DVT presents a significant healthcare burden; therefore, early diagnosis and the initiation of anticoagulant therapy are essential to reduce the risk of morbidity and mortality as well as prevent complications [35]. Contrast venography is usually considered the “gold standard” for DVT diagnosis; however, point-of-care compression ultrasound is currently regarded as the first-line imaging tool in the emergency department since it is more safe, cost-effective, and non-invasive [36, 37]. The current study shows that both 2 and 3-point POCUS have high sensitivity, specificity, PPV, and NPV for DVT diagnosis. Compared to the 2-point and 3-point ultrasound techniques, the pooled data for other POCUS protocols (Complete compression ultrasound and whole-leg duplex ultrasound) seem to result in higher sensitivity and specificity. Furthermore, our analysis shows that the time from triage to DVT diagnosis when using POCUS in the emergency department is significantly reduced compared to when reference tests are carried out in the radiology department.

The diagnostic results reported in our study are supported by a more recent meta-analysis that compared 2-point and 3-point POCUS and had fewer included studies than ours. The results of that meta-analysis showed high sensitivity and specificity for both 3-point (90% and 95%) and 2-point POCUS (91% and 98%) [38]. Similarly, a previous meta-analysis pooling data for all POCUS protocols (Complete compression ultrasound, 2-point, and 3-point) reported sensitivity and specificity of 95% and 96%, respectively [39]. In addition, a meta-analysis evaluating the accuracy of EP-performed ultrasound reported the ultrasound was able to diagnose DVT with a 94.8% weighted mean sensitivity and 96.2% weighted mean specificity [40]. Despite all these results pointing to high sensitivity and specificity, it should be noted that high heterogeneity exists. Therefore, these findings should be interpreted with caution. We also noticed that some included studies recorded relatively low specificity and sensitivity values. For example, Abbasi and colleagues recorded as low as 41.2% sensitivity for DVT diagnosis [10]. The low accuracy reported in this study was attributed to the fact the POCUS was carried out by second-year emergency residents who had low hours of training. Similarly, Zitek and colleagues recorded a 57.1% sensitivity when using the 2-point protocol [32]. The low sensitivity was also attributed to the fact the operators of that study were less experienced and skilled in ultrasound.

Since training level has been attributed to low diagnostic performance, it is essential to discuss the role of education when carrying out POCUS to diagnose DVT. Our regression analysis showed that the level of training was not a significant source of heterogeneity in the specificity and sensitivity analysis. Moreover, the pooled data shows that POCUS performed by both experienced and inexperienced EP has comparable specificity and sensitivity. However, research shows that inadequate training could result in omission errors, where DVT may not be treated when it is falsely excepted, and commission errors, where anticoagulant therapy is initiated when DVT is falsely confirmed [40]. Even though the exact training and experience required to diagnose DVT is uncertain, The American College of Emergency Physicians guidelines suggests that for clinical decision-making, POCUS training should be done for at least over a two-day course [41]. Furthermore, Blaivas reported that 10 min of training is insufficient for DVT diagnosis but reiterated that when emergency physicians are trained properly, they can accurately diagnose DVT in the emergency department [42]. To support this hypothesis, Blavais and colleagues later reported that 2-h didactic education followed by hands-on training for three hours and previous experience on POCUS has a very high correlation with vascular studies (0.9 kappa and 98% (95% CI: 95.4–100%) agreement). However, the education curriculum currently varies. For this reason, Fox and colleagues called for more uniform and universal training of EP to use POCUS in DVT diagnosis [43].

Our meta-analysis results have also shown that POCUS is advantageous in reducing the time from triage to DVT diagnosis compared to reference tests in the radiology department. These results are reinforced by a Malaysian study of 63 patients, which reported that bedside ultrasound significantly shortened the time between ED arrival and confirmation of DVT (2.24 ± 0.43 h and 17.28 ± 4.77 h, p < 0.001) [44]. In addition, studies claim that POCUS can improve the time to disposition (being discharged from the ED or Hospital). Seyedhosseini and colleagues reported that the time between triage and the disposition of patients was significantly shorter for patients in the emergency department POCUS group compared to the radiologist group (69 min (28–138) vs. 142 min (91–233), respectively; p < 0.001). Similarly, Chu and colleagues reported a significantly shorter disposition time when using bedside ultrasound (p < 0.001) [44]. On the other hand, El-Gazzar and colleagues reported that the time EP took to diagnose DVT was significantly shorter as opposed to the time taken by a radiologist (6.68 ± 1.81 vs. 5.76 ± 1.62 min, respectively; p < 0.001) [20]. Similarly, zitek and colleagues reported that ultrasounds carried out in the ED were completed 84 min before the ultrasound in the radiology department was made available [32]. The significant reduction in time to diagnosis and disposition in ED-performed POCUS reported in these studies can be attributed to the fact that ultrasound devices are usually readily available in the ED for 24 h.

Point-of-care compression ultrasound in DVT diagnosis is also subject to various pitfalls. The first limitation is the location of DVT. Research shows that the 2 and 3-point POCUS protocols cannot diagnose calf vein thrombosis, but whole-leg compression ultrasound carried out in the radiology department can. This means that the 2 or 3-point compression can miss to diagnose some DVTs that would have been detected when using the whole-leg compression technique. However, previous research suggests that the 2-point compression protocol may be as sensitive as the complete compression in diagnosing DVT from the inguinal ligament up to the calf [45]. In addition, DVT in the bedside ultrasound can be mistaken for a Baker’s Cyst or lymph nodes; therefore, it is essential that landmarks such as ensuring the vein is usually closer to the artery are identified. Lymph nodes have also been mistaken for the common femoral vein, thus increasing the rate of false negatives. For instance, Zitek and colleagues reported that a total of 22 false negatives were observed when carrying out the 2-point POCUS, of which one false negative was attributed to the fact that the resident sonographer mistook the lymph node for the common femoral vein, thus contributing to the low sensitivity [32].

In addition, the acute clot has been mistaken for chronic clots. Research shows that an abnormal compression ultrasound may continue to be seen in up to 70% of DVT patients after one year [46]. The thrombus age is usually inferred from the clot echogenicity, of which older clots tend to be more echo dense. However, this skill is generally left to radiologists with advanced skills. Another common error not reported in many studies is inadequate visualization of the popliteal vein. Zitek and colleagues reported that 8 of the 22 false negatives resulted from inadequate popliteal vein visualization [32]. The videos analyzed in that study showed that most residents had mistaken the popliteal vein with the superficial vein. Moreover, one of the residents had mistaken the popliteal vein with a hyperechoic thrombus for the tibial vein. Given this high-frequency error, ultrasound educators should keenly ensure that the learners understand the popliteal vein to help avoid this error in future and improve the diagnostic performance of POCUS.

In the evaluation of DVT, an accurate determination of the pre-test probability for a clot is also crucial. The widely accepted guidelines have recommended using validated scores and D-dimer in evaluating the likelihood of DVT diagnosis among patients bestowing indicative symptoms [47]. As reported in our previous case report of a 51-year-old male with type 2 diabetes and hypertension, after D-dimer testing was done, a clinical judgment suspected DVT as one of the differential diagnoses; thus, doppler ultrasound was carried out and found a distended and non-compressible intramuscular calf muscle which was suggestive of acute thrombosis[48]. Research also shows that using algorithms that incorporate pre-test probability assessment with a sensitive D-dimer test reduces the number of imaging studies carried out [49, 50]. The most commonly validated score system is the Well’s scoring system, of which a ≥ 2 score is indicative of a high pre-test probability of DVT. Studies incorporating a POCUS protocol with Well’s scores and D-dimer testing seem to have a high diagnostic performance. For instance, in the study by Garcia and colleagues, the 3-point ultrasound was led by a well’s criteria and D-dimer testing, and this led to high sensitivity, specificity, and accuracy of 93.2%, 90%, and 91.7%, respectively[9]. Similarly, an Egyptian study conducted the Well’s criteria and D-dimer testing before the 3-point POCUS and found that the sensitivity, specificity, and accuracy of POCUS examination for DVT diagnosis were high (94.12%, 92.42%, and 93.0%, respectively)[20].

Compared to the most recent systematic review and meta-analysis [38] and other two previous meta-analyses [39, 40], our study has more number and most recent studies that assess the role of POCUS in DVT diagnosis. Unlike the study by Lee et al. [38], we were able to evaluate the effect of the level of training on the observed heterogeneity. Our regression analysis showed that the level of training did not contribute to the heterogeneity, and the sensitivity and specificity of both experienced and inexperienced was comparable. This indicates that even EP with the most miniature training can diagnose DVT using POCUS with a certain degree of accuracy. However, the regression analysis also showed that the presence of emergency medicine (EM) attending significantly contributed to heterogeneity, and the pooled specificity was always higher when the EM attending was present. This is to show that even though low training levels can give good outcomes, to obtain better outcomes, it is essential that EM attendings with POCUS experience are present during the POCUS examinations.

The current review was subject to several limitations. First, the eligibility criteria only allowed the inclusion of English-published studies, thus introducing selection bias in our analysis. Secondly, the meta-analysis results showed high heterogeneity; however, the risk of bias assessment revealed a low risk of bias, meaning that the bias did not influence our results. Thirdly, in the meta-regression analysis, we classified the training levels as either experienced or inexperienced and found that the level of training did not influence the heterogeneity. However, the number of hours to train the EP varied from study to study, which, if analyzed, might result in significant heterogeneity. Lastly, very few studies have evaluated the diagnostic performance of the whole-leg compression and complete compression techniques in the emergency department despite our results pointing out that the sensitivity and specificity are higher compared to those of 2-point and 3-point compression techniques. Therefore, further studies should be carried out to support this evidence fully.