What lab work should I order for a suspected Deep Vein Thrombosis (DVT)?

Laboratory Testing for Suspected DVT

Order a D-dimer test (highly sensitive assay preferred) combined with clinical pretest probability assessment using the Wells score to guide your diagnostic approach for suspected DVT. 1

Initial Clinical Assessment

Before ordering any labs, stratify your patient's pretest probability using the Wells score into low, moderate, or high risk categories, as this determines which laboratory tests are appropriate 2, 1:

• Low pretest probability (≤0 points): DVT prevalence approximately 2-5% 3, 4
• Moderate pretest probability (1-2 points): DVT prevalence approximately 20-34% 2, 3
• High pretest probability (≥3 points): DVT prevalence >60% 3, 4

Laboratory Testing Algorithm by Pretest Probability

Low Pretest Probability Patients

Order either a moderately sensitive or highly sensitive D-dimer as your initial test 2, 1:

• A negative D-dimer result excludes DVT with 100% sensitivity and 100% negative predictive value—no further testing needed 3, 5
• If D-dimer is positive, proceed to compression ultrasound 2
• The D-dimer approach is preferred over proceeding directly to ultrasound (Grade 2B-2C) 2

Moderate Pretest Probability Patients

Order a highly sensitive D-dimer test as the preferred initial laboratory test 2, 1:

• A negative highly sensitive D-dimer excludes DVT with 100% sensitivity and 100% negative predictive value 3
• If D-dimer is positive, proceed to proximal compression ultrasound 2
• Alternatively, you may skip D-dimer and proceed directly to ultrasound, though D-dimer is suggested as the initial approach (Grade 2C) 2, 1

High Pretest Probability Patients

Skip D-dimer testing entirely and proceed directly to proximal compression ultrasound or whole-leg ultrasound 1:

• D-dimer has limited utility in high pretest probability patients, as the positive predictive value is only 80% and most will require imaging regardless 3
• The high prevalence of DVT (>60%) makes imaging the most efficient initial approach 2, 4

Special Clinical Situations Where D-Dimer Should Be Avoided

Do not order D-dimer in patients with conditions that cause false-positive elevations 1, 6:

• Chronic lower extremity edema: Proceed directly to ultrasound, as D-dimer will likely be falsely elevated even without DVT 6
• Active malignancy: D-dimer has reduced specificity; consider proceeding directly to imaging 1, 7
• Pregnancy: Initial evaluation with proximal compression ultrasound is recommended over D-dimer 1
• Advanced age, inflammation, recent surgery, or hospitalization: These conditions elevate D-dimer independent of thrombosis 1

D-Dimer Test Selection

When ordering D-dimer, specify a highly sensitive assay (such as ELISA-based tests like VIDAS or Auto Dimer) rather than point-of-care rapid tests 2, 8:

• Highly sensitive assays achieve 84-100% sensitivity for DVT 5, 8
• Less sensitive rapid assays (like Nycocard) show only 63% sensitivity and are inadequate for ruling out DVT 8
• The standard cutoff is <500 ng/mL for a negative result 5

Common Pitfalls to Avoid

• Don't order D-dimer in high pretest probability patients—you're wasting time and resources, as they need imaging regardless of the result 1, 6
• Don't rely on D-dimer in patients with chronic edema or malignancy—the false-positive rate makes it clinically unhelpful 6, 7
• Don't use less sensitive D-dimer assays—only highly sensitive assays have adequate negative predictive value 2, 8
• Don't assume a positive D-dimer confirms DVT—specificity ranges from only 42-57%, so imaging is still required for confirmation 3, 8

Additional Laboratory Considerations

Beyond D-dimer for acute diagnosis, no other routine laboratory tests are required for the initial diagnostic workup of suspected DVT 2, 1. Complete blood count, coagulation studies (PT/INR, aPTT), and thrombophilia testing are not part of the acute diagnostic algorithm and should not delay imaging if indicated 2.