Impaired fibrinolysis and postoperative thromboembolism in orthopedic patients

Elevated plasma levels of plasminogen activator inhibitor-1 are associated with risk of future incident venous thromboembolism

BACKGROUND

Plasminogen activator inhibitor-1 (PAI-1), the main inhibitor of fibrinolysis, is frequently elevated in obesity and could potentially mediate the risk of venous thromboembolism (VTE) in obese subjects. However, whether PAI-1 is associated with VTE remains uncertain.

OBJECTIVE

To investigate the association between plasma PAI-1 levels and risk of future incident VTE and whether PAI-1 could mediate the VTE risk in obesity.

METHODS

A population-based nested case-control study, comprising 383 VTE cases and 782 age- and sex-matched controls, was derived from the Tromsø Study cohort. PAI-1 antigen levels were measured in samples collected at cohort inclusion. Logistic regression was used to calculate odds ratios (ORs) with 95% confidence intervals (CIs) for VTE across PAI-1 tertiles.

RESULTS

The VTE risk increased dose-dependently across PAI-1 tertiles (P for trend <.001) in the age- and sex-adjusted model. The OR of VTE for the highest versus lowest tertile was 1.73 (95% CI 1.27-2.35), and risk estimates were only slightly attenuated with additional stepwise adjustment for body mass index (BMI; OR 1.59, 95% CI 1.16-2.17) and C-reactive protein (CRP; OR 1.54, 95% CI 1.13-2.11). Similar results were obtained for provoked/unprovoked events, deep vein thrombosis, and pulmonary embolism. In obese subjects (BMI of ≥30 kg/m2 vs. <25 kg/m2 ), PAI-1 mediated 14.9% (95% CI 4.1%-49.4%) of the VTE risk in analysis adjusted for age, sex, and CRP.

CONCLUSION

Our findings indicate that plasma PAI-1 is associated with increased risk of future incident VTE and has the potential to partially mediate the VTE risk in obesity.

Sleep apnea in total joint arthroplasty patients and the role for cardiac biomarkers for risk stratification: an exploration of feasibility

Obstructive sleep apnea (OSA) is highly prevalent in patients undergoing total joint arthroplasty (TJA) and is a major risk factor for postoperative cardiovascular complications and death. Recognizing this, the American Society of Anesthesiologists urges clinicians to implement special considerations in the perioperative care of OSA patients. However, as the volume of patients presenting for TJA increases, resources to implement these recommendations are limited. This necessitates mechanisms to efficiently risk stratify patients having OSA who may be susceptible to post-TJA cardiovascular complications. We explore the role of perioperative measurement of cardiac troponins (cTns) and brain natriuretic peptides (BNPs) in helping determine which OSA patients are at increased risk for post-TJA cardiovascular-related morbidity.

A prospective study on role of tranexamic acid in reducing postoperative blood loss in total knee arthroplasty and its effect on coagulation profile

Total knee arthroplasty(TKA) is associated with extensive postoperative blood loss. Despite various studies proving the efficacy of Tranexamic Acid (TEA) with single or multiple boluses of different sizes with or without subsequent infusions, no consensus has been reached on the dose of tranexamic acid to be administered or the duration of treatment. In this study, we have investigated in a homogenous healthy population undergoing total knee arthroplasty, if administration of a high dose of tranexamic acid has a blood sparing effect. They were found to be significant with high power concluding a decrease in total blood loss in patients who were administered Tranexamic Acid (TEA) during Total Knee Arthroplasty (TKA). A dose regimen of 15 mg/kg every 8 h for 24 h would seem appropriate as longer administration of TEA was not accompanied by further reduction in blood loss.

Biomarkers of Coagulation and Fibrinolysis during Cemented Total Hip Arthroplasty with Pre- versus Postoperative Start of Thromboprophylaxis

Venous thrombosis is common in elective hip surgery, and prophylaxis is recommended. Clinical trials suggest that the drug dose and timing of initiating prophylaxis significantly influence antithrombotic effectiveness and safety. We studied the time course and gradient of plasma coagulation and fibrinolysis during total hip arthroplasty (THA) in twenty patients that were randomly assigned to have the first dose of 5000 IU dalteparin subcutaneously (sc) injected 12 hours before or 6 hours after surgery. Baseline characteristics were similar in both groups. Specific biomarkers on coagulation (prothrombin fragment 1+2 (F1+2)) and fibrinolytic activity (plasmin/α2-antiplasmin complex (PAP) and D-dimer) were collected at six events during hospitalization and analysed. There were no significant group differences in the biomarkers at any time point. The highest concentrations were measured 6 hours after surgery and before the first postoperative injection. A marked decrease followed at the first postoperative day, and then a second increase in plasma concentrations was observed 6 days after surgery. This study showed that activation of coagulation and fibrinolysis by the operative trauma was the same when the first dose of dalteparin was injected 12 hours before or 6 hours after surgery.

Hypertensive patients show delayed wound healing following total hip arthroplasty

BACKGROUND

Prolonged wound-discharge following total hip arthroplasty (THA) is associated with an increased risk of infection. However, the potential role of hypertension in prolonging the duration of wound healing in this population has not yet been investigated. The aim of the present study was to compare healing in this population that has not yet been investigated. The aim of the present study was to compare hypertensive and normotensive THA patients in terms of the length of time required to achieve a dry wound and the length of stay in the hospital.

METHODS

One hundred and twenty primary THA patients were evaluated. Pre-operative clinical history and physical examination revealed that 29 were hypertensive and 91 were normotensive. The two groups were statistically matched using optimal propensity score matching. The outcomes of interest were the number of days until a dry wound was observed and the duration of hospital stay.

RESULTS

The average systolic blood pressures were 150.1 mmHg and 120.3 mmHg for the hypertensive and normotensive groups, respectively. The mean number of days until the wound was dry was 3.79 for the hypertensive group and 2.03 for the normotensive group. Hypertensive patients required more days for their wounds to dry than normotensive patients (odds ratio  =  1.65, p<0.05). No significant difference in the duration of hospital stay was found between the two groups.

CONCLUSIONS

Hypertensive patients had a higher risk of prolonged wound discharge after THA than their normotensive counterparts. Patients with prolonged wound drainage are at greater risk for infection. Clinicians should pay particular attention to infection-prevention strategies in hypertensive THA patients.

Increased platelet, leukocyte, and endothelial cell activity are associated with increased coagulability in patients after total knee arthroplasty

BACKGROUND

Orthopedic surgery, especially total knee and total hip arthroplasty, is considered a risk factor for peri-operative venous thromboembolism.

OBJECTIVES

This study evaluates how accelerated inflammatogenic cellular interactions and the subsequent production of tissue factor and CD40 ligand play an important role in the pathogenesis of venous thromboembolism.

PATIENTS AND METHODS

Twenty-four patients undergoing total knee arthroplasty were randomly assigned to groups with (Ti; n = 12) and without (Tn; n = 12) pneumatic tourniquet inflation.

RESULTS

Numbers of leukocyte-platelet aggregates, especially those comprising monocytes-platelets in central venous blood from the Ti group, were increased during the peri-operative period (P < 0.01), and returned to the baseline level at 24 h after starting surgery. Levels of PAC-1, P-selectin, CD40 ligand, tissue factor, Mac-1 expression on monocytes including monocyte-platelet aggregates, and the number of microparticles including those of endothelial cell origin were noticeably increased in central venous blood from the Ti group (P < 0.01). Whole blood coagulability was also obviously increased in central venous blood from the Ti group (P < 0.01). Furthermore, the concentrations of venous plasma tissue factor antigen, CD40 ligand, platelet factor 4, beta-thromboglobulin, the soluble fibrin monomer complex and prothrombin fragment 1+2 were also increased (P < 0.05).

CONCLUSIONS

This study showed that platelet, leukocyte and endothelium activities as well as their interactions are enhanced during the peri-operative period of total knee arthroplasty, particularly in venous blood from the lower half of the body, which consequently augments blood coagulability. Further, tourniquet inflation during surgery exaggerates these responses.

Extended monitoring of hemostatic activation after varicose vein surgery under general anesthesia

BACKGROUND

Postoperative heparin prophylaxis after stripping of the long saphenous vein is a matter of controversial discussion, and practices vary by surgeon and country.

OBJECTIVE

The aim of this study was to assess the extent of hypercoagulability by continued monitoring of activation markers of coagulation and fibrinolysis for a period of 3 weeks after stripping of the long saphenous vein and concomitant phlebectomy.

METHODS

Including 21 patients, the following markers were measured preoperatively and on postoperative day 1, 2, 3, 7, 14, and 21: Activation products of coagulation: thrombin-antithrombin complex (TAT), thrombus precursor protein (TPP), and prothrombin-fragment F1+2 (F1+2), and markers of fibrinolysis: plasmin-alpha(2)-antiplasmin complexes (PAP), D-Dimer, tissue plasminogen activator (t-PA) antigen, and plasminogen activator inhibitor 1 (PAI-1) antigen.

RESULTS

TAT levels increased significantly until day 3 (p=.008) and normalized within 14 days. TPP levels increased significantly until day 7 (p=.02), decreasing to initial values within 21 days. PAP complexes increased significantly until day 2 (p=.02) reducing to baseline within the observation period. D-Dimer levels increased immediately after surgery (p<.001) until day 14 (p<.001) and returned to baseline until day 21.

CONCLUSIONS

Significant hemostatic activation after varicose vein surgery was observed and persisted until 3 weeks postoperatively, indicating that heparin prophylaxis for 2 to 3 weeks is advisable for at-risk patients.

Surgical stress induces acute coronary release of tissue-type plasminogen activator in the pig

BACKGROUND

Tissue-type plasminogen activator (t-PA) is an endothelium derived key enzyme in the initiation of endogenous fibrinolysis. Acute regulated release of active t-PA occurs within minutes in response to threatening thrombotic vessel occlusion. The aim of this study was to investigate the impact of surgical stimulation on the kinetics of t-PA release in the coronary vascular bed in the pig.

METHODS

In anaesthetised pigs (n=16), arterio-venous concentration gradients of t-PA, and plasma flows (retrograde thermodilution) were obtained across the coronary vascular bed before (control) and at 1, 3, 5 and 10 min after sternotomy.

RESULTS

At control, no significant coronary net flux (release or uptake) of t-PA was observed, while sternotomy induced a rapid net release of total t-PA (132.6 ng x min(-1)), with an associated increase in active t-PA (93.6 ng x min(-1)). This response, evident already after 1 min, showed a peak at 5 min and returned towards baseline levels within 10 min. No concurrent alterations in aortic levels of active t-PA were found and haemodynamic variables were unaltered.

CONCLUSION

The rapidly increasing and transient net coronary release of t-PA after sternotomy suggests that the endothelium actively promotes local endogenous fibrinolysis during surgery. Such events could reflect a dynamic responsiveness to protect the coronary circulation during stress.

Effect of tourniquet use on activation of coagulation in total knee replacement

Total knee replacement often is performed with tourniquet application. The advantages of a dry field, including fixation, are well known, but it still is debatable if tourniquet application increases deep vein thrombosis. Measurement of coagulation markers is a well accepted method of studying thrombogenesis activation intraoperatively and postoperatively. Twenty patients undergoing total knee replacement with subarachnoid anesthesia were assigned randomly to two groups: either with tourniquet application (Group I) or without tourniquet application (Group II). There were no differences between patients in the two groups in terms of age, gender, diagnosis (all had osteoarthritis), operative time, and total (intraoperative and postoperative) blood loss. Markers for thrombin generation and fibrinolysis were measured. Blood samples were drawn at four times: baseline before the operation; after bone cuts; after cement fixation (Group II) or 2 minutes after tourniquet deflation (Group I); and 1 hour after surgery. Markers of thrombin generation and fibrinolysis showed a significant increase from baseline in all the patients. In Group II these markers started to increase during surgery, whereas in Group I the increase occurred at the end of the procedure when the tourniquet was deflated. The total amount of thrombin generation was significantly higher in Group II (without tourniquet), whereas fibrinolysis was significantly greater in Group I. Total knee replacement is accompanied by a hypercoagulative state with or without the use of a tourniquet, but it seems to be higher when the tourniquet is not used. In addition, tourniquet application may increase fibrinolysis.

Course of molecular hemostatic markers during and after different surgical procedures

STUDY OBJECTIVE

To establish the most vulnerable time of thrombi formation with regard to the plasmatic (noncellular) part of the coagulatory and fibrinolytic systems.

DESIGN

Nonrandomized observational study.

SETTING

A surgical and an orthopedic unit and the central laboratory of a university hospital.

PATIENTS

61 consenting ASA physical status I and II inpatients undergoing four different types of surgery: total hip replacement (THR): 16 patients; hemicolectomy: 15 patients; endoscopic cholecystectomy: 15 patients; subtotal thyroid resection: 15 patients.

INTERVENTIONS

The time course of 11 procoagulatory and fibrinolytic parameters was examined during the different types of surgery. Blood samples were drawn on the day before surgery, directly before the induction of general anesthesia, 1 to 2 hours postoperatively, and on the mornings of postoperative days 1, 2, 3, 4, and 5.

MEASUREMENTS AND MAIN RESULTS

The coagulation samples were centrifuged within 1 hour of collection at 2,300 g for 15 minutes at 4 degrees C. Hemoglobin, hematocrit, platelets, fibrinogen, prothrombin time, activated partial thromboplastin time, thrombin time, antithrombin III, and protein C were determined immediately on laboratory arrival of the samples. The samples were aliquoted at -70 degrees C. They were thawed within 2 weeks and prepared for the following assays: thrombin-antithrombin III complexes (TAT-complexes), D-dimers, and plasminogen activator inhibitor type 1. Maximum activation of coagulation is not reached until 2 hours postoperatively and slowly decreases until normal values are reached around the fifth postoperative day. Parameters displaying the greatest changes are TAT-complexes and D-dimers. The type of surgery with the most pronounced changes was total hip replacement, followed by hemicolectomy, cholecystectomy, and subtotal thyroid resection.

CONCLUSION

The total hip replacement and hemicolectomy groups show similar and strong activation of the procoagulatory and fibrinolytic systems. Much less pronounced are the changes during endoscopic cholecystectomy and subtotal thyroid resection. Maximum activation occurs 1 to 2 hours postoperatively.

Changes in the fibrinolytic system in patients with peripheral arterial occlusive disease undergoing percutaneous transluminal angioplasty

We have investigated fibrinolytic parameters in 33 patients with peripheral arterial occlusive disease (PAOD) before and 6, 24, and 48 hours after percutaneous transluminal angioplasty (PTA) as well as in 35 gender-matched healthy controls, whose mean age was not significantly different from the mean age of the patients. PAOD patients had significantly higher plasma levels of t-PA antigen (12.0+/-4.9 vs. 9.2+/-5.5 ng/ml), PAI-1 antigen (34.8+/-22.1 vs. 27.2+/-23.6 ng/ml), PAI-1 activity (10.0+/-6.5 vs. 8.0+/-8.0 U/ml), PCI (188.2+/-55.6 vs. 134.1+/-75.5% as compared with normal human plasma), and fibrinogen (420.2+/-92.6 vs. 261.9+/-32.7 mg/dl) as compared with controls. After angioplasty, fibrinolytic parameters and fibrinogen levels increased, reaching higher than preintervention levels 24 and 48 hours after the intervention. Six months after initially successful PTA, restenosis was demonstrated in 14 out of 33 patients (42%). Patients with late restenosis had significantly higher levels of PAI-1 activity 24 and 48 hours after PTA, as compared with patients with late patency (24 hours: 16.1+/-8.0 vs. 10.0+/-7.4; 48 hours: 16.5+/-7.9 vs. 12.0+/-7.0; p<0.05 for both time points). The results suggest that impaired fibrinolysis early after PTA might be a cause or marker of a disturbed repair process of vascular injury, leading to restenosis.

Hemostatic markers in surgery: a different fibrinolytic activity may be of pathophysiological significance in orthopedic versus abdominal surgery

Without prophylaxis, patients subjected to major abdominal surgery have a risk of deep vein thrombosis of approximately 30%, while the rate varies between 40% and 60% in orthopedic surgery. The reasons for this discrepancy are not completely understood. The present study was designed to compare the pre- and postoperative behavior of different coagulation and fibrinolysis parameters in patients undergoing both types of surgery, receiving low molecular weight heparin prophylaxis. Samples were taken before operation and on postoperative days 1, 3, and 7. The following parameters were assessed: prothrombin fragment 1 + 2, thrombin-antithrombin III complexes, fibrinopeptide A, tissue plasminogen activator, plasminogen activator inhibitor, plasmin-alpha 2-antiplasmin complexes, and fibrin degradation products. We found a significant increase in the clotting markers postoperatively compared with preoperative values (P < 0.05), both in abdominal and orthopedic surgery, indicating a marked hemostatic activation which remained until postoperative day 7. A significant increase in plasminogen activator inhibitor (P < 0.01) and a decrease in tissue plasminogen activator and plasmin-alpha 2-antiplasmin complexes was also observed early after operation. The plasminogen activator inhibitor activity decreased, while tissue plasminogen activator and plasmin-alpha 2-antiplasmin levels increased significantly on days 3 and 7 (P < 0.05). Fibrin degradation products significantly increased throughout the postoperative period (P < 0.01). Preoperatively, we found higher plasminogen activator inhibitor activity and lower tissue plasminogen activator and plasmin-alpha 2-antiplasmin complexes (P < 0.05) in patients undergoing hip replacement compared with abdominal surgery. Fibrin degradation products were also significantly lower on postoperative day 3 in patients undergoing hip replacement (P < 0.01). We suggest that the lower preoperative fibrinolytic activation observed in patients undergoing orthopedic surgery compared with abdominal surgery might have pathophysiological consequences. Our results also indicate that the hemostatic activation persists beyond the 7th postoperative day despite prophylaxis.

Aortic cross-clamping influences regional net release and uptake rates of tissue-type plasminogen activator in pigs

BACKGROUND

The key regulator of intravascular fibrinolysis, tissue-type plasminogen activator (t-PA), is released from a dynamic endothelial storage pool. The aim of the study was to investigate regional t-PA net release and uptake rates in response to infra-renal aortic cross-clamping (AXC) and declamping (DC).

METHODS

Anesthetized pigs were studied during 5 min of AXC, followed by a 35-min declamping (DC) period. Arterio-venous concentration gradients of total and active t-PA, as well as respective plasma flows, were simultaneously obtained across the preportal, hepatic, coronary and pulmonary vascular beds. Plasma levels of total t-PA (ELISA with purified porcine t-PA as standard), and active t-PA (spectrophotometric functional assay) were determined.

RESULTS

Prior to AXC, we found a high net release rate of total t-PA across the preportal vascular bed (1700 ng.min-1 P < 0.001), and a high hepatic net uptake (4900 ng.min-1, P < 0.001), while coronary and pulmonary t-PA net fluxes were small and variable. AXC per se did not induce significant alterations in net fluxes of t-PA. Following DC, preportal and coronary net releases of total t-PA increased (to 2900 ng.min-1 and 60 ng.min-1, respectively). Despite an increase in hepatic net uptake of total t-PA (to 6100 ng.min-1) after DC, a significant increase in hepatic venous total t-PA occurred.

CONCLUSIONS

The release and uptake of t-PA is indicated to be dynamic and organ-specific. DC induces an acute profibrinolytic reaction in preportal organs. The high hepatic t-PA uptake capacity restricts preportal profibrinolytic events to affect the systemic circulation.

The effects of gynaecological surgery on coagulation activation, fibrinolysis and fibrinolytic inhibitor in patients with and without ketorolac infusion

The effects of gynaecological surgery on the fibrinolytic and inhibitor mechanisms were followed up for 24 h post-operatively in patients receiving a single dose of ketorolac infusion (n = 18) as compared with those not receiving ketorolac infusion (n = 11). A pre-operative state of lower mean t-PA activity and higher PAI-1 levels with increased platelet activation than that reported in normal subjects were observed in both groups of patients. Increased t-PA activity upon anaesthetic induction together with a decreased level at 24 h post-operation was seen in both groups. However, fibrinolytic 'shut-down' was not evident as significant increase in D-dimer levels was observed post-operatively, suggesting an enhanced lytic state concurrent with an enhanced activation of coagulation and diminished platelet activation although beta-TG remained above the normal level; plasmin from this enhanced lytic state affects platelet adhesion and cleaves platelet glycoprotein Ib thus inhibit release reaction. Ketorolac infusion elicited a significant response in PAI-1 activity within 24 h post-operation and this was not seen in the non-ketorolac group in spite of the rising trend by 24 h post-operation which did not achieve statistical significance. There were no statistical significant differences in blood loss and duration of surgery between the two groups of patients. Overall, both groups of patients showed similar haemostatic changes post-operatively for 24 h, a longer duration of post-operative study would have revealed any subtle changes in the molecular markers of thrombosis which was not the objective of this study.

Can laboratory testing improve screening strategies for deep vein thrombosis at an emergency unit?

OBJECTIVES

To study various markers of blood coagulation and fibrinolysis in relation to the extension of deep vein thrombosis (DVT), and to compare the diagnostic usefulness of these markers as screening tests for excluding DVT.

DESIGN

A clinical study of patients admitted to an emergency unit.

SETTING

Ostra Hospital, Göteborg, Sweden.

SUBJECTS

One hundred and five patients with a clinical suspicion of DVT.

MAIN OUTCOME MEASURES

Phlebography was used as the reference method for a diagnosis of DVT. Small distal thromboses as well as large proximal thromboses were included. Plasma D-dimer as well as other markers of coagulation and fibrinolysis were analysed.

RESULTS

Twenty-eight proximal and 20 distal DVTs were found. Plasma D-dimers (one ELISA and two latex assays), fibrin monomer, prothrombin fragment 1 + 2 (F1+2), thrombin-antithrombin III complex (TAT) and the t-PA-PAI-1 complex were all significantly correlated to the extension of DVT, whilst fibronectin, tissue-type plasminogen activator (t-PA), single-chain urokinase-type plasminogen activator (scru-PA) and plasminogen activator inhibitor 1 (PAI-1) were not. The sensitivity was 94% for the D-dimer ELISA and one of the latex methods (latex-B), at a specificity of 60% and 68%, respectively. The negative predictive value was 92% for ELISA and 93% for latex-B, and both assays showed a negative predictive value of 100% for proximal DVTs. Fibrin monomer, F1+2, TAT, D-dimer (latex-S) and the t-PA-PAI-1 complex all showed lower negative predictive values (88, 84, 79, 78 and 65% respectively).

CONCLUSIONS

Sensitivity and negative predictive values for a latex assay (D-dimer latex-B) was similar to that of a D-dimer ELISA: With a sensitivity of 94% (100% for proximal DVTs) such a latex assay may be included in a screening strategy for DVT at an emergency unit. However, the safety of such an approach has to be tested in other prospective studies.

Sequential intrapulmonary and systemic activation of coagulation and fibrinolysis during and after total hip replacement surgery

Hip joint replacement surgery, using acrylic cement for prosthesis fixation, is associated with intraoperative cardiorespiratory dysfunction, and a high frequency of postoperative proximal deep vein thrombosis (DVT). Levels of prothrombin fragments 1+2 (F1+2), tissue plasminogen activator antigen (t-PA), plasminogen activator inhibitor 1 activity (PAI-1), D-dimer and interleukin 6 (IL-6) were measured in arterial (AB) and mixed venous blood (MVB) in five patients during and after total hip replacement operation with acrylic cement prosthesis fixation. Sequential peaks of F1+2, t-PA, PAI-1 and IL-6 appeared, starting with activation of coagulation during preparation of bone, closely followed by activation of fibrinolysis. Later, this was counteracted by an antifibrinolytic response and increase of IL-6. After a fibrinolytic shutdown on the third postoperative day as evidenced by a drop in t-PA and D-dimer concentrations, a second wave of coagulation was seen at the end of the first week. The present model, with frequent sampling of blood entering and leaving the lungs, confirms our earlier findings of the lung as a key organ in promoting coagulation following traumatic activation.

Factors affecting deep vein thrombosis rate following total knee arthroplasty under epidural anesthesia

A retrospective review was performed of 448 consecutive patients undergoing primary, unilateral, bicondylar, and cemented total knee arthroplasty under epidural anesthesia by three surgeons to determine factors contributing to deep vein thrombosis rate. All had venography on the fourth or fifth postoperative day and received aspirin and elastic stockings as their only thromboprophylaxis. The overall deep vein thrombosis rate was 41% (2% had proximal clots). The rate of deep vein thrombosis was not related to obesity, history of heart disease, hypertension, prior malignancy, smoking, diagnosis of osteoarthritis, duration of surgery, type of local anesthetic used, or the use of postoperative epidural analgesia. The rate of deep vein thrombosis varied significantly between surgeons: one surgeon had an overall deep vein thrombosis rate of 58% (proximal thrombi, 4%) whereas the other two surgeons had a deep vein thrombosis rate of 35% (proximal clot thrombi, 1%). A number of possible mechanisms to explain the variation in deep vein thrombosis rates between surgeons are provided.

[Identification of persons with high risk of venous thrombosis]

The aim of identification of patients with high risk of deep venous thrombosis is a selective and more efficiency prophylaxis. Deficiencies of coagulation inhibiting and fibrinolytic proteins are implicated in less than 10% of patients. However there are several clinical situations with an increased risk of thrombosis. Thus the characterization of blood abnormalities as screening tests for the diagnosis of patients at risk is an important goal.