Should I use aPTT or anti-Xa levels to monitor my patient on IV heparin infusion?

Despite more than half a century of use unfractionated heparin (UFH), the optimal method to monitor its anticoagulation effect remains unclear, with arguments for and against continued use of activated partial thromboplastin time, aPTT) vs switching to antifactor Xa heparin assay (anti-Xa HA). 1-4

The advantage of aPTT include decades of use and familiarity by providers, and its relative accessibility, ease of automation and cost.1 Its disadvantages include variation among the sensitivities of different aPTT reagents as well as susceptibility to factors that do not reflect intrinsic heparin activity (eg, liver dysfunction, hypercoagulable states). 1,2 Thus patients may receive unnecessarily high or low heparin doses because of physiologic and non-physiologic influences on aPTT.

In contrast, since anti-XA HA measures the inhibition of a single enzyme (factor Xa)1, it is a more direct measurement of heparin activity, with less variability and minimal interference by certain biological factors (eg, lupus anticoagulants). Anti-Xa monitoring may also improve the time to therapeutic anticoagulation and lead to fewer dose adjustments compared to aPTT monitoring.2

The disadvantages of anti-Xa HA include inaccuracy in the setting of hypertriglyceridemia (>360 mg/dL), hyperbilirubinemia (total bilirubin >6.6 mg/dL), recent use of low molecular weight heparin, fondaparinux and direct oral factor Xa inhibitors. Its relative expense and generally less laboratory availability among healthcare facilities may also limit its use in monitoring patients on therapeutic UFH. 1-3

Somewhat unsettling is the frequent discordance between aPTT and anti-Xa values having been reported in 46% to 60% of instances that may result in either thromboembolic or bleeding complications. 1,4 One study reported that aPTT may be therapeutic only 35% of the time that anti-Xa is also therapeutic! 2

What’s clearly missing are definitive studies that can shed light on the clinical impact of these intriguing findings on patient outcomes. So stay tuned!

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  1. Guervil DJ, Rosenberg AF, Winterstein AG, et al. Activated partial thromboplastin time versus antifactory Xa heparin assay in monitoring unfractionated heparin by continuous intravenous infusion. Ann Pharmacother 2011;45:861-68.
  2. Whitman-Purves E, Coons, JC, Miller T, et al. Performance of Anti-factor Xa versus activated partial thromboplastin time for heparin monitoring using multiple nomograms. Clinical and Applied Thromosis/Hemostasis 2018;24:310-16.
  3. Fruge KS, Lee YR. Comparison of unfractionated heparin protocols using antifactory XA monitoring or activated partial thrombin time monitoring. Am J Health-System Pharmacy. 2015; 72: S90-S97,
  4. Samuel S, Allison TA, Sharaf S, et al. Antifactor XA levels vs activated partial thromboplastin time for monitoring unfractionated heparin. A pilot study. J Clin Pharm Ther 2016;41:499-502.
  5. doi:10.1111/jcpt.12415.
Should I use aPTT or anti-Xa levels to monitor my patient on IV heparin infusion?

What is the significance of a prolonged activated partial thromboplastin time (aPTT) in my patient with suspected antiphospholipid syndrome (APS)?

APS is an acquired hypercoagulable state which presents classically as recurrent arterial and/or venous thrombosis and is a major cause of late first- and second-trimester spontaneous fetal loss. In addition to thrombotic complications, diagnosis of APS requires the presence of ≥ 1 of the following antiphospholipid antibodies on 2 occasions ≥12 weeks apart: 1) anti-ß2-glycoprotein 1 antibodies; 2) anticardiolipin antibodies; and 3) lupus anticoagulant (LA)1.  

An unexpected prolongation of aPTT may be a clue to the presence of APS and may be explained by the in vitro prevention of the assembly of the prothrombinase complex—which catalyzes the conversion of prothrombin to thrombin— by LA2,3.  

Because the phospholipid component of the reagent used in aPTT tests determines its sensitivity to LA, aPTT results may vary, influenced by the type and concentration of phospholipids used in the assay. Other factors such as acute phase reaction associated with increased fibrinogen and factor VIII levels may also impact the results by shortening the aPTT and potentially masking a weak LA2.




  1. Giannakopoulos B, Passam F, Ioannou Y, Krilis SA. How we diagnose the antiphospholipid syndrome.Blood. 2009;113:985-94.
  2. 2. Abo SM, DeBari VA. Laboratory evaluation of the antiphospholipid syndrome. Ann Clin Lab Sci 2007;37:3-14.
  3. Smock KJ, Rodgers GM. Laboratory identification of lupus anticoagulants. Am J Hematol. 2009;84(7):440-2.



Contributed by Ricardo Ortiz, medical student, Harvard Medical School

What is the significance of a prolonged activated partial thromboplastin time (aPTT) in my patient with suspected antiphospholipid syndrome (APS)?