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!

Liked this post? Download the app on your smart phone and sign up below to catch future pearls right into your inbox, all for free! Thank you!

Subscribe to Blog via Email

Enter your email address to subscribe to this blog and receive notifications of new posts by email.


  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?

Is diabetes mellitus (DM) an independent risk factor for venous thromboembolism (VTE)?

Although DM was originally thought to be an independent risk factor for DM1,2, more recent data suggest otherwise.

A population-based study involving residents of Olmsted County, Minnesota, calculated the incidence of VTE among patients with DM over a 25-year period and found it to be higher than that of controls .   However, in the same study, after controlling for hospitalization for major surgery or medical illness and nursing home confinement, no association between DM and VTE was found2  .  

A recent systematic review and meta-analysis of case-control and cohort studies involving over 1 million patients found no significant association between DM and VTE when controlled for common risk factors (eg, obesity, sedentary life style, smoking, hypertension, or dyslipidemia)3.  The authors concluded that DM and its complications are not independent risk factors for incident VTE.  

Thus, it appears that much of the risk of DVT in DM may be related to its comorbidities and the need for hospitalization, surgery or nursing home stay.


  1. Petrauskiene V, Falk M, Waernbaum I, et al. The risk of venous thromboembolism is markedly elevated in patients with diabetes. Diabetologia 2005;48:1017-21.
  2. Heit JA, Leibson CL, Ashrani AA, et al. Is diabetes mellitus an independent risk factor for venous thromboembolism? A population-based case-control study. Thromb Vasc Biol 2009; 29:1399-1405.
  3. Gariani K, Mavrakanas T, Combescure C, et al. Is diabetes mellitus a risk factor for venous thromboembolism? A systematic review and meta-analysis of case-control and cohort studies. Eur J Intern Med 2016;28:52-58.
Is diabetes mellitus (DM) an independent risk factor for venous thromboembolism (VTE)?

Is there a seasonal variation in the incidence of cardiovascular (CV) events or venous thromboembolism (VTE)?

Seasonal variation, primarily characterized by a winter peak, has been reported for acute CV events, such as acute myocardial infarction (AMI) and sudden death, aortic rupture or dissection, and ischemic or hemorrhagic stroke, and VTE (1). A meta-analysis involving patients with VTE, primarily with a diagnosis of pulmonary embolism, revealed a 20% absolute increase in the incidence of VTE during January (1).  

Potential physiological mechanisms for these observations include increased sympathetic activity, decreased loss of fluids and sodium, increase in LDL cholesterol, increase in serum fibrinogen levels and other coagulation markers and C-reactive protein, and lower vitamin D levels due to shorter daylight hours during winter months (1,2).  At least in the case of AMI in the U.S., the higher incidence in winter is not affected by climate (2).  

Respiratory virus infections as a cause of acute inflammation leading to  CV or VTE events is another intriguing explanation (3). Indeed, influenza vaccination has been associated with reduction in hospitalization for cardiac disease and stroke among the elderly (4) and, in patients with cardiovascular disease, a reduction in death due to combined cardiovascular disease events such as heart attacks and strokes (5).




  1. Dentali F, Ageno W, Rancan E, et al. Seasonal and monthly variability in the incidence of venous thromboembolism. A systematic review and a meta-analysis of the literature. Thromb Haemost 2011;106:439-447.
  2. Spencer FA, Goldberg RJ, Becker RC, et al. Seasonal distribution of acute myocardial infarction in the Second National Registry of Myocardial Infarction. J Am Coll Cardiol 1998;31:1226-33.h ttps://
  3. Woodhouse PR, Khaw KT, Plummer M, et al. Seasonal variations of plasma fibrinogen and factor VII activity in the elderly: winter infections and death from cardiovascular disease. Lancet 1994;343:435-39.
  4. Nichol KL, Nordin J, Mulloly J, et al. Influenza vaccination and reduction in hospitalization for cardiac disease and stroke among the elderly. N Engl J Med 2003; 348:1322-1332.
  5. Clar C, Oseni Z, Flowers N, et al. Cochrane Database of Systematic Reviews 2015. DOI: 10.1002/14651858.CD005050.pub3h ttp://  





Is there a seasonal variation in the incidence of cardiovascular (CV) events or venous thromboembolism (VTE)?