are many additional variables that have been identified that will affect the aPTT time. Medications such as warfarin, direct thrombin inhibitors, and the DOACs as well or oral contraceptives or herbal supplements will all interfere with the aPTT test. Pre-analytic variables, often due to patient or sample collection that will negatively impact the quality of data include: volume, transport time, citrate concentration present in collection tube used, the patient’s hematocrit, and the presence of hemolysis, icterus, or lipemic samples. Laboratory based analytic variables affecting the aPTT time include: instrumentation and reagent sensitivity, centrifugal speed and forces generated in plasma preparation, the presence of residual platelets which may lead to the neutralization of heparin, the reagent type (such as silica or ellagic acid) used as the activator in the assay itself, the concentration of calcium chloride, lot-to-lot variations, time to sample processing, storage temperature, and method of thawing frozen samples.  There are several physiological factors that will alter the aPTT time, not all come with an increased bleeding risk. The presence of hemophilia A or B, von Willebrand Disease, and a Factor XI deficiency all come with an increased bleeding risk.  However, lupus anticoagulant, Factor XII (the Hageman Factor) deficiency, Prekallikrein (Fletcher Factor), and high molecular weight kininogen (Fitzgerald Factor) deficiencies are all associated with aPTT defects but not with an increased bleeding risk.

Previously the laboratory had to have assays with a different calibration curve to assay each different type of heparin. It also was a problem for our institution in determining the type of heparin to be analyzed. We first have a communication problem. One of the first things we did is go to an on-line web site uabcoag.net. We went to the “confusing test name” page and saw how the University of Alabama at Birmingham Coagulation Laboratory informed their clinicians when confronted with the issue of the proper test orders.


Advantages to using anti-FXa over aPTT

The advantages of using the anti-Xa assay over the aPTT for monitoring subjects on heparin are many. The lack of sensitivity of the anti-Xa method to oral anticoagulants given in conjunction with heparin, different concentrations of sodium citrate in collection tubes, minimal blood draws other than a 9:1 blood to anticoagulant ratio, little to no influence of a lupus anticoagulant, elevated fibrinogen or factor VIII levels, or liver disease are advantages the aPTT can’t compete with (1) (2). We have discovered since that elevated levels of plasma hemoglobin levels, elevated bilirubin and triglycerides in neonates on ECMO can cause issues with chromogenic methods for monitoring heparins (3). The sensitivity of clottable assays to heparin also greatly varies depending on the reagent/instrument combinations, patient response to heparin, and the source of heparin. To date there have been very few protocols that describe how pre-analytical variables affect the anti-Xa chromogenic assay for both UFH and the various LMWH’s (4) (5).

Our ability to monitor UFH and various LMWH’s has improved in the last few years by being able to use a single calibration curve for monitoring subjects. The use of the so called “hybrid curve” which combined calibrators of varying concentrations of UHF and LMWH heparanoids has been accomplished in several studies (1) (6) (7) (8). Researcher’s compared UFH, enoxaparin, tinzaparin, nadroparin and tinzaparin measurements using a single calibration curve with success when compared to a calibration curve for each individual heparin.  At our institution we exposed the anti-Xa chromogenic heparin assay to a number of pre-analytic variables during the specimen collection process. Then we compared individual calibration curves for heparin and enoxaparin with a hybrid curve measuring both types of anticoagulant in each specimen. The hybrid curves results matched those determined on the individual UFH and LMWH calibration curves (9) (10). Since we accomplished these protocols and published the findings several companies have developed and received FDA clearance to distribute the anti-FXa chromogenic assay using a “hybrid curve.” Other companies have them under development.

Additionally, the anti-Factor Xa tests have long been available in a liquid format. This allows for long stability times onboard the analyzer. It also reduces waste and decreases reagent management which allows for lab personnel to spend more time on more productive tasks and eases some storage concerns.

From a clinical perspective, there are advantages of the anti-Factor Xa test as well as it may allow the achievement of the heparin therapeutic range faster (HTR). In a study of 268 subjects on unfractionated heparin, randomized to two groups, the group monitored with anti-Factor Xa was in range 67% of the time, while those monitored with the aPTT test were in range 33% of the time. A subsequent study using a dosing nomogram shows that 80% of the anti-Factor Xa group reaches HTR within 16 hours and 87% achieved within 24. A third study showed that only 43% HTR of subjects using aPTT only with longer times to reach therapeutic levels in 24 hours (11) (12).

If your facility hasn’t switched from the APTT to the anti-FXa for monitoring heparin consider the following comparisons. Most facilities have the instrumentation to perform automated chromogenic assays 24/7. Economic impact studies have found that the anti-Xa assay saved the cost of repeat assays because a higher percentage of patients achieved therapeutic ranges within 24 hours, some as few as 6 hours (13). The same study found that the one-year costs of monitoring patients using the heparin assay equaled APTT costs. Another study performed at the Moses Cone Memorial Hospital in North Carolina found that 48% of patients monitored with the APTT reached therapeutic range in 24 hours compared to 90% when using the anti-Xa method. One study found that 62% of 197 patients achieved therapeutic levels at 7-9 hours and 87% at 16-24 hours using the heparin procedure (14).


Rationale for change to anti-Factor Xa—Cost

Using the anti-Factor Xa test has allowed for great cost savings. One lab saw a shorter time to therapeutic target (TTT) compared to the aPTT. 54% of patients monitored by anti-Factor Xa were within range within 6 hours and 74% were within range within 24 hours. Those monitored with the aPTT showed 27% within range of 6 hours and 63% within range within 24 hours. Those patients monitored with anti-Factor Xa also required fewer dosing changes within 24 hours; an average of 1.7 changes for aPTT monitored patients and an average of 1.0 change for anti-Factor Xa. Another lab saw an average TTT of 28 hours with Anti-Factor Xa and 48 hours for aPTT. They also saw 0.8 dose changes using anti-Factor Xa compared to 1.6 dose changes for the aPTT. Anti-Factor Xa appears to be a more precise measure of heparin than the aPTT test, quicker to HTR, and reduces the number of sample draws required (15). Depending on the condition, the use of anti-Factor Xa has reduced blood transfusions by 4.7-17.5% (16)!


Anti-Factor Xa is not the same as the Factor X test!!

In 2008 in ASCLS Today I described a real-world scenario of being in a coagulation laboratory running through the mundane world of performing PT/INR, APTT, fibrinogen and d-dimer assays when I got the request for a factor X level from the surgical ICU unit at the hospital where I worked PRN. Immediately I suspected this to be an incorrect request.  Since true factor X deficiencies are very rare. I called the unit to inquire about the disposition of the patient. “Does the patient have vitamin K deficiency, ingested old style rat poison, overdose of warfarin or is there some other coagulation problem? “What kind of anticoagulant drug are you monitoring?” The disembodied voice on the other end of the line said the subject was on heparin and they needed a factor X level stat. I then was able to figure out that the unit really wanted a chromogenic anti-Xa level, a test used to monitor heparin dosage. I then asked the person the type of heparin the patient was receiving. “Are they on unfractionated heparin (UFH) or on a low molecular weight heparin (LMWH) such as enoxaparin?” Dead silence occurred then the voice said, “I don’t know they are getting some type of heparin.” This is still a problem in many laboratories. Thankfully we now have some solutions to this problem.


Closing Thoughts

Currently there is no standardized method to compare aPTTs and the comparison studies for anti-Xa methods are limited. However, due to the problems in obtaining proper specimens for calibrating the APTT method against the anti-Xa procedure it is evident that using a method specific for the presence of heparin against a global assay makes the choice easy.

Fondaparinux (Arixtra®) is a synthetic analog of the antithrombin-binding pentasaccharide found in UFH or LMWH and has a higher specific anti-FXa activity than any of the other hepararinoids. If monitoring fondararinux a different calibrator containing that pentasaccharide is required to monitor the medication. These calibrators are now cleared in Europe and Canada but are RUO in the United States. This is the only heparinoid that calibration curves can be determined with “spiked” plasma specimens because it is not a pro drug.

The time has come to make the make the switch to using a specific assay for monitoring all heparins. Many laboratories working in association with their pharmacy using a weight-based heparin protocol can now safely monitor their patient’s anticoagulant therapy with a more specific method (17).

Clinical Data from the last 10-20 years have begun to show that a conversion from APTT to anti-FXa monitoring may offer a smoother dose-response curve, such that levels remain more stable, requiring fewer blood samples and dosage adjustments. Given the minimal increased acquisition cost of the anti-FXa reagents it can be argued that the anti-FXa is a cost-effective method for monitoring UFH (18).

In later articles RA Marlar describes issues and recommendations when monitoring UFH therapy. This was referred to many times in the previous pages reviewing various methods when a lab needs to determine the heparin therapeutic range and help labs in establishing an APTT/heparin therapeutic range for different hospital settings (19). In other studies on enoxaparin dosing and anti-FXa monitoring in special populations studying subjects who are renal-impaired, extremes of body weight and subjects who are pregnant or pediatric patients interesting data was discovered. Since the APTT can’t be used for LMWH testing the anti-FXa was found to be subtherapeutic in overweight subjects and supratherapeutic in underweight and neonates and pediatric patients. Therefore, special dosing has to be prescribed with LMWH and anti-FXa testing (20) (21).

A final note to consider is the monitoring of heparins in patients that have low levels of antithrombin (AT). It is known that the heparinoids cause their anticoagulant effect with activation of antithrombin. In APTT testing with UFH the assay may not be affected in subjects that have decreased levels of anti-thrombin. Therefore, the APTT results may not be in the TTR with these low levels of AT. A recent study looked at the ability to monitor patients with decreased AT levels with the anti-FXa assay. In this study they used 34 AT-deficient patients and 17 family controls that were spiked with UFH and LMWH (nadroparin).  Antithrombin, Beta-AT and anti-Xa activities were assayed. Their results found that reduced antithrombin activity caused clinically and statistically reduced anti-FXa levels. Clinicians may need to adjust dosing in AT deficient subjects because the UFH and LMWH levels may not be in the necessary TTR (22).

Further studies are still ongoing, however; I think we have enough data that show that when measuring all the heparinoids that the chromogenic anti-FXa method may be the best option.


Works Cited

  1. Effects of pre-analytical variables on the anti-activated factor X chromogenic assay when monitoring unfractionated heparin and low molecular weight heparin anticoagultion. McGlasson, David L, et al., et al. 3, 2005, Blood Coagulation & Fibrinolysis, Vol. 16, pp. 173-176.
  2. Preanalytical and Postanalytical Variables: The Leading Causes of Diagnostic Error in Hemostasis. Favaloro, Emmanuel J, Lippi, Giuseppe and Adcock, Dorothy M. 7, 2008, Seminars in Thrombosis and Hemostasis, Vol. 34, pp. 612-634.
  3. Evaluation of Heparin Assay for Coagulation Management in Newborns Undergoing ECMO. Khaja, Wassia A, et al., et al. 6, 2010, American Journal of Clinical Pathology, Vol. 134, pp. 950-954.
  4. Interlaboratory agreemtn in the monitoring of unfractionated heparin using the anti-factor Xa-correlated activated partial thromboplastin time. Cuker, A, et al., et al. 1, 2009, Journal of Thrombosis and Haemostasis, Vol. 7, pp. 80-86.
  5. Interlaboratory agrrement in the monitoring of unfractionated heparin using teh anti-factor Xa-correlated activated partial thromboplastin time: a buttal. Kitchen, S, et al., et al. 12, 2009, Journal of Thrombosis and Haemostasis, Vol. 7, p. 2157.
  6. Meyers, BJ. Suppl 1, 1998, Blood, Vol. 92, p. 123b.
  7. Gilbert, M. 2001, Thrombosis and Haemostasis, Vol. Suppl 1, p. CD3224.
  8. Chan, AKC and et al. 2007, Thrombosis and Haemostasis, Vol. Suppl 1, pp. P-S128.
  9. Using a Single Calibration Curve With the Anti-Xa Chromogenic Assay for Monitoring Heparin Anticoagulation. McGlasson, David L. 5, 2005, Lab Medicine, Vol. 36, pp. 297-299.
  10. Comparison of two anti-Xa assas using a single calibration curve for monitoring heparin anticoagulation. McGlasson, David L and Fritsma, George A. Suppl 2, 2007, Journal of Thrombosis and Haemostasis, Vol. 5, pp. P-T-092.
  11. The Heparin Anti-Xa Therapeutic Range. Smythe, Maureen A, Mattson, Joan C and Koerber, John M. 1, 2002, Chest, Vol. 121, pp. 303-304.
  12. Antifactor Xa Levels versus Activated Partial Thromboplastin Time for Monitoring Unfractioanted Heparin. Vandiver, Jeremy W and Vondracek, Thomas G. 6, 2012, Pharmacotherapy, Vol. 32, pp. 546-558.
  13. Evans, E W. 2000, Clinical Hem, Vol. 14, pp. 8-9.
  14. Achieving Target Antifactor Xa Activity with a Heparin Protocol Based on Sex, Age, Height, and Weight. Roskborough, Terry K and Shepherd, Michele F. 6, 2004, Pharmacotherapy, Vol. 24, pp. 713-719.
  15. Comparison of unfractioanted heparin protocols using antifactor Xa monitoring or activated partial thrombin time monitoring. Fruge, Kristian S and Lee, Young R. Suppl 2, 2015, American Journal of Health Systems Pharmacy, Vol. 72, pp. 590-597.
  16. A comparison of red blood cell transfusion utilization between anti-activated factro X and activated partial thromboplastin monitoring in patients receiving unfractionated heparin. Belk, K W, Laposata, M and Craver, C. 11, 2016, Journal of Thrombosis and Haemostasis, Vol. 14, pp. 2148-2157.
  17. Weight-based heparin protocol using antifactor Xa monitoring. Smith, Michael L and Wheeler, Kathryn E. 5, 2010, American Journal of Health-System Pharmacy, Vol. 67, pp. 371-371.
  18. A Comparative Trial of Anti-Factor Xa Levels Versus the Activated Partial Thromboplastin Time for Heparin Monitoring. Vandiver, Jeremy W and Vondracek, Thomas G. 2, 2015, Hospital Practice, Vol. 41, pp. 16-24.
  19. Activated Partial Thromboplastin Time Monitoring of Unfractionated Heparin Therapy: Issues and Recommendations. Marlar, Richard A, Clement, Bernadette and Gausman, Jana. 3, 2017, Seminars in Thrombosis & Hemostasis, Vol. 43, pp. 253-260.
  20. Enoxaparin Dosing at Extremes of Weght: Literature Review and Dosign Recommendations. Sebaaly, Jamie and Covert, Kelly. 9, 2018, Annals of Pharmacotherapy, Vol. 52, pp. 898-909.
  21. Enoxaparin Dosing and AntiXa Monitoring in Specialty Popuylations: A Case Series of Renal-Impaired, Extremes of Body Weight, Pregnant, and Pediatric Patients. Ahuja, Tania, et al., et al. 10, 2018, P.T., Vol. 43, pp. 609-614.
  22. Monitoring of heparins in antithrombin-deficient patients. Croles, Frederik Nanne, et al., et al. 2019, Thrombosis Research, Vol. 175, pp. 8-12.