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3. Calf DVT: To scan or not to scan?

By Ellie Smith, Vascular Scientist - North Bristol NHS Trust

The debate about the benefit of scanning calf veins for deep vein thrombosis (DVT) remains a controversial topic which has been debated by clinicians for a number of years; Kakkur et al (1969) produced findings on the natural progression of calf DVT and the debate has continued since.  Influencing factors for the assessment of the calf veins includes the balance of the risks and benefits of treatment (Masuda et al, 2010), sensitivity and specificity of the ultrasound scan below knee, and the competence of the individual performing the scan. The variation in scanning protocols across NHS Trusts, with evidence to support both viewpoints, means the debate continues. This could potentially impact patient care if a consistent approach is not employed, which begs the question- to scan or not to scan (for calf DVT)?

NICE guidance

The NICE guidance for venous thromboembolism diagnosis and treatment is based on findings by the Guideline Development Group, with ongoing summary of evidence from multiple sources to ensure best practice based on current evidence.   NICE guidance for diagnosis of deep vein thrombosis clearly recommends proximal leg DVT scanning, considering proximal leg to be popliteal vein and above, with repetition if required between 6 and 8 days post initial scan (NICE, 2015).  This is based on the risk of anticoagulation vs the risk of clot propagation in an individual, with statistics supporting the idea that calf vein clot in many patients may be self-limiting in 60-75% of patients (Kearon, 2003).  However, the guidance suggests that every patient with a negative above knee scan with a positive D dimer test should be rescanned in 6-8 days.  This could incorporate a huge number of patients who may have 2 or more points on the Wells system, and a raised D-dimer for any number of reasons, including pregnancy, infection, malignancy, trauma or recent surgery.  Many of these patients are currently excluded from having a DVT at the initial full leg scan, but with more limited scanning this may not be safe or appropriate to do, and therefore a larger number of patients may return for repeat scans. The context for the NICE guidance states the guidelines do not cover people below the age of 18 nor pregnant women, therefore, does this mean we should be scanning calf veins in this cohort of patients?  Unfortunately, the diagnosis or elimination of DVT requires a ‘better to be safe than sorry’ approach, meaning the burden of this may fall on vascular laboratories in the form of repeated imaging.

Anticoagulation risk vs clot propagation risk

Another point to consider is the benefit of treating calf DVTs, as anticoagulation carries its own inherent risks.  Scanning the calves of patients with other explanations for leg pain, such as cellulitis, recent leg surgery or injury may discover a blood clot which in reality may be asymptomatic, and this may result in the patient being treated for an incidental finding.  At North Bristol NHS Trust, the Trauma and Orthopaedic consultants in particular are reluctant to treat calf DVT, and therefore, is it sensible to scan this area when this will not be treated regardless of the scan result?

The statistics around risks of clot progression and PE are highly variable and the typical progression of calf DVT is based on observational studies (Fleck et al, 2017). PE is most likely in the early stages of DVT when the clot has not adhered to the vessel wall (Myers & Clough), and therefore it may be seen to be more important to diagnose DVT early to prevent this.  The annual incidence of venous thromboembolism (VTE) in the UK is approximately 2 in 1000 with increasing likelihood with increasing age, and the risk of diagnosed PE is typically 7-8 per 10,000 (NICE, 2012).  Propagation of calf DVT to PE is difficult to quantify, due to a limited number of blinded non-interventional studies that examine the typical patterns of progression (Palareti, 2014), however it is quoted that 90% of PEs are thought to originate from lower limb veins (Katz et al, 2014). According to Palareti (2014), the presence of calf DVT was found in 7-11% of patients with suspected PE, however these figures appear variable within the evidence base. Stubbs et al (2018) report distal veins are involved in 40% of DVTs; by not assessing calf veins, this indicates that almost half of the lower limb clots currently diagnosed would not be detected.  However, this may not be significant; Kearon (2003) states that proximal extension of isolated and symptomatic calf DVT occurs in a quarter to a third of incidences, meaning a relatively small proportion of calf DVT scans may result in a proximal DVT. 

The accuracy of calf DVT ultrasound is poorly reported; the sensitivity and specificity of above knee DVT is reported to be 97% and 94% respectively (Zierler, 2004).  It has been quoted by Zierler (2004) that scientists can scan 80-98% of calf veins effectively, with sensitivity and specificity in these studies being greater than 90% (Rose et al, 1990).  Therefore, if we can scan these veins accurately, and if a significant number of deep vein clot may present in these veins, shouldn’t we do so? Some would disagree, and based on propagation risk, this is a reasonable argument.   According to Righini et a (2006), scanning the below knee veins for DVT doubles the number of people being given anticoagulation, and performing distal ultrasound risks overtreatment of patients with limited evidence that this is indicated in calf DVT. It is difficult to predict in which patients an isolated calf DVT will propagate and who should be fully assessed.  The CACTUS trial (Righini et al, 2016), a double blinded randomised trial looking at the use of anticoagulant and placebo treatments in low risk patients with calf DVT, found that there were no significant differences in the groups in regards to proximal DVT extension or venous thromboembolic events.  However, there was an increased risk of bleeding in the anticoagulated patient group, indicating that blanket anticoagulation for isolated calf DVT may not be in the best interests of all patients.

Rescanning patients at 7 days

A consistent concern of many services is the requirement for rescanning patients at 7 days, and whether these numbers would be manageable. In terms of service and time management, scanning the calf veins after the proximal leg scan extends the scan by approximately 5 minutes, depending on complexity, however the return of patients after 7 days would require a rescan for which 15-30 minutes may be allotted, depending on local protocol. Often clinicians are satisfied if above knee DVT has been excluded in patients that are low suspicion of DVT, and therefore patients may not return for a 7 day rescan.  However, if symptoms deteriorate, patients can often be scanned numerous times, which may occur more regularly if calf scans are not being performed.  Another element to consider is the cost of repeated scanning. If two scans are required, this increases the cost to the CCG, as the tariff for the scan remains the same for proximal or full leg scans. This could be seen as beneficial for vascular laboratories, as income and activity would be increased.  However, many vascular departments are currently understaffed, and the additional workload may create issues relating to capacity for repeated scans.

Patient satisfaction and management

A large part of scanning the calf is in order to alleviate patient concern, as this is typically where patients complain of pain. If we are no longer scanning calf veins, will patients have their worries alleviated by the scan, or could they represent to A&E insistent on further investigation? It may make it difficult for doctors to form a differential diagnosis, such as a muscle tear or collection, if we are unable to exclude a DVT as the cause for pain.  Patient health and wellbeing is essentially at the centre of what we do, and perhaps if we do not scan calf vessels, many of the patients we examine may be denied a straightforward diagnosis for their pain, and this may have an impact on the amount of people we rescan; the NICE guidance suggests a 7 day rescan, but perhaps patients will represent before this time period to complain of increasing pain, and the issue then arises of how often these patients should be scanned. Could this also increase patient complaints, particularly in patients who have previously had scans where the calf vessels were assessed, feeling we are being obtuse by refusing to scan the problematic area? Something else to consider is patients with co-morbidities which may be affected by a DVT diagnosis. We scan many patients who have active malignancy, and in these cases, could an undiagnosed blood clot impact on their treatment schedules, i.e. would their chemotherapy or radiotherapy be delayed for a week until a DVT is either excluded or diagnosed? This would have an impact not only on patient wellbeing but on hospital services, and could have a widespread impact. Could this delay discharge of inpatients, unhappy to go home until the problem with their leg has been diagnosed? 

Clinician opinion

The input of clinicians is an important consideration when determining whether to scan calf veins for DVT. On discussion with vascular surgeons at North Bristol Trust, they feel it is important to know there is a clot present, regardless of the treatment, as it can reassure patients and allow them to have a diagnosis.  If the clot is not treated, the patients can also have an understanding of the reasons for not treating the calf thrombosis.  It is also useful to know if a patient has got a DVT for future consideration, as a previous DVT increases the risk of recurrent VTE and would be taken into account in future medical management. It can also influence other treatments or patient lifestyle; if a DVT is found a patient may have stockings or clexane to fly, whereas this may be a difficult decision to make if a DVT has not been diagnosed.  In patients who may be having treatment for other pathology, i.e. cancer, a treatment regime may be changed if a blood clot is found, such as a change to the chemotherapy drug offered.  This could have a significant impact on these patients.

It is also possible that there would be significant dissatisfaction from clinicians who currently expect a full leg scan when querying a DVT; by taking away a service which they currently have, this may create problems, and could this cause pressure to be put on scientists to perform the scan outside of Trust protocol? If performing scans outside of Trust protocol or NICE guidance recommendations, this could have medico-legal implications, which may encourage Trusts to adhere to the guideline of scanning only proximal veins.

Deskilling of scanning staff

A further consideration is the deskilling of scanning staff. Ultrasound is a safe and effective tool to diagnose calf vein thrombosis when being utilised by trained sonographers; calf DVT identification has sensitivity and specificity of >90% with duplex ultrasound when performed by a trained professional (Rose et al, 1990). Would this skill potentially be lost if this scan is not being performed with regularity? On the other hand, a calf vein scan can be complex; an inexperienced sonographer attempting to scan the calf vessels may miss a subtle DVT, which may be more of a risk to patients than repeated scanning of the proximal veins.

Conclusion

In conclusion, there are arguments for and against the scanning of calf veins for DVT. Ultimately, due to the risk of further complications, it is important to scan the calf veins OR rescan at a week as per NICE guidelines. The job of the vascular scientist is to provide information to the clinician, who will ultimately decide if anticoagulant treatment is in the best interest of the patient, and when scanning calf veins, we are providing the maximum information for the clinician to review.  However, inexperience and scan limitations can cause reduced confidence with calf vein assessment, and in these circumstances a proximal vein scan and seven day rescan may be in the best interest of the patient.  The wellbeing of patients is first and foremost in our work, and we should consider this when determining whether or not to scan the calf vessels.

We would appreciate the input of committee members to this debate- please send your thoughts on the subject matter above via the PSC committee through the newsletter.

With thanks to the vascular team at North Bristol NHS Trust for their input to this debate.

References

Fleck, D., Albadawi, H., Wallace, A., Knuttinen, G., Naidu, S., Oklu, R. (2017). Below-knee deep vein thrombosis (DVT): diagnostic and treatment patterns. Cardiovascular diagnosis & therapy, 7 (Suppl 3): S134- S139. doi: 10.21037/cdt.2017.11.03

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NICE (2015). Venous thromboembolic diseases: diagnosis, management and thrombophilia testing. https://www.nice.org.uk/guidance/cg144, accessed 20/06/19.

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Righnini, M. et al (2006). Clinical relevance of distal deep vein thrombosis. Review of literature data. Thromb Haemost. 2006 Jan;9 95(1):56-64.

Righini, M. et al. (2016). Anticoagulant therapy for symptomatic calf deep vein thrombosis (CACTUS): a randomised, double-blind, placebo-controlled trial. The Lancet, Volume 3 Issue 12 Pages e556-562, https://doi.org/10.1016/S2352-3026(16)30131-4, accessed 24/06/19.

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