A blood D-dimer test should be performed. American College of Chest Physicians guidelines suggest that a clinical pretest probability assessment of deep venous thrombosis (DVT) should be the first part of the diagnostic process for a first lower extremity DVT, rather than performing an imaging study in all patients. The pretest probability that this patient has a DVT is low. Applying the Wells criteria for DVT, this patient's score is 0, making DVT an unlikely diagnosis. The criteria include a medical history indicating increased risk for venous thromboembolism (VTE) (such as previously documented DVT; paralysis, paresis, recent cast immobilization of legs; recent bedridden state for ≥3 days; major surgery) and leg symptoms highly suggestive of DVT (calf swelling ≥3 cm; swelling of the entire leg; unilateral pitting edema; localized tenderness along the deep venous system). Because this patient has none of these factors and no alternative explanation for his leg symptoms, obtaining a D-dimer test is an appropriate next step. If the test is negative, no further testing is needed, because a VTE has been ruled out. However, if the D-dimer is positive, an imaging study is then indicated. The imaging test of choice is duplex ultrasonography of the leg. When using the D-dimer test for pretest probability assessments, a moderately or highly sensitive assay should be used. Physicians must be aware of the sensitivity of the test used in local laboratories. Lower sensitivity assays have not been validated as useful for predicting pretest probability.

Long-term anticoagulants should not be prescribed without a confirmed diagnosis of VTE. However, in a patient with high or intermediate clinical suspicion of DVT, beginning anticoagulation while awaiting diagnostic test results is appropriate. In a patient with low clinical suspicion of acute DVT, such as this patient, withholding anticoagulant therapy while awaiting the test result is suggested.

If a D-dimer test is not available or is positive, performing duplex ultrasonography would be the appropriate next step.

Magnetic resonance venography is a noninvasive diagnostic method that is as accurate as contrast venography for DVT diagnosis. However, it is not the study of choice for DVT diagnosis because of the complexities of performing the test and its high cost relative to duplex ultrasonography. Additionally, initial imaging is not indicated in this patient with a low risk of DVT before further assessment with D-dimer testing.