As more experience has been gained with compression ultrasonography, much has been learned about the factors that effect its accuracy. The sensitivity and specificity of ultrasonography are highly technician dependent, and more experience with administering the test has been shown to correlate with greater accuracy. Garino et al 15 reported one technician had a 0% sensitivity in identifying seven venogram-proven proximal clots in 121 patients, and another technician had a 100% sensitivity (and 98% specificity) in identifying five proximal clots in 84 patients. Westrich et al 62 reported that the sensitivity of ultrasound (compared with venography) improved from 75% to 85% and the accuracy of the test improved from 91% to 95% after 2 years of experience by the same ultrasound technician and radiologist. Ultrasonography is a more sensitive test for proximal than distal clots. Most pulmonary emboli after total knee arthroplasty probably originate in the popliteal or femoral venous systems, but pulmonary emboli also have been reported to arise from calf clots after total knee arthroplasty. 19 Ultrasonography also may miss clots in pelvic veins. The fact that ultrasound may not detect calf or pelvic clots is important because this means a negative ultrasound does not ensure that a patient has no risk of pulmonary embolism.
The results of using ultrasonographic screening for deep venous thrombosis after total knee arthroplasty before hospital discharge as part of a routine patient treatment algorithm have been reported by several authors. 18,31,70 With such protocols, the authors have had a low rate of complications related to venous thromboembolic disease and related to the prophylaxis and treatment of venous thromboembolic disease. However, there is only a small amount of information about whether such a screening protocol reduces the rate of overall morbidity and mortality related to prevention and treatment of venous thromboembolic disease compared with protocols for patient treatment that do not include routine screening for venous thromboembolism. The best information on this subject comes from a prospective, double-blind, randomized, controlled trial in which 1024 patients who had elective total hip arthroplasty or total knee arthroplasty were randomized to receive bilateral compression ultrasound tests or sham bilateral compression ultrasound tests before hospital discharge. 2,51 Patients routinely were treated with Coumadin for venous thromboembolism prophylaxis while in the hospital and were followed up for 90 days after the operation. Patients who had asymptomatic clots at discharge were treated with anticoagulation and patients without clots had the Coumadin discontinued at discharge. In the group of 518 patients who were screened, asymptomatic proximal clots were identified (and treated) in 13 patients (2.5%). In the same group, four additional patients ultimately had a symptomatic proximal deep venous thrombosis develop and one patient who had been treated with anticoagulation had a major bleeding episode; the event rate in the group that was screened therefore was five of 513 or 1%. In the group of 506 patients who had a sham ultrasound, three patients had symptomatic proximal clots develop and two had nonfatal pulmonary emboli develop; the event rate in the patients in the sham group was five of 506 or 1%. The authors concluded that with routine Coumadin prophylaxis, the rate of venous thromboembolic disease is sufficiently low that the use of screening ultrasound is not justified.
The information summarized in the aforementioned sections shows that venous thromboembolic disease after total knee arthroplasty is common, usually asymptomatic, most commonly is present in the calf but also can involve the proximal venous system, and can be detected in many cases, particularly for proximal clots (that seem to represent the greatest embolism risk) using noninvasive tests.
The known potential of asymptomatic proximal clots to lead to pulmonary emboli in combination with the development of noninvasive, widely available, screening tests for asymptomatic clots has provided the rationale for proposing routine surveillance for deep venous thrombosis after total knee arthroplasty. The potential value of surveillance is that it might reduce venous thromboembolismrelated morbidity and mortality by two mechanisms. First, screening could identify patients with clots and allow treatment of clots before they become symptomatic or lead to embolism. Second, screening could identify patients without clots and allow them to safely discontinue pharmacologic venous thromboembolic prophylaxis and thereby avoid the risk, cost, and inconvenience associated with longer use of prophylactic agents. At the same time, routine screening has some potential disadvantages including the potential to lead to undertreatment or overtreatment of some patients and the cost associated with the test.
To date, the best information available on the value of screening comes from a large, prospective, randomized controlled trial comparing compression ultrasound with a sham test before hospital discharge. 2,51 The study failed to show an advantage of routine ultrasound screening in reducing the rate of adverse events related to venous thromboembolism or its prevention and treatment. The study provides a strong argument against the routine use of screening after total hip arthroplasty and total knee arthroplasty. However, to interpret the study properly, and to place it in context, the limitations of the study must be understood. The study is only one cohort of patients treated by one group of surgeons with a specific perioperative protocol. In the study, all patients routinely were treated with Coumadin during hospitalization and the rate of proximal deep venous thrombosis was low. The lower the rate of disease, the lower the likelihood that routine screening will be of value. Additionally in that study, the mean length of hospitalization was 9 days, considerably longer than that commonly reported for patients who currently are treated in North America. The study included patients who had hip and knee arthroplasty but does not distinguish between them. Importantly, the rate and location of venous thromboembolic disease and the natural history of the disease in patients who had hip and knee arthroplasty are not identical. Interpretation of this prospective randomized study should be made with this significant limitation in mind.
The value of routine screening is determined by many factors including the frequency of asymptomatic clots in a patient group; the risk of embolization from those clots if the patient does not receive specific treatment other than the routine prophylaxis being used; the rate of complications associated with treatment of asymptomatic clots; the sensitivity and specificity of the screening test being used; and the rate of complications associated with routine prophylaxis of all patients after hospital discharge. If a specific perioperative protocol is associated with a relatively high rate of asymptomatic proximal clots, screening may be of greater value. Similarly, if the routine venous thromboembolism prophylaxis program in place provides little protection against asymptomatic clots (if pharmacologic prophylaxis routinely is stopped after a brief hospital stay) the value of screening also might be greater. Likewise, if the routine venous thromboembolism program in place for all patients leads to a relatively high rate of complications then the value of screening (and the value of being able to spare many patients a longer duration of routine prophylaxis) would be greater. However, if the treatment protocol for asymptomatic clots identified by screening has a high complication rate, the relative benefit of screening is less. Finally, if the screening test used in a specific hospital is not sufficiently sensitive, some patients may be undertreated whereas if the screening test is not sufficiently specific, some patients may be overtreated.
The value of routine surveillance for deep venous thrombosis at the time of hospital discharge after total knee arthroplasty is unproven, and is cast into doubt by the best study on the subject. 2,51 However, the value of a surveillance protocol in reducing venous thromboembolic disease-related morbidity and mortality, and the cost-effectiveness of such a protocol probably depends on many features of a specific hospital and surgeon’s practice and perioperative protocol. These include the type and duration of venous thromboembolism prophylaxis in routine use, the rate of asymptomatic and symptomatic venous thromboembolic disease associated with that protocol, and the accuracy of the screening tests used for surveillance.
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Richard S. Laskin, MD; Cecil H. Rorabeck, MD; William Healy, MD; and Robert Barrack, MD, Guest Editors