What laboratory tests are ordered for a suspected Deep Vein Thrombosis (DVT)?

Laboratory Tests for Suspected Deep Vein Thrombosis (DVT)

The primary laboratory test for suspected DVT is a D-dimer test, which should be ordered in conjunction with clinical pretest probability assessment to guide the diagnostic approach. 1

Diagnostic Algorithm Based on Pretest Probability

Clinical Pretest Probability Assessment

• Use a structured clinical model (such as Wells score) to categorize patients into low, moderate, or high pretest probability for DVT 1

For Low Pretest Probability Patients:

• Order a moderately or highly sensitive D-dimer test as the initial test 2, 1
• If D-dimer is negative, no further testing is needed as DVT can be safely excluded 2
• If D-dimer is positive, proceed to proximal compression ultrasound (CUS) 2

For Moderate Pretest Probability Patients:

• Order a highly sensitive D-dimer test or proceed directly to ultrasound 2, 1
• If D-dimer is negative, DVT is excluded 2
• If D-dimer is positive, proceed to proximal CUS 2

For High Pretest Probability Patients:

• Proceed directly to proximal compression ultrasound or whole-leg ultrasound 1
• D-dimer testing is less useful in high probability patients due to high false positive rates 1

Special Considerations

Pregnant Patients

• For pregnant patients with suspected DVT, initial evaluation with proximal compression ultrasound is recommended over D-dimer testing 2, 1
• If initial proximal CUS is negative, suggest further testing with either serial proximal CUS (day 3 and day 7) or a sensitive D-dimer 2

Upper Extremity DVT

• For suspected upper extremity DVT, initial evaluation with combined modality ultrasound (compression with either Doppler or color Doppler) is suggested over D-dimer testing 2, 1

Recurrent DVT

• In patients with suspected recurrent lower extremity DVT, initial evaluation with proximal CUS or a highly sensitive D-dimer is recommended 2
• If highly sensitive D-dimer is positive, proceed to proximal CUS 2

Patients with Comorbid Conditions

• Initial ultrasound testing may be preferred over D-dimer in patients with conditions likely to cause elevated D-dimer levels (pregnancy, inflammation, malignancy, advanced age) 1, 3

Limitations and Pitfalls

• D-dimer has high sensitivity but poor specificity for DVT, making it useful for ruling out but not ruling in DVT 4, 5
• The failure rate of D-dimer as a stand-alone test is approximately 1.8% for all DVTs and 0.6% for proximal DVTs, emphasizing the importance of combining it with clinical assessment 6
• D-dimer performance decreases with increasing prevalence of DVT in the tested population 7
• Different D-dimer assays have varying sensitivities and specificities; highly sensitive assays are preferred for ruling out DVT 5, 7

Efficiency Considerations

• Using D-dimer with clinical probability assessment can reduce the need for ultrasound testing by approximately 30% in outpatient settings 3
• This approach is most efficient in outpatient populations where DVT prevalence is typically 20% or less among clinically suspected cases 4