Investigations
1st investigations to order
quantitative D-dimer level
Test
Suitability of D-dimer testing depends on:[12][26][27]
Whether the patient’s Wells score has categorised the patient as ‘DVT likely’ (Wells ≥2) or ‘DVT unlikely’ (Wells <2)
AND
The availability of venous ultrasound.
If the patient is pregnant, do not use D-dimer testing (or the Wells score) to confirm or rule out DVT.[26][27] See Suspected DVT in pregnancy under Diagnosis recommendations for more information.
If D-dimer testing is indicated, consider:[12]
Fully quantitative point-of-care D-dimer testing if laboratory facilities are not available (e.g., in a primary care setting)
An age-adjusted D-dimer test threshold if the patient is aged over 50 years.
Practical tip
If D-dimer testing is required, always order this before giving anticoagulation; a false negative D-dimer result can occur if blood is drawn after giving anticoagulation.[26]
DVT likely (Wells ≥2)
Venous ultrasound is the first-line investigation for patients deemed ‘DVT likely’.[12][26][27] See Venous ultrasound below. However, request D-dimer testing if the result of venous ultrasound is:[12]
Negative
OR
Not available within 4 hours. After the D-dimer test, start interim therapeutic anticoagulation, and organise a venous ultrasound with the result available within 24 hours.[12]
DVT unlikely (Wells <2)
Request D-dimer testing for all patients categorised as ‘DVT unlikely’.[12][26][27]
If the D-dimer level has been taken but the result is not available within 4 hours, start interim therapeutic anticoagulation.[12]
If the D-dimer level is elevated, organise:[12]
A venous ultrasound, with the result available within 4 hours
OR
Interim therapeutic anticoagulation, and a venous ultrasound with the result available within 24 hours.
A normal D-dimer level excludes the diagnosis of DVT; consider alternative causes.[12]
Practical tip
Be aware that an elevated D-dimer level is non-specific and is frequently abnormal in patients without DVT who are older, are acutely ill, have underlying hepatic disease, have an infection, or are pregnant.
Result
normal (DVT excluded if Wells score <2); elevated (proceed to imaging)
venous ultrasound
Test
Venous ultrasonography is the first-line method of imaging.[12][26]
Choose to use venous ultrasound based on whether the patient’s Wells score has categorised them as ‘DVT likely’ or ‘DVT unlikely’, and the result of D-dimer testing (if this is indicated).[12] A positive venous ultrasound confirms the diagnosis of DVT.[12][26][27]
In the UK, different centres may use either proximal or whole-leg venous ultrasound. Check your local protocol to determine the recommended strategy.
The UK National Institute for Health and Care Excellence (NICE) recommends using proximal venous ultrasound only.[12] However, the European Society of Vascular Surgery recommends whole-leg ultrasound if you suspect distal (calf) DVT.[26] Both strategies have advantages and disadvantages.
If a whole-leg ultrasound confirms distal (calf) DVT, check your local protocol for recommendations on whether and how to start anticoagulation; there is debate in the guidelines.
In the UK, common practice is to start anticoagulation unless the patient has a high risk of bleeding or the DVT is not extensive (<5 cm). Seek advice from a haematologist if the patient has a high bleeding risk or the DVT is not extensive. See Confirmed distal (calf) DVT under Management recommendations for more information.
DVT likely (Wells ≥2)
Organise a venous ultrasound, with the result available within 4 hours.[12][60]
If the result of a proximal-leg ultrasound is negative, order D-dimer testing.[12] If the result of D-dimer testing is:[12]
Positive, organise a repeat venous ultrasound 6 to 8 days later but do not start interim therapeutic anticoagulation. If this repeat ultrasound is negative, consider alternative causes[12]
Negative, consider alternative causes.
If the result of a whole-leg ultrasound is negative:
Check your local protocol because recommendations vary; some UK centres advocate a repeat ultrasound in this scenario
However, the European Society for Vascular Surgery does not recommend a repeat ultrasound, and advises to consider alternative causes.[26]
If the result of the venous ultrasound is not available within 4 hours:[12]
Order a D-dimer test and then offer interim therapeutic anticoagulation
Organise a venous ultrasound with the result available within 24 hours.
DVT unlikely (Wells <2)
If the patient has a positive D-dimer result, organise a venous ultrasound with the result available within 4 hours if possible.[12]
If the result will not be available within 4 hours, start interim therapeutic anticoagulation and organise a venous ultrasound with the result available within 24 hours.[12]
If the result of the venous ultrasound is negative, check your local protocol for advice on next steps because practice varies in the UK. NICE in the UK recommends stopping interim anticoagulation (if this has been started) and considering other causes.[12] However, some UK centres may consider repeating the ultrasound (even if whole-leg ultrasound has been used) to look for missed distal (calf) DVT that is extending proximally.
Result
abnormal B-mode image: inability to fully compress lumen of vein using ultrasound transducer; normal B-mode image: all vein segments fully compressible, non-diagnostic
abnormal Doppler: reduced or absent spontaneous flow, lack of respiratory variation, intraluminal echoes, or colour flow patency abnormalities
full blood count
Test
Order as a baseline level before starting anticoagulation.[12][26] Do not wait for the result before starting anticoagulation.[12] However, ensure you have reviewed (and acted on if necessary) the results within 24 hours of starting anticoagulation.[12]
A component of the assessment of bleeding risk while using anticoagulation. For example, marked thrombocytopaenia or severe anaemia can be contraindications to anticoagulation.
May detect abnormalities such as underlying haematological malignancy (e.g., anaemia, leucopenia).
A high platelet count may suggest essential thrombocytosis or a myeloproliferative disorder. Exceedingly low platelet count may preclude the use of some anticoagulants. Heparin therapy can be associated with heparin-induced thrombocytopenia; platelet counts should be measured at baseline and regularly throughout treatment.
Result
baseline levels
urea and creatinine
Test
Order as a baseline level before starting anticoagulation.[12][26] Do not wait for the result before starting anticoagulation.[12] However, ensure you have reviewed (and acted on if necessary) the results within 24 hours of starting anticoagulation.[12]
Doses of some anticoagulants (e.g., low molecular weight heparin, fondaparinux, apixaban, rivaroxaban, dabigatran, edoxaban) may need to be adjusted or discontinued in patients with renal impairment, so baseline values should be obtained.
Result
baseline levels
liver function tests
Test
Order as a baseline level before starting anticoagulation.[12][26] Do not wait for the result before starting anticoagulation.[12] However, ensure you have reviewed (and acted on if necessary) the results within 24 hours of starting anticoagulation.[12]
Note that choice of anticoagulant for a patient with hepatic impairment depends on the underlying cause and severity of hepatic impairment.
May detect abnormalities associated with underlying provoking factor (e.g., cancer).
Result
baseline levels
clotting screen
Test
Order a clotting screen, which should include prothrombin time (PT) and activated partial thromboplastin time (aPTT), as a baseline level before starting anticoagulation.[12][26] Do not wait for the result before starting anticoagulation.[12] However, ensure you have reviewed (and acted on if necessary) the results within 24 hours of starting anticoagulation.[12]
Result
baseline levels
Investigations to consider
CT/MRI venography
Test
Note that these recommendations do not apply to pregnant patients - see Suspected DVT in pregnancy under Diagnosis recommendations if your patient is pregnant.
Consider CT (or MRI) venography if venous ultrasound is not available or inconclusive.[26][27] Contrast venography (using x-ray) is now rarely used, except when other investigations are inconclusive, or catheter-based treatment is considered.[26]
Other indications for CT (or MRI) venography include:
Detection of more proximal thrombosis if this is clinically suspected or suggested by flow patterns on Doppler ultrasound[90]
Detection of other medical conditions that may be an alternative cause of the patient’s symptoms and/or increase the risk of DVT, such as extrinsic venous compression syndromes or pelvic malignancies[26][27]
Diagnosis of pulmonary embolism before insertion of filter devices (although this is not required in practice if anticoagulation has been started)[27]
Planned endovascular treatment.[27]
CT may also be more accurate than ultrasound at detecting thrombosis in larger veins of the abdomen and pelvis.[26][90] However, it requires the use of iodine contrast, and involves radiation exposure (a significant concern, particularly in younger patients).[26] MRI has shown similar sensitivity and specificity to venous ultrasound for diagnosis of DVT, but has been evaluated in far fewer studies, using a variety of different techniques.[27]
Result
presence of an intraluminal filling defect
further investigation for unprovoked DVT
Test
An unprovoked DVT is a DVT in a patient who had no pre-existing, major, transient provoking risk factor in the prior 3 months.[12]
Major provoking risk factors include: major surgery; trauma; significant immobility (bedbound, unable to walk unaided, or likely to spend a substantial proportion of the day in bed or in a chair); pregnancy or puerperium; use of oral contraceptive/hormone replacement therapy).
An unprovoked DVT may be suggestive of an underlying condition so further investigations are sometimes warranted.
Undiagnosed cancer
In any patient diagnosed with unprovoked DVT who is not known to have cancer:[12]
Review medical history
Review baseline blood tests including full blood count, renal and hepatic function, prothrombin time (PT), and activated partial thromboplastin time (aPTT)
Offer a physical examination.[26]
Do not offer further investigations for cancer for patients with an unprovoked DVT unless they have relevant clinical symptoms or signs.[12] Occult cancer is present in approximately 3% to 5% of patients with an unprovoked DVT.[93]
These recommendations are from the National Institute for Health and Care Excellence in the UK. However, note that the European Society for Vascular Surgery recommends clinical examination and sex-specific cancer screening (but without routine extensive screening for cancer) if the patient has an unprovoked DVT.[26]
Thrombophilia testing
Consider testing for hereditary thrombophilia in patients who don't have an identifiable risk factor and have a first-degree relative who has had a VTE, if it is planned to stop anticoagulation.[12][26]
Do not routinely offer thrombophilia testing to first-degree relatives of people with a history of DVT and thrombophilia.[12]
Consider testing for antiphospholipid antibodies in patients who have had an unprovoked DVT if it is planned to stop anticoagulation treatment.[12][26] In practice, this is usually only done if the patient is under 50 years of age.
Practical tip
Be aware that tests for hereditary thrombophilia and antiphospholipid antibodies can be affected by anticoagulation; specialist advice may be needed.[12]
Result
may show underlying cause
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