Describe the diagnostic approach for suspected lower-extremity DVT.
This approach is valid for evaluating a first-episode or recurrent lower extremity DVT.
Calculate the pretest probability (PTP) using Wells criteria for DVT.
Check D-dimer first for low PTP (initial D-dimer is not diagnostically helpful for intermediate and high PTP).
Negative (< 500 ng/mL): DVT ruled out
Positive (≥ 500 ng/mL): Possible DVT; Proceed to venous US.
Obtain venous ultrasound (US) for intermediate or high PTP, or low PTP with positive D-Dimer
Negative US
Intermediate and low PTP: DVT ruled out
High PTP: Repeat venous US within a week if no alternate diagnosis.
Positive US: DVT confirmed; Screen for an underlying cause if no risk factors for DVT are identified on initial evaluation.
Inconclusive US: Consider venography, CT venography, or MR venography.
When is a D-dimer not helpful?
A negative D-dimer can help rule out DVT without venous ultrasound in patients with low pretest probability. It is not helpful for patients with intermediate or high pretest probability.
Diagnostic approach (picture).
Describe the Indication, Interpretation and accuracy of D-Dimer.
Based on the patient's pretest probability, the initial test to evaluate for DVT may be either D-dimer or compression ultrasound.
D-dimer
Indication: preferred initial test for nonpregnant patients with a low PTP of DVT (Wells score = 0)
Interpretation
Cutoff for normal range is typically 500 ng/mL
Some centers use age-adjusted D-dimer cutoffs
Accuracy
High sensitivity (∼ 96%)
Low specificity (∼ 36%)
Not reliable for ruling out DVT in patients with intermediate or high PTP
In which patients does a D-dimer rule out DVT?
In patients with a low pretest probability of DVT, a negative D-dimer (< 500 ng/mL) rules out DVT.
However, a positive D-dimer alone does not confirm DVT.
Describe the indications and procedure of lower extremity venous ultrasound.
Indications
Preferred initial test for patients with moderate or high PTP of lower extremity DVT (Wells score ≥ 1)
Preferred initial test for pregnant or postsurgical patients even if the PTP of DVT is low
Next diagnostic step in patients with a low PTP of lower extremity DVT but a positive D-dimer
Procedures
Compression ultrasound: The vein is identified and external pressure is directly applied over it with the probe.
Proximal leg: allows for evaluation of the femoral and popliteal veins up to the trifurcation
Whole leg: allows for evaluation of the proximal leg PLUS distal calf veins (beyond the trifurcation)
Venous duplex ultrasound: can be added to compression ultrasound
Involves the addition of color Doppler
Allows for better evaluation of noncompressible deep veins
List supportive ultrasound findings of DVT.
Noncompressibility of the obstructed vein
Intraluminal hyperechoic mass
Distention of the affected vein
On Doppler imaging
Absent venous flow (complete obstruction) or abnormal venous flow (partial obstruction)
Inadequate augmentation of venous flow on distal calf compression or Valsalva maneuver
Of recurrent DVT: thrombosis in a new venous segment or a > 4 mm increase in noncompressibility of the obstructed vein
Describe the accuracy of lower extremity venous ultrasound.
operator- and technique-dependent
Whole leg study by radiology: high sensitivity and specificity (∼ 95%) for proximal DVT; lower sensitivity and specificity (∼ 65%) for distal DVT
Point of care ultrasound study (POCUS) by trained nonradiologists: equivalent accuracy for detection of proximal DVT, but distal DVT may be missed
What is the most accurate test for diagnosing DVT?
Compression ultrasound of the whole leg with color Doppler (i.e., duplex scanning) is the most accurate test for diagnosing DVT.
If appropriately trained, consider performing a POCUS study to quickly rule in a proximal DVT. If the study is negative, further investigations (e.g., a whole leg ultrasound study by radiology) may be necessary.
List important routine laboratory studies.
These are recommended to assess organ function and bleeding risk prior to anticoagulation.
CBC
BMP
Liver chemistries
Coagulation studies
Assess the patient's baseline coagulation status.
Screen for subtherapeutic INR in patients on VKA therapy with suspected recurrent DVT.
Describe the indications and findings of Venography, CT venography, or MR venography.
Inconclusive findings on compression US
Inability to perform compression US due to, e.g., obesity, significant lower limb edema, immobilization cast
Finding: intraluminal filling defect
Which patients may require additional evaluation?
Patients with the following may require additional evaluation: unprovoked DVT, unexplained recurrent VTE, and/or a history suggestive of a hypercoagulable state or occult malignancy.
List factors for thrombophilia screening.
Thrombosis in patients < 45 years
Unusual thrombus localization
Positive family history
Recurrent or multiple thromboses
Recurrent pregnancy loss
Describe the indications and investigations for screening for occult malignancy.
Indications: unprovoked VTE (esp. patients > 50 years), recurrent VTE, unusual thrombus location
Investigations: routine age-appropriate cancer screening recommended
In addition to routine laboratory studies: consider e.g., urinalysis, FOBT, serum calcium levels
Consider CXR, colonoscopy, mammogram, digital rectal exam, Pap smear
List the most common DDs.
Superficial thrombophlebitis
Muscle or soft tissue injury (i.e., posttraumatic swelling or hematoma)
Lymphedema
Venous insufficiency
Ruptured popliteal cyst
Cellulitis
Compartment syndrome
In patients with suspected cellulitis AND risk factors for DVT or no response to antibiotics, consider compression ultrasound to rule out DVT
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