Individual risk factors predictive of venous thromboembolism in patients with temporary lower limb immobilization due to injury: a systematic review

Long-term patient outcome is affected by deep venous thrombosis after Achilles tendon rupture repair

PURPOSE

The aim of the study was to examine the impact of a deep venous thrombosis (DVT) on patient outcomes 3 years after Achilles tendon rupture (ATR) repair and if there were continued improvements between 1 and 3 years after surgery. A secondary aim was to determine risk factors associated with impaired patient outcomes in the long term.

METHODS

This cohort study consisted of 181 ATR-repaired patients, from two large randomized clinical trials, who attended a 3-year follow-up evaluation. Patients were postoperatively randomized to two different weight-bearing interventions compared with immobilization in a below-knee plaster cast for 2 weeks. During immobilization, screening for DVT was performed with Doppler ultrasound. At 1 and 3 years postoperatively, functional- and patient-reported outcomes were evaluated by the validated heel-rise test and self-reported questionnaire, Achilles tendon Total Rupture Score (ATRS).

RESULTS

In total, 76 out of 181 (42%) patients exhibited a DVT at the 2- or 6-week screening after ATR surgery. Suffering from a DVT during immobilization resulted at 3 years in a worse limb symmetry index (LSI) of heel-rise total work compared to patients without DVT, adjusted for age (DVT mean LSI 68% vs. no DVT 78%, p = 0.027). At 3 years, patients with a DVT during immobilization displayed lower ATRS (DVT median 88 vs. no DVT 93, p = 0.046), which was not significant after adjustment for age. However, patients with DVT exhibited an improvement in ATRS, LSI total work, and LSI maximum height between 1 and 3 years, which was not seen among patients without DVT. Independent risk factors for reduced patient functional outcomes at 3 years were older age, greater calf muscle hypotrophy, and suffering a DVT.

CONCLUSIONS

DVT during immobilization affects patients' long-term functional outcomes 3 years after ATR repair. Clinicians should adequately address risk factors contributing to impaired patient outcomes in the long term, including calf muscle hypotrophy, DVT, and older age.

LEVEL OF EVIDENCE

Level Ⅲ.

Venous thromboembolism (VTE) developing after ankle sprain. Comparison with VTE after knee arthroplasty

BACKGROUND

Venous thromboembolism (VTE) is a critical complication after non-major trauma or surgery. While the risk and severity of VTE following major orthopedic surgery is well-documented, there is significant knowledge gap regarding, non-major trauma such as ankle sprains.

METHODS

We analyzed data from the RIETE registry to assess the clinical characteristics, VTE prophylaxis usage, and outcomes in patients with VTE following ankle sprain versus those post elective knee arthroplasty. We aimed to assess the risk and severity of VTE in a population traditionally considered at lower risk. Risk stratification was performed using the TRiP(cast) score.

RESULTS

Among 1,250 patients with VTE, those with ankle sprain (n = 459) were much younger than those post knee arthroplasty (n = 791), less often female, had fewer comorbidities, and received VTE prophylaxis less often (27% vs. 93 %). During anticoagulation, 26 patients developed recurrent VTE, 31 had major bleeding, and 12 died (fatal PE 3, fatal bleeding 2). There were no differences between the two groups in the rates of VTE recurrences (rate ratio (RR): 1.65; 95%CI: 0.69-3.88) or death (RR: 1.12; 95%CI: 0.33-3.46), but patients with VTE after ankle sprain had a lower rate of major bleeding (RR: 0.39; 95%CI: 0.13-0.99).

CONCLUSIONS

Ankle sprain patients are often undertreated for VTE prophylaxis and have similar severity of VTE than those undergoing elective knee surgery, indicating the need for a more customized approach to VTE management.

A Comprehensive Review of Risk Factors and Thrombophilia Evaluation in Venous Thromboembolism

Venous thromboembolism (VTE), which encompasses deep vein thrombosis (DVT) and pulmonary embolism (PE), is a significant cause of morbidity and mortality worldwide. There are many factors, both acquired and inherited, known to increase the risk of VTE. Most of these result in increased risk via several common mechanisms including circulatory stasis, endothelial damage, or increased hypercoagulability. Overall, a risk factor can be identified in the majority of patients with VTE; however, not all risk factors carry the same predictive value. It is important for clinicians to understand the potency of each individual risk factor when managing patients who have a VTE or are at risk of developing VTE. With this, many providers consider performing a thrombophilia evaluation to further define a patient's risk. However, guidance on who to test and when to test is controversial and not always clear. This comprehensive review attempts to address these aspects/concerns by providing an overview of the multifaceted risk factors associated with VTE as well as examining the role of performing a thrombophilia evaluation, including the indications and timing of performing such an evaluation.

A Comprehensive Review of Risk Factors for Venous Thromboembolism: From Epidemiology to Pathophysiology

Venous thromboembolism (VTE) is the third most common cause of death worldwide. The incidence of VTE varies according to different countries, ranging from 1-2 per 1000 person-years in Western Countries, while it is lower in Eastern Countries (<1 per 1000 person-years). Many risk factors have been identified in patients developing VTE, but the relative contribution of each risk factor to thrombotic risk, as well as pathogenetic mechanisms, have not been fully described. Herewith, we provide a comprehensive review of the most common risk factors for VTE, including male sex, diabetes, obesity, smoking, Factor V Leiden, Prothrombin G20210A Gene Mutation, Plasminogen Activator Inhibitor-1, oral contraceptives and hormonal replacement, long-haul flight, residual venous thrombosis, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, trauma and fractures, pregnancy, immobilization, antiphospholipid syndrome, surgery and cancer. Regarding the latter, the incidence of VTE seems highest in pancreatic, liver and non-small cells lung cancer (>70 per 1000 person-years) and lowest in breast, melanoma and prostate cancer (<20 per 1000 person-years). In this comprehensive review, we summarized the prevalence of different risk factors for VTE and the potential molecular mechanisms/pathogenetic mediators leading to VTE.

Venous thromboembolism risk following temporary immobilisation after injury: evaluation of the Aberdeen VTE risk tool

BACKGROUND

Temporary lower limb immobilisation following injury is a risk factor for symptomatic venous thromboembolism (VTE). Pharmacological thromboprophylaxis can mitigate this risk but it is unclear which patients benefit from this intervention. The Aberdeen VTE risk tool was developed to tailor thromboprophylaxis decisions in these patients and this evaluation aimed to describe its performance in clinical practice. Secondarily, diagnostic metrics were compared with other risk assessment methods (RAMs).

METHODS

A prospective cohort service evaluation was conducted. Adult patients (≥16 years) managed with lower limb immobilisation for injury who were evaluated with the Aberdeen VTE risk tool prior to discharge from the ED were identified contemporaneously between February 2014 and December 2020. Electronic patient records were scrutinised up to 3 months after removal of immobilisation for the development of symptomatic VTE or sudden death due to pulmonary embolism (PE). Other RAMs, including the Thrombosis Risk Prediction for Patients with cast immobilisation (TRiP(cast)) and Plymouth scores, were assimilated retrospectively and diagnostic performance compared.

RESULTS

Of 1763 patients (mean age 46 (SD 18) years, 51% women), 15 (0.85%, 95% CI 0.52% to 1.40%) suffered a symptomatic VTE or death due to PE. The Aberdeen VTE tool identified 1053 (59.7%) patients for thromboprophylaxis with a sensitivity of 80.0% (95% CI 54.8% to 93.0%) and specificity of 40.4% (95% CI 38.1% to 42.6%) for the primary outcome. In 1695 patients, fewer were identified as high risk by the TRiP(cast) (33.3%) and Plymouth (24.4%) scores, but with greater specificity, 67.0% and 75.6%, respectively, than dichotomous RAMs, including the Aberdeen VTE tool.

CONCLUSION

Routine use of the Aberdeen VTE tool in our population resulted in an incidence of symptomatic VTE of less than 1%. Ordinal RAMs, such as the TRiP(cast) score, may more accurately reflect VTE risk and permit more individually tailored thromboprophylaxis decisions but prospective comparison is needed.

Qualitative analysis of randomized controlled trials informing recommendations for venous thromboembolism prophylaxis after distal lower extremity injuries

Purpose:

The purpose of this study is to assess the quality of evidence to stratify recommendations for chemoprophylaxis following distal lower extremity trauma.

Methods:

Literature review identified primary studies investigating venous thromboembolism (VTE) chemoprophylaxis following traumatic injury distal to the knee. Inclusion criteria were randomized controlled trials in adult patients treated with and without operative intervention. Each primary study was assessed by the Consolidated Standards of Reporting Trials 2010 checklist and Modified Coleman methodology score.

Results:

Literature review resulted in 462 studies, of which 9 met inclusion and exclusion criteria. All studies included low molecular weight heparin as a treatment group with 2 (22%) also including a treatment group with a direct factor Xa inhibitor. Five studies (56%) used placebo as a control group. The mean Modified Coleman Methodology score was 63% (range 51%–72%), a categorical rating of Fair. The mean Consolidated Standards of Reporting Trials score was 78% (range 56%–97%). Most studies (89%) screened all asymptomatic subjects for deep venous thrombosis. Statistical significance in VTE incidence among prophylactic treatment groups was not achieved in 78%.

Conclusions:

Development of consensus for VTE prophylaxis recommendations following traumatic injury distal to the knee is complicated by heterogenous study populations, low incidence of VTE in study populations, and inconsistent definitions of clinically important VTE. Low molecular weight heparin is not consistently superior for preventing VTE. Chemoprophylaxis should be considered on an individual basis in the presence of additional risk factors, although an externally validated, evidence-based risk assessment tool does not currently exist.

Level of Evidence: IV, therapeutic

Fixation of intraoperative proximal femoral fractures during THA using two versus three cerclage wires - a biomechanical study

Background

Intraoperative proximal femoral fractures (IPFF) are relevant complications during total hip arthroplasty. Fixation using cerclage wires (CW) represents a minimally-invasive technique to address these fractures through the same surgical approach. The goal of treatment is to mobilise the patient as early as possible, which requires high primary stability. This study aimed to compare different cerclage wire configurations fixing IPFF with regard to biomechanical primary stability.

Methods

Standardised IPFF (type II, Modified Mallory Classification) were created in human fresh frozen femora and were fixed either by two or three CW (1.6 mm, stainless steel). All cadaveric specimens (n = 42) were randomised to different groups (quasi-static, dynamic) or subgroups (2 CW, 3 CW) stratified by bone mineral density determined by Dual Energy X-ray Absorptiometry. Using a biomechanical testing setup, quasi-static and dynamic cyclic failure tests were carried out. Cyclic loading started from 200 N to 500 N at 1 Hz with increasing peak load by 250 N every 100 cycles until failure occurred or maximum load (5250 N) reached. The change of fracture gap size was optically captured.

Results

No significant differences in failure load after quasi-static (p = 0.701) or dynamic cyclic loading (p = 0.132) were found between the experimental groups. In the quasi-static load testing, all constructs resisted 250% of the body weight (BW) of their corresponding body donor. In the dynamic cyclic load testing, all but one construct (treated by 3 CW) resisted 250% BW.

Conclusions

Based on this in vitro data, both two and three CW provided sufficient primary stability according to the predefined minimum failure load (250% BW) to resist. The authors recommend the treatment using two CW because it reduces the risk of vascular injury and shortens procedure time.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04956-5.

Risque thromboembolique veineux chez les patients traumatisés d’un membre inférieur nécessitant une immobilisation : vers une approche individualisée

Un traumatisme d’un membre inférieur nécessitant une immobilisation est une situation à risque de développement de maladie thromboembolique veineuse (MTEV). Cependant, les recommandations et les pratiques varient notablement d’un pays à un autre et d’un centre à un autre. Cette revue narrative a pour objectifs de décrire l’épidémiologie, la prévention et les algorithmes de prédiction de la MTEV chez les patients traumatisés d’un membre inférieur nécessitant une immobilisation. L’incidence de la MTEV varie selon les études du fait de la grande hétérogénéité des patients inclus (de l’entorse de cheville à une lésion chirurgicale) et du fait des différents critères d’évaluation utilisés. L’incidence des événements thromboemboliques veineux symptomatiques est estimée à 2,0 % (intervalle de confiance à 95 % : 1,3 à 2,7). L’efficacité de la thromboprophylaxie a été démontrée dans des méta-analyses récentes. Cependant, la confiance à accorder à ces résultats est médiocre, car de nombreux essais présentaient des faiblesses méthodologiques. L’étude la plus importante et la plus récente ne montre pas de bénéfice de la prévention par héparine de bas poids moléculaire sur les événements symptomatiques dans une population non ciblée. Ces résultats suggèrent d’adopter une démarche personnalisée en réservant la prophylaxie aux patients à risque. Plusieurs scores existent pour évaluer le risque thromboembolique individuel. La prise en compte des caractéristiques du patient, du traumatisme et de l’immobilisation permet d’identifier un large sous-groupe de patients chez qui la prévention ne semble pas utile et un sous-groupe de patients à haut risque où la prévention devrait être prescrite, voire renforcée. En conclusion, lors d’un traumatisme d’un membre inférieur nécessitant une immobilisation, l’indication d’une thromboprophylaxie devrait être guidée par l’évaluation individuelle du risque thrombotique.

Venous Thromboembolism in Patients Discharged From the Emergency Department With Ankle Fractures: A Population-Based Cohort Study

STUDY OBJECTIVE

Temporary lower limb immobilization may be a risk for venous thromboembolism. The purpose of this study was to examine the 90-day incidence of venous thromboembolism among patients discharged from an emergency department (ED) with ankle fractures requiring temporary immobilization. Secondary objectives were to examine individual factors associated with venous thromboembolism in this population and to compare the risk of venous thromboembolism in patients with ankle fractures against a priori-selected control groups.

METHODS

This was a retrospective cohort study using province-wide health datasets from Ontario, Canada. We included patients aged 16 years and older discharged from an ED between 2013 and 2018 with closed ankle fractures requiring temporary immobilization. We estimated 90-day incidence of venous thromboembolism after ankle fracture. A Cox proportional hazards model was used to evaluate risk factors associated with venous thromboembolism, censoring at 90 days or death. Patients with ankle fractures were then propensity score matched to 2 control groups: patients discharged with injuries not requiring lower limb immobilization (ie, finger wounds and wrist fractures) to compare relative hazard of venous thromboembolism.

RESULTS

There were 86,081 eligible patients with ankle fractures. Incidence of venous thromboembolism within 90 days was 1.3%. Factors associated with venous thromboembolism were older age (hazard ratio [HR]: 1.18; 95% confidence interval [CI]: 1.00 to 1.39), venous thromboembolism or superficial venous thrombosis history (HR: 5.18; 95% CI: 4.33 to 6.20), recent hospital admission (HR: 1.33; 95% CI: 1.05 to 1.68), recent nonankle fracture surgery (HR: 1.58; 95% CI: 1.30 to 1.93), and subsequent surgery for ankle fracture (HR: 1.80; 95% CI: 1.48 to 2.20). In the matched cohort, patients with ankle fractures had an increased hazard of venous thromboembolism compared to matched controls with finger wounds (HR: 6.31; 95% CI: 5.30 to 7.52) and wrist fractures (HR: 5.68; 95% CI: 4.71 to 6.85).

CONCLUSION

The 90-day incidence of venous thromboembolism among patients discharged from the ED with ankle fractures requiring immobilization was 1.3%. These patients had a 5.7- to 6.3-fold increased hazard compared to matched controls. Certain patients immobilized for ankle fractures are at higher risk of venous thromboembolism, and this should be recognized by emergency physicians.

Thromboembolic risk stratification by TRiP(cast) score to rationalise thromboprophylaxis in patients with lower leg trauma requiring immobilisation: a study protocol of the casting stepped-wedge cluster randomised trial

INTRODUCTION

Patients with lower limb trauma requiring orthopaedic immobilisation may be at risk of venous thromboembolism but opinions differ about who may benefit from thromboprophylactic anticoagulant treatment.The aim of this CASTING study is to demonstrate the safety of thromboprophylaxis based on the Thrombosis Risk Prediction for patients with cast immobilisation (TRiP(cast) score with regards to the 3-month incidence of symptomatic venous thromboembolism events in low-risk patients not receiving thromboprophylaxis, as well as the usefulness of this strategy on the rate of patients receiving anticoagulant treatment in comparison to current practice.

METHODS AND ANALYSIS

CASTING will be a stepped-wedge cluster randomised controlled clinical trial, performed in 15 emergency departments in France and Belgium. With their informed consent, outpatients admitted to one of the participating emergency departments for a lower limb trauma requiring orthopaedic immobilisation without surgery will be included. All centres will begin the trial with the 'observational period' and, every 2 weeks, 1 centre will be randomly assigned to switch to the 'interventional period' and to apply the TRiP(cast) score, in which only patients with a score ≥7 will receive thromboprophylactic anticoagulant treatment. The primary endpoint is the rate of clinical thromboembolic events within 90 days following the inclusion of low-risk patients not receiving thromboprophylaxis.

ETHICS AND DISSEMINATION

The protocol has been approved by the Comité de Protection des Personnes Sud I (Ethics Review ID-RCB: 2019-A01829-48) for France and the Comité d'éthique hôpital-facultaire Saint Luc (N° B403201941338) for Belgium. It is carried out in accordance with the Declaration of Helsinki and Good Clinical Practice guidelines. The findings of this study will be disseminated in peer-reviewed journals and at scientific conferences.

TRIAL REGISTRATION NUMBER

NCT04064489.

Venous thromboembolism after lower extremity orthopedic surgery: A population-based nationwide cohort study

BACKGROUND

Venous thromboembolism (VTE) causes morbidity and mortality in the general population. Several events occur after lower limb orthopedic surgery, but the contribution from various types of lower limb surgery is not well known.

OBJECTIVE

To investigate the postoperative incidence of VTE for all types of lower extremity orthopedic surgery compared with the background population.

METHODS

Individual-level linkage of Danish nationwide register data for all Danish residents with first-time orthopedic surgery of the lower limb (1996-2017) and, for each of these, four controls from the general population matched on age, sex, and history of VTE. Adjusted hazard ratios (HR) compared the postoperative risk of VTE to the matched controls.

RESULTS

In total 7203 of the 1 012 823 patients with a first orthopedic procedure had a VTE within 180 days after surgery, corresponding to a postoperative cumulative incidence of 0.71% (95% confidence interval [CI], 0.70-0.73). The cumulative incidence of VTE among controls was 0.11% (95% CI, 0.11-0.12). The HR of VTE within the first 30 days after surgery below knee level was 20.5 (95% CI, 17.9-23.5) compared with matched controls. The HRs of VTE after minor distal procedures (eg, meniscectomy and arthroscopies) were 2.9 (95% CI, 1.9-4.4) to 7.1 (95% CI, 6.4-8.0).

CONCLUSION

All types of lower limb orthopedic surgery including minor distal procedures were associated with higher rates of VTE compared with matched controls, in particular within the first 30 days after surgery.

What's new in VTE risk and prevention in orthopedic surgery

A State of the Art lecture titled "What's New in VTE Risk and Prevention in Orthopedic Surgery" was presented at the ISTH congress in 2019. Patients undergoing orthopedic surgery have long been recognized to be at increased risk of venous thromboembolism (VTE) and were among the first patient groups to be studied in VTE prophylaxis trials. From the late 1950s to 2010s, prophylaxis trials in major orthopedic surgery tended to focus on venographic deep vein thrombosis and assessed thromboprophylaxis in all patients based on a population approach. In general, anticoagulants were favored over mechanical prophylaxis or aspirin, and longer-duration prophylaxis was favored over shorter durations. As discussed in this paper, more recently, orthopedic prophylaxis has started to become more nuanced and individualized. Modern trials are focusing on symptomatic VTE as outcomes; there has been a resurgence in interest in aspirin for prophylaxis, and there has been a slow move to studying ways to evaluate VTE risk in patients undergoing orthopedic surgery and recommending thromboprophylaxis to patients based on individual attributes, in whom risk stratification and weighing of benefit versus risk of thromboprophylaxis is becoming key. We also touch on VTE risk and guideline recommendations to prevent VTE in 2 other commonly encountered orthopedic populations: patients undergoing knee arthroscopy and those with distal leg fractures. Finally, we summarize relevant new data on this topic presented during the 2019 ISTH annual congress in Melbourne.

Thromboprophylaxis in lower limb immobilisation after injury (TiLLI)

Venous thromboembolic disease is a major global cause of morbidity and mortality. An estimated 10 million episodes are diagnosed yearly; over half of these episodes are provoked by hospital admission/procedures and result in significant loss of disability adjusted life years. Temporary lower limb immobilisation after injury is a significant contributor to the overall burden of venous thromboembolism (VTE). Existing evidence suggests that pharmacological prophylaxis could reduce overall VTE event rates in these patients, but the proportional reduction of symptomatic events remains unclear. Recent studies have used different pharmacological agents, dosing regimens and outcome measures. Consequently, there is wide variation in thromboprophylaxis strategies, and international guidelines continue to offer conflicting advice for clinicians. In this review, we provide a summary of recent evidence assessing both the clinical and cost effectiveness of thromboprophylaxis in patients with temporary immobilisation after injury. We also examine the evidence supporting stratified thromboprophylaxis and the validity of widely used risk assessment methods.

Early mobilization does not reduce the risk of deep venous thrombosis after Achilles tendon rupture: a randomized controlled trial

PURPOSE

The hypothesis was that early functional mobilization would reduce the incidence of deep venous thrombosis (DVT) during leg immobilization after Achilles tendon rupture surgery. A secondary aim was to evaluate if the amount of weightbearing and daily steps influenced the risk of sustaining a DVT.

METHODS

One-hundred and fifty patients with Achilles tendon rupture repair were randomized to treatment with early functional mobilization, encouraging full weightbearing and ankle motion in orthosis, or treatment-as-usual, i.e., 2 weeks of unloading in plaster cast followed by 4 weeks weightbearing in orthosis. At 2 and 6 weeks postoperatively, all patients were screened for DVT using compression duplex ultrasound. During the first 2 weeks postoperatively, patient-reported loading, pain and step counts were assessed.

RESULTS

At 2 weeks, 28/96 (29%) of the patients in early functional mobilization group and 15/49 (31%) in the control group (n.s) had sustained a DVT. At 6 weeks, the DVT rate was 35/94 (37%) in the early functional mobilization and 14/49 (29%) in the control group (n.s). During the first postoperative week, the early functional mobilization group reported low loading and higher experience of pain vs. the control group (p = 0.001). Low patient-reported loading ≤ 50% (OR = 4.3; 95% CI 1.28-14.3) was found to be an independent risk factor for DVT, in addition to high BMI and higher age.

CONCLUSIONS

Early functional mobilization does not prevent the high incidence of DVT during leg immobilization in patients with Achilles tendon rupture as compared to treatment-as-usual. The low efficacy of early functional mobilization is mainly explained by postoperative pain and subsequent low weightbearing. To minimize the risk of DVT, patients should be encouraged to load at least 50% of body weight on the injured leg 1 week after surgery.

LEVEL OF EVIDENCE

Therapeutic, level 1.

Individual risk factors predictive of venous thromboembolism in patients with temporary lower limb immobilization due to injury: a systematic review

Essentials Thromboprophylaxis after lower limb injury is often based on complex risk stratification. Our systematic review identified variables predicting venous thromboembolism (VTE) in this group. Age and injury type were commonly reported to increase the odds of VTE (odds ratio 1.5-3.48). We found limited evidence to support the use of other risk factors within prediction models. SUMMARY: Background Patients immobilized after lower limb injury are at risk of venous thromboembolism (VTE). There is international variation in the use of thromboprophylaxis for such patients. Risk-based strategies have been adopted to aid decision making in many settings. The accuracy of these strategies is unclear. Objectives A systematic review was undertaken to identify all individual patient-identifiable risk factors linked to any VTE outcome following lower limb immobilization. Methods Several electronic databases were searched from inception to May 2017. Any studies that included a measurement of VTE as a patient outcome in adults requiring temporary immobilization (e.g. leg cast or brace in an ambulatory setting) for an isolated lower limb injury and reported risk factor variables were included. Descriptive statistics and thematic analysis were used to synthesize the evidence. Results Our database search returned 4771 citations, of which 15 studies reporting outcome data on 80 678 patients were eligible for analysis. Risk-factor associations were reported through regression analyses, non-parametric tests and descriptive statistics. All studies were assessed as at moderate or serious risk of bias using the ROBINS-I risk of bias tool. Advancing age and injury type were the only individual risk factors demonstrating a reproducible association with increased symptomatic and/or asymptomatic VTE rates. Several risk factors currently used in scoring tools did not appear to be robustly evaluated for subsequent association with VTE within these studies. Conclusions Clinicians should be aware of the limited evidence to support individual risk factors in guiding thromboprophylaxis use for this patient cohort.