Laboratory Tests for Suspected DVT
The primary laboratory test to order for suspected DVT is a highly sensitive D-dimer, but only after clinical pretest probability assessment determines the patient has low or moderate probability—high probability patients should skip D-dimer and proceed directly to imaging. 1, 2
Pretest Probability Assessment Guides Laboratory Testing
Before ordering any labs, stratify the patient using a validated clinical decision rule (such as Wells score) into low, moderate, or high pretest probability 3, 1:
- Low probability patients (~5% DVT prevalence): Order moderately sensitive or highly sensitive D-dimer as the initial test 3, 1
- Moderate probability patients (~17% DVT prevalence): Order highly sensitive D-dimer as the preferred initial test 3, 2
- High probability patients (~53% DVT prevalence): Skip D-dimer entirely and proceed directly to compression ultrasound 1, 4
D-Dimer Testing Strategy
A negative highly sensitive D-dimer in low or moderate probability patients excludes DVT without further testing. 3, 2 This approach is safe, with only 0.5% of patients developing proximal DVT during 90-day follow-up when DVT was initially excluded using this strategy 5.
Critical Limitations of D-Dimer
D-dimer has important false-positive triggers that reduce its utility 4, 2:
- Pregnancy (order ultrasound first instead) 2
- Active malignancy 4
- Inflammation or infection 4
- Advanced age 4
- Recent surgery or trauma 4
In patients with these conditions, skip D-dimer and proceed directly to ultrasound imaging. 3, 2
Baseline Laboratory Tests Before Treatment
Once DVT is confirmed by imaging, order these essential baseline tests before initiating anticoagulation 1:
- Complete blood count with platelet count 1
- Coagulation profile (PT/INR, aPTT) 1
- Comprehensive metabolic panel (to assess kidney and liver function) 1
Do not delay anticoagulation to obtain additional imaging when DVT is already confirmed. 1
Special Population Considerations
Suspected Recurrent DVT
Start with proximal compression ultrasound or highly sensitive D-dimer 1
Pregnant Patients
Begin with proximal compression ultrasound—avoid D-dimer as the initial test 1, 2
Upper Extremity DVT
Use combined modality ultrasound rather than D-dimer 1, 2
Patients Already on Anticoagulation
Be aware that D-dimer may be falsely negative in patients on therapeutic anticoagulation, creating diagnostic challenges 6. Consider proceeding directly to ultrasound in this population.
Common Pitfalls to Avoid
Never use D-dimer as a standalone test in high pretest probability patients—it should not be used to rule out DVT in this population, as the negative predictive value drops to only 78-86% 4, 7.
Never rely on clinical assessment alone to rule out DVT—objective testing is crucial to avoid fatal pulmonary embolism or unnecessary anticoagulation 4.
Recognize that D-dimer testing is most accurate in low pretest probability patients (negative predictive value 99.5%) and least accurate in high pretest probability patients (negative predictive value 85.7%) 7.