Sonographic imaging of calf veins in patients with suspected deep vein thrombosis: a cost-effective strategy?

The paper by Atni et al. [1] comes at a time of discord. Abbreviated protocols for the sonographic evaluation of the leg veins, with limited sampling at one site in the common femoral and popliteal veins, are now being encouraged. Atni et al. [1] would have us evolve in the opposite direction and perform a comprehensive and time-consuming sonographic examination of the legs that includes the calf veins. Proponents of the abbreviated examination protocol would argue that the calf veins are not important clinically [2]. Proponents of calf vein imaging argue that patients with calf vein thrombi should be treated [3]. These arguments are important in an era where outcomes analysis and a utilitarian approach to the use of medical resources are gaining momentum. These issues cannot be resolved by correlative studies looking at the accuracy of venous sonography for calf vein imaging. The data presented by Atni et al. [1] and their brief review of the pertinent literature can, however, be used to give some perspective on the role of sonographic imaging of the lower extremity veins. The paper by Atni et al. [1] gives us some information on the accuracy of sonographic imaging for two very different clinical scenarios. There is a great difference in the pathophysiology, and in the sonographic appearance, of thrombi that form in symptomatic and asymptomatic populations. Symptomatic patients who present with pain and unilateral swelling of the leg will overwhelmingly have obstructive thrombi. Asymptomatic patients, mostly enrolled from high-risk patients seen in the postoperative period, will tend to have nonobstructive thrombi. These nonobstructive thrombi will also, on average, be smaller than those seen in the symptomatic patients. At first glance, the results from Atni et al. [1] are somewhat confusing. The expected accuracy for calf vein imaging should be lower in the asymptomatic patients based on the logic that the larger the thrombus, the more likely that the thrombus will be seen by sonography [4]. Ihis finding is notwhatAtni et al. [1] report. Their data suggest that the accuracy of calf vein imaging is better for the asymptomatic population (86% versus 92%; symptomatic versus asymptomatic patients). This result is likely a statistical fluke. The confidence intervals are broad, given that only eight cases of isolated symptomatic calf vein thrombi were seen; the true-positive rates overlap because of the small number of cases. The failure to visualize small, nonobstructive thrombi is also confirmed by Atni et al. [1]. Two of the three femoral vein thrombi, all approximately 1 cm in size, were missed. It is interesting that the prevalence of femoral vein thrombi is so low in these postoperative patients. This finding confirms the heavy weighting toward knee surgery (97 of 1 10 cases; 88%). Patients with knee surgery are known to preferentially develop thrombi below the knee, whereas hip surgery patients have a predilection for both femoral and below-the-knee venous thrombi. In the symptomatic patients, the results confirm what is already in the literature. The sensitivity of calf vein imaging is likely above 80%. In fact, looking at three published series for isolated calf vein thrombi [5-7] in addition to the currently published one (Table 1) gives us an estimated sensitivity for calf vein imaging of 93% (confidence interval, 66-100%). The confidence limits remain large. It does appear that techniques that focus mainly on color flow imaging do not perform as well as those that rely on gray-scale imaging [8].