What diagnostic test should be ordered for a suspected deep vein thrombosis (DVT) in an adult patient with symptoms such as leg pain, swelling, or redness, and possibly a history of immobility, surgery, or previous DVT?

Diagnostic Testing for Suspected Deep Vein Thrombosis (DVT)

For suspected DVT in the leg, order compression ultrasound (CUS) of the proximal veins or a highly sensitive D-dimer test based on clinical probability assessment, with the specific approach determined by whether the patient has low, moderate, or high pretest probability of DVT. 1, 2

Clinical Probability Assessment First

Before ordering any test, apply a structured clinical prediction rule such as the Wells score to categorize the patient's pretest probability as low, moderate, or high. 2 This stratification is essential because clinical features alone are insufficient for diagnosis and the testing strategy differs based on probability. 2, 3

Testing Strategy by Pretest Probability

Low Pretest Probability (~5-6% prevalence)

• Order a highly sensitive D-dimer test as the initial and only test needed. 1, 2
• If the D-dimer is negative, no further testing is required—DVT is effectively ruled out. 1, 2, 3
• If the D-dimer is positive, proceed to proximal CUS. 1

Moderate Pretest Probability (~20-30% prevalence)

• Order either a highly sensitive D-dimer OR proximal CUS as the initial test, with D-dimer slightly preferred. 1
• The choice depends on local availability, costs, and whether the patient has comorbid conditions that elevate D-dimer levels (such as cancer, pregnancy, or recent surgery), in which case starting with ultrasound is preferred. 1
• If D-dimer is negative, no further testing is needed. 1
• If D-dimer is positive, proceed to proximal CUS. 1
• If proximal CUS is negative but D-dimer was positive, repeat proximal CUS in 1 week. 1

High Pretest Probability (>60% prevalence)

• Order proximal CUS or whole-leg ultrasound directly without D-dimer testing. 1, 2
• D-dimer should NOT be used as a standalone test in high-probability patients because it cannot safely rule out DVT in this population. 1, 2
• If the ultrasound is positive, treat for DVT without confirmatory venography. 1
• If proximal CUS is negative, order additional testing with either highly sensitive D-dimer, whole-leg ultrasound, or repeat proximal CUS in 1 week. 1

Ultrasound Options

Proximal compression ultrasound (CUS) assesses the femoral and popliteal veins for compressibility and is the most widely used imaging modality. 2, 4, 5 Studies show sensitivity and specificity exceeding 95% for proximal DVT. 4, 5

Whole-leg ultrasound should be preferred in specific situations: 1, 2

• Patients unable to return for serial testing
• Severe symptoms suggesting calf DVT
• Extensive unexplained leg swelling where iliac vein thrombosis is suspected

Follow-Up Testing Algorithms

If proximal CUS is negative and clinical suspicion remains:

• With negative D-dimer: no further testing needed. 1
• With positive D-dimer: repeat proximal CUS in 1 week OR perform whole-leg ultrasound. 1
• After negative serial proximal CUS (initial plus 1-week repeat): no further testing needed. 1

Special Circumstances

When ultrasound is impractical (leg casting, excessive subcutaneous tissue, morbid obesity preventing adequate compression assessment) or nondiagnostic, order CT venography, MR venography, or MR direct thrombus imaging as alternatives. 1, 2

For extensive unexplained leg swelling with negative proximal or whole-leg ultrasound and positive D-dimer, image the iliac veins to exclude isolated iliac DVT. 1, 2

Critical Pitfalls to Avoid

• Never rely on clinical features alone for diagnosis—they are nonspecific and have little individual diagnostic value. 2, 3
• Never use D-dimer as the only test in high pretest probability patients—proceed directly to imaging. 1, 2
• Never skip serial testing when indicated (negative initial proximal CUS with positive D-dimer or high clinical suspicion). 1, 2
• Never forget to image iliac veins in patients with extensive unexplained leg swelling when proximal ultrasound is negative. 1, 2

Practical Summary

The diagnostic approach is risk-stratified: low-risk patients get D-dimer only, moderate-risk patients get D-dimer or ultrasound (D-dimer preferred for cost-effectiveness), and high-risk patients bypass D-dimer and go straight to ultrasound imaging. 1, 2, 3 This algorithm safely rules out DVT while minimizing unnecessary testing and costs.