Laboratory monitoring of low-molecular-weight heparin therapy

Antithrombotic and Antiplatelet Drug Toxicity

Anticoagulant and antiplatelet drugs target a specific portion of the coagulation cascade or the platelet activation and aggregation pathway. The primary toxicity associated with these agents is hemorrhage. Understanding the pharmacology of these drugs allows the treating clinician to choose the correct antidotal therapy. Reversal agents exist for some of these drugs; however, not all have proven patient-centered outcomes. The anticoagulants covered in this review are vitamin K antagonists, heparins, fondaparinux, hirudin derivatives, argatroban, oral factor Xa antagonists, and dabigatran. The antiplatelet agents reviewed are aspirin, adenosine diphosphate antagonists, dipyridamole, and glycoprotein IIb/IIIa antagonists. Additional notable toxicities are also reviewed.

Perioperative management of antithrombotic treatment

End-stage liver disease is characterized by multiple and complex alterations of hemostasis that are associated with an increased risk of both bleeding and thrombosis. Liver transplantation further challenges the feeble hemostatic balance of patients with decompensated cirrhosis, and the management of antithrombotic treatment during and after transplant surgery, which is particularly difficult. Bleeding was traditionally considered the major concern during and early after surgery, but it is increasingly recognized that transplant recipients may also develop thrombotic complications. Pathophysiology of hemostatic complications during and after transplantation is multifactorial and includes pre-, intra-, and postoperative risk factors. Risk stratification is important, as it helps the identification of high-risk recipients in whom antithrombotic prophylaxis should be considered. In recipients who develop thrombosis during or after surgery, prompt treatment is indicated to prevent graft failure, retransplantation, and death.

Monitoring anti-Xa levels in patients with cancer-associated venous thromboembolism treated with bemiparin

OBJECTIVE

To analyze the relationship between therapeutic (weight-adjusted) dose of bemiparin and anti-Xa activity in patients with venous thromboembolism (VTE) and cancer in comparison with a cohort of patients with VTE without cancer, and its relationship with outcomes.

MATERIALS AND METHODS

This is a prospective cohort study that comprised a cohort of patients with cancer-associated VTE and a cohort of non-cancer patients with VTE, all of them treated with bemiparin. The ethics committee approved the study and informed consent was obtained from the patients.

RESULTS

One hundred patients were included (52 with cancer and 48 without cancer), with a median follow-up of 9.8 months. Mean anti-Xa activity was 0.89 (± 0.33) UI/mL in oncological patients and 0.83 (± 0.30) UI/mL in non-cancer patients (mean difference - 0.05 95% CI - 0.18; 0.06). A multiple linear regression model showed that anti-Xa peak was associated with the dose/kg independently of possible confounding variables (presence of cancer, age, sex and eGFR-estimated Glomerular Filtration Rate), in a way that for every 1 UI of dose/kg increase, the anti-Xa peak activity increased 0.006 UI/mL (95% CI 0.003; 0.009) (p < 0.001). The predictive capacity of anti-Xa peak in the oncology cohort showed an area under the ROC curve of 0.46 (95% CI 0.24-0.68), 0.70 (95% CI 0.49-0.91) and 0.74 (95% CI 0.44-0.94) for death, first bleeding and recurrence of VTE, respectively, and none was statistically significant.

CONCLUSION

In patients with venous thromboembolism treated with bemiparin, anti-Xa levels were not influenced by the presence of cancer.

Extended enoxaparin monotherapy for acute symptomatic pulmonary embolism

We investigated the efficacy and safety of extended enoxaparin monotherapy in symptomatic patients with acute pulmonary embolism (PE). We randomized 40 patients in a 1:1 allocation to enoxaparin monotherapy (1 mg/kg twice daily for 10-18 days, and then 1.5 mg/kg once daily until day 90) ( n = 20) or to enoxaparin 1.0 mg/kg twice daily as a bridge to warfarin with a target international normalized ratio of 2.0-3.0 for 90 days (at least 10 doses of enoxaparin overlapping with warfarin for at least 4 days) ( n = 20). All patients underwent echocardiography, cardiac troponin I (TnI), and brain natriuretic peptide testing to identify patients with an increased likelihood of adverse clinical outcomes. The end-points were newly diagnosed deep venous thrombosis (DVT) or PE and bleeding events through day 90. In 15 patients on extended enoxaparin therapy, we used repeated measure analysis of variance (ANOVA) to investigate differences in anti-Xa levels obtained at 2, 4, 8 and 12 weeks. The patients’ mean age was 52 ± 17 years; the most common comorbidities were obesity (58%), hypertension (30%), concomitant DVT (30%) and cancer (15%). Twelve (30%) patients had elevated cardiac TnI >0.1 mg/l and 11 (28%) had moderate or severe right ventricular dysfunction on echocardiography. Ten (25%) patients received thrombolysis with a continuous infusion of 100 mg alteplase prior to randomization. During a 90-day follow-up, one patient from the enoxaparin monotherapy group suffered symptomatic distal DVT; one from the warfarin group had recurrent symptomatic PE (p= 1.0). None of the study patients had major hemorrhage; two warfarin group patients had minor bleeding compared with none in the enoxaparin monotherapy group (p= 0.49). Repeated measure ANOVA did not reveal significant differences in anti-Xa levels over time (p= 0.217). In patients with acute symptomatic PE, extended enoxaparin monotherapy is feasible and warrants further investigation in a large clinical trial.

Elderly Medical Patients Treated with Prophylactic Dosages of Enoxaparin

BACKGROUND

The safety and optimal use of prophylactic treatment with low-molecular-weight heparins in elderly patients with impaired renal function remain undefined.

METHODS

The primary aim of this study was to analyse, in 'real life', the influence of renal function, as assessed by creatinine clearance (CL(CR)), on the level of anti-Xa activity in medical hospitalised elderly patients receiving prophylactic dosages of enoxaparin. Consecutive hospitalised acutely ill medical patients aged >or=75 years receiving daily dosages of enoxaparin 4000 IU for up to 10 days were prospectively enrolled in two centres. Peak anti-Xa activity was measured at the beginning and during the course of therapy.

RESULTS

One hundred and twenty-five patients (31 men, 94 women), mean age 87.5 +/- 6.3 years, mean bodyweight 56.4 +/- 11.9 kg and mean CL(CR) 39.8 +/- 16.1 mL/min, were enrolled in the study. The mean maximum anti-Xa activity (day 1 to day 10) [anti-Xa(max1-10)] was 0.64 +/- 0.23 IU/mL (range 0.24-1.50 IU/mL). Weak negative correlations were found between CL(CR) and anti-Xa(max) and between bodyweight and anti-Xa(max). Mean anti-Xa(max) was slightly but significantly higher in patients with CL(CR) of 20-30 mL/min compared with patients with CL(CR) of 31-40, 41-50 or 51-80 mL/min (0.72 versus 0.61, 0.61 and 0.60 IU/mL, respectively), and in patients weighing <50 kg compared with patients weighing 50-60 kg or >60 kg (0.74 vs 0.64 and 0.52 IU/mL, respectively). Serious bleeding occurred in five patients, but anti-Xa(max) values in these patients were not different to those in patients without bleeding (p = 0.77). Individual anti-Xa(max) at the beginning or during the course of treatment was measured in the subgroup of 58 patients in whom anti-Xa activity was measured at least once during the study. The mean anti-Xa(max) value was slightly but significantly higher during the course of the therapy than at the beginning of the study (0.63 +/- 0.26 IU/mL vs 0.56 +/- 0.23 IU/mL, p = 0.012).

CONCLUSION

Only CL(CR) <30 mL/min and bodyweight <50 kg were associated with significantly higher anti-Xa(max) values. The clinical relevance of these increases remains questionable. No conclusions about the safety of enoxaparin in elderly medical patients can be drawn from these findings.

How useful is the monitoring of (low molecular weight) heparin therapy by anti-Xa assay? A laboratory perspective

We have conducted a series of laboratory-based surveys to assess variability in assay results utilized to monitor heparin anticoagulant therapy. These surveys involved laboratories participating in the Haematology component of the Royal College of Pathologists of Australasia Quality Assurance Program (RCPA QAP). Thirty five of 646 laboratories that were sent a preliminary questionnaire indicated that they performed anti-Xa assays and these laboratories were sent a panel of four plasma samples. These plasma samples contained respectively: (i) no added heparin, (ii) low molecular weight heparin (LMWH), enoxaparin, added to a level of approximately .5 U/mL, (iii) unfractionated heparin added to a level of approximately .5 U/mL, and (iv) LMWH added to a level of approximately 1.0 U/mL. Tests to be performed were the activated partial thromboplastin time (APTT), the thrombin time (TT), fibrinogen, and anti-Xa. As expected, returned results for APTT and TT showed some elevation in heparinized samples while fibrinogen assays were not affected. Anti-Xa assays yielded the following results (median [range]): (i) .01 [0-.11], (ii) .43 [.33-.80], (iii) .23 [.10-.49], and (iv) .90 [.60-1.30]. Thus, although median values were close to those anticipated, there was a wide variation in returned results. In a repeat exercise a few months later laboratories were also asked about their therapeutic ranges (TRs) and provided with an additional vial of LMWH-spiked (1.0 U/mL) plasma labeled as 'heparin-standard' to be used as an assay calibrant. TRs varied substantially between laboratories, from low ranges of .2-.4 to high ranges of .8-1.2. Anti-Xa assay results were similar to those of the first survey: (median [range]): (a) repeat testing: (i) .02 [0-.28], (ii) .47 [.34-.80], (iii) .25 [.14-.58], (iv) .95 [.65-1.31]; (b) repeat testing using survey provided 'heparin-standard': (i) .02 [0-.24], (ii) .55 [.4-.83], (iii) .28 [.10-.63], (iv) 1.00 [.9-1.16]. Thus using the provided 'heparin-standard' yielded lower variability in results for LMWH. In conclusion, the high variability of anti-Xa assay results coupled with the widely variable TRs suggests that therapeutic heparin monitoring is poorly standardized, and this raises some concerns over the clinical value of such monitoring.

Safety profile of different low-molecular weight heparins used at therapeutic dose

Low-molecular weight heparins (LMWHs) have been shown to be as safe and effective as unfractionated heparin (UFH) for the treatment of acute venous thrombosis and non-life-threatening pulmonary embolism. Different reports have shown that LMWHs may also be used to treat patients with unstable angina or non-Q-wave infarction. The safety of LMWHs used at therapeutic dose has been widely studied in pivotal clinical trials and analysed in several meta-analyses. However, despite the wide development and use of LMWHs, several issues regarding the safety and optimal use of LMWHs remain unanswered. The main adverse effect of LMWHs is bleeding and it is uncertain whether a weight-adjusted dosage regimen without laboratory monitoring can be used in patients with a high risk of bleeding, such as patients with renal failure, elderly patients, obese patients or pregnant women. These patients are usually excluded from clinical trials and only a few studies, not sufficiently powered to estimate efficacy and safety, have been carried out in these special populations. Most of the available data comes from pharmacokinetic or population pharmacodynamic studies or clinical reports. Results in patients with renal impairment who are not undergoing haemodialysis suggest that a reduction in calculated creatinine clearance levels is associated with an increased risk of accumulation of anti-Xa activity, the extent of which differs depending on the individual LMWH and the extent to which the compound is cleared by the kidney. The limited data available regarding the use of therapeutic doses of LMWHs in obese patients suggest that there is no need to cap the dose at a maximal allowable dose. Long-term (3-month) treatment with LMWHs appears to be as effective and safe as oral anticoagulant therapy for the treatment of venous thromboembolism. It appears that each LMWH is a distinct compound with unique pharmacokinetic and pharmacodynamic profiles. Until more data are available regarding these special populations, periodic monitoring of anti-Xa activity levels may be recommended to detect accumulation and/or an overdose and minimise the bleeding risk. The non-haemorrhagic adverse effects of the LMWHs include heparin-induced thrombocytopenia (HIT) and osteoporosis. The incidence of HIT appears to be lower with LMWHs than with UFH; there is currently not enough data to compare the frequency of HIT between the various LMWHs. LMWHs also appear to carry a lower risk of causing osteoporosis than UFH. In conclusion, studies that include special population patients are required to make conclusive recommendations concerning the safety and monitoring of the different LMWHs.

The influence of extreme body weight on clinical outcome of patients with venous thromboembolism: findings from a prospective registry (RIETE)

BACKGROUND

Data evaluating the safety of using weight-based dosing of low-molecular-weight heparin (LMWH) in either underweight or obese patients with venous thromboembolism (VTE) are limited. Thus, recommendations based on evidence from clinical trials might not be suitable for patients with extreme body weight.

PATIENTS AND METHODS

Patients with objectively confirmed, symptomatic acute VTE are consecutively enrolled into the Registro Informatizado de la Enfermedad TromboEmbólica (RIETE) registry. For this analysis, data from patients in the following ranges of body weight were examined: <50, 50-100, and >100 kg. Patient characteristics, underlying conditions, treatment schedules and clinical outcomes during the first 15 days of treatment were compared.

RESULTS

As of August 2004, 8845 patients with acute VTE were enrolled from 94 participating centers. Of these, 169 (1.9%) weighed <50 kg, 8382 (95%) weighed 50-100 kg and 294 (3.3%) weighed >100 kg. Patients weighing <50 kg were more commonly females, were taking non-steriodal antiinflammatory drugs (NSAIDs), and had severe underlying diseases more often than patients weighing 50-100 kg. Their incidence of overall bleeding complications was significantly higher than in patients weighing 50-100 kg (odds ratio 2.2; 95% CI: 1.2-4.0). Patients weighing >100 kg were younger, most commonly males, and had cancer less often than those weighing 50-100 kg. Incidences of recurrent VTE, fatal pulmonary embolism or major bleeding complications were similar in both groups.

CONCLUSIONS

Patients with VTE weighing <50 kg have a significantly higher rate of bleeding complications. The clinical outcome of patients weighing over 100 kg was not significantly different from that in patients weighing 50-100 kg.